Laboratory Exercise:

Horizontal, Tilted and Folded Rock Layers Name ________________________

 

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The Geology of the Grand Canyon

 

         How was it formed?

         Where did all the rock come from?

         Why does it look like it does?

         When did all this happen?


Grand Canyon rock layers
clickable image map

thumbnail image
Slopes of Dox Formation near Unkar Creek


How was it formed?

The truth is that no one knows for sure though there are some pretty good guesses. The chances are that a number of processes combined to create the views that you see in todays Grand Canyon. The most powerful force to have an impact on the Grand Canyon is erosion, primarily by water (and ice) and second by wind. Other forces that contributed to the Canyon's formation are the course of the Colorado River itself, vulcanism, continental drift and slight variations in the earths orbit which in turn causes variations in seasons and climate.

Water seems to have had the most impact basically because our planet has lots of it and it is always on the move. Many people cannot understand how water can have such a profound impact considering that the Canyon is basically located in a desert. This is one of the biggest reasons that water has such a big impact here. Because the soil in the Grand Canyon is baked by the sun it tends to become very hard and cannot absorb water when the rains do come. When it does rain the water tends to come down in torrents which only adds to the problem. The plants that grow in the Grand Canyon tend to have very shallow root systems so that they can grab as much water as possible on those rare occasions when it does rain. Unfortunately these root systems do nothing to deter erosion by holding the soil in place. Now you've got lots of water, no place for it to go, but down to the Colorado River, and nothing holding the soil and rock in place. The result is frequently a flash flood roaring down a side canyon that can move boulders the size of automobiles, buses and even small houses. If automobiles, buses and small houses are in the way then it will take them too. Luckily no one builds houses in the Grand Canyon so that's not a problem but there are a few autos, vans and buses sitting at the bottom of the Colorado. This mass that moves down a side canyon during a flash flood is more like a fast flowing concrete than water and it can be very dangerous. You should always be well informed of weather conditions when you are hiking through side canyons in the Grand Canyon.

After erosion by liquid water the next most powerful force is probably its solid form, ice. In the colder months, especially on the north rim, water seeps into cracks between the rocks. These cracks can be caused by seismic activity, or by the constant soaking and drying of the rocks. When the water freezes it expands and pushes the rocks apart and widens the cracks. Eventually rocks near the rim are pushed off the edge and fall into the side canyons. These rocks sometimes hit other rocks and are stopped but on occasion one fall by a large rock will cause a cascading effect and create a rock fall that will alter the landscape drastically in the side canyon. Debris from rock falls piles up at the bottom of the side canyons and is then carried down to the Colorado River the next time there is a flash flood. Rock falls frequently take out sections of trail in the Grand Canyon requiring the Park Service to close these trails until they can be repaired.

Once the ice had pushed the rocks off the edge and the water in the flash floods has carried them down to the river, then the Colorado itself takes over. The erosive action of the Colorado has been severely constrained by the building of the Glen Canyon Dam, which ended the annual spring floods, but there is still a lot of water flowing relatively quickly through a very narrow gorge. Before building the dam the Colorado River had spring floods that would exceed a flow rate of 100,000 CFS. All of that snow melting in the Colorado Rockies came pouring down through the Grand Canyon in May and June, every year, like clock-work. These spring floods were considerably larger than today's "trickle" of 8,000-10,000 CFS at low water and even the 20,000 CFS peak flow rates.

The Colorado's spring floods used to carry away all of the debris that was deposited in the main channel by the flash floods, but todays mediocre flow rates have a tough time doing the job. It still gets done to some extent, it just takes a lot longer. In the process of moving the rocks and sediment down the river to the Pacific Ocean the bed of the river is scoured by all of this fast moving debris which slowly eats away at the banks and bed of the river. This causes the river to widen and cut down deeper into the lower rock layers. Another cause for the slowing of the erosive force of the Colorado River is the fact that it is now trying to cut through harder granites and schists found at the bottom of the Canyon instead of the softer limestones, sandstones and shales near the top. This rock takes a lot longer to erode and a slower moving river means it takes even longer.


Where did all of the rock come from?

Geologists have this question pretty much wrapped up, aside from some missing layers, or unconformities, that have been completely eroded away. Again there were a number of forces at work and this is where continental drift, vulcanism and climatic change come into play.

The fact that the Earth's continents are not fixed in place but rather float on a sea of molten rock, means that they move around quite a bit, relatively speaking. The surface of the Earth is composed of about twenty of these "plates" which form its crust. Seven of these plates are very large and consist of entire continents or sea floors and the rest are smaller in comparison. The plates are average out to be about 50 miles or 80 kilometers thick and float on top of the Earth's mantle. The plate which contains the Grand Canyon, the North American plate, was at one time considerably further south than its present location and therefore had a much different climate. In time it has gradually moved north and rotated about ninety degrees to its present location and configuration.

Continental drift animated gif

The continents in motion, the red dot indicates the approximate location of the Grand Canyon region.

 

Click here or on the image above for more information on the continental drift theory.

Click here to visit the USGS site This Dynamic Earth: the Story of Plate Tectonics

The North American Plate is moving west and is colliding with the Pacific Plate which is moving towards the northwest. The Pacific Plate is also expanding from its middle and its eastern edge is being subducted beneath the North American Plate as it comes into contact with it. Oceanic plates are typically subducted beneath continental plates because they area heavier. As pressure increases while they are being subducted they tend to get heavier still and to some extent they start to fall and pull more plate along with them. As the Pacific Plate moves beneath the North American Plate the rock of which it is composed is superheated and water is released and begins to rise. This water, which is extremely hot, causes lighter minerals to melt and forms lava which feeds the chain of volcanoes on the eastern edge of the Pacific Rim which runs from Alaska to Chile.

Continental plates

The conflict between the plates is also frequently responsible for mountain building activity. As the plates are forced together they sometimes buckle which causes mountain ranges to be formed along the contact point. This is how the Rocky Mountains, the Sierra Nevada and the costal mountains of California were formed and how the Aleutian Island are being formed today. A much older range of mountains, which geologists suspect were much higher than todays Rocky Mountains and may even have rivaled the Himalayas, now forms the base of the Grand Canyon. The rocks that made up these mountains are about 1.7 billion years old, or about one-third the age of our planet. These mountains have long since eroded away and sedimentary deposits have covered them over.

The sediments that covered the roots of these ancient mountains were deposited by a series of advancing and retreating ocean coast lines. As the climate of our planet warms and cools the median sea level of the planet rises and falls due to the melting and freezing of the polar caps. When the sea level rises, land areas which are close to the coast and relatively low in altitude are sometimes submerged. This was the case with the land area of the Grand Canyon and is why so many different sedimentary rock layers exist. Each of these was formed by a different period in which the ocean moved in and covered the land, stayed for a while, and then retreated again. Limestone deposits are created when the ocean moves in and slates, shales and mudstone deposits are created when the ocean moves out and the area is covered by silts washing into the retreating ocean.

How do we know this?

Well, the fact is that most of the rock in the Grand Canyon is composed of sedimentary rock which can only be formed at the bottom of the ocean or in shallow coastal plains. The Kaibab Limestone which is the current top of the Grand Canyon is composed mostly of a sandy limestone, with some sandstone and shale thrown in for good measure. This means that it was probably formed in a shallow sea near the coast. The fact that it contains fossils of creatures that used to live in the ocean, like brachiopods, coral, mollusks, sea lilies, worms and fish teeth, only tends to reinforce this belief. The intrusion of sandstone and shales into this layer means that at times the layer was also above the surface of the water but still very close to the edge. Sandstones are solidified sand which are typically fields of sand dunes or beaches, and shales are solidified mud which are common to river deltas. By dating the fossils found in the rock of the Kaibab Limestone, geologists have determined that it is approximately 250 million years old, and this is the youngest layer.

So where are the younger rocks?

The younger rocks have already been eroded away by the forces of nature, at least in the immediate vicinity of the Grand Canyon. Some of the younger layers, like the Navajo Sandstone of which the Vermilion Cliffs and the rock of Zion National Park are composed, can be found in the region north of the Grand Canyon. Going even further north results in even younger rocks as can be seen in Bryce Canyon. The area from Bryce Canyon down to Grand Canyon is typically referred to as the Grand Staircase.

Colorado Plateau
Cross sectional view of the Colorado Plateau
showing the Grand Staircase


Why does it look like it does?

The reason that it looks the way it does is due to the sequence in which the events that help to create it happened. We already know that there was once a very tall chain of mountains in the area that occupied the Grand Canyon. These mountains were, over many millions of years, eventually eroded away to form a level plain. Fluctuations in climate then caused the oceans to move in over successive periods and each time a new rock layer was deposited. The rock layers were deposited one on top of the other and sometimes there were long periods in between in which some of the upper layers were eroded away, sometimes completely.

And now the Colorado River comes into play. The ancestral "Colorado River" came into being when the Rocky Mountains to the east of the Grand Canyon were formed, at sometime around 60-70 million years ago, as the primary western drainage for these mountains. Over millions of years the course of this ancestral river changed its course a number of times as the terrain around it was altered. The course of the ancestral Colorado River probably started in Colorado and at one point it entered the region of Marble Canyon, but that is about all that can be agreed upon at this point.

Some geologists believe that very young rock layers to the west of the Grand Canyon, dated at only 5 and 10 million years old, and through which the Colorado now flows, indicate that the river could not have been flowing there prior to that time. The river had to cut through these layers after they were deposited. The search for another exit for the Colorado River from the Grand Canyon has been a hotly debated issue. Some geologists believe that it flowed out of Marble Canyon where the Little Colorado now enters, others believe that it exited near present day Diamond Creek and still others believe that it exited through massive caves in the Redwall Limestone. The most likely exit at this point seems to be up through Kanab Creek which would have had the ancestral river flowing back up into Utah and then across Nevada and California to the Pacific.

At around 17 million years ago, while the river was flowing across this ancient landscape, the land mass know as today's Colorado Plateau began to uplift. The uplift was caused by pressures deep with the Earth and may have been caused by additional conflict between the North American Plates and the Pacific Plates. This process continued until around 5 million years ago which interestingly enough is the date of the sedimentary layers just west of the plateau. At its greatest height the Colorado Plateau was once about three miles above sea level. The rise of the plateau probably prevented the seas from submerging it again and instead the topmost layers were eroded away and carried into the sea. The most favorable currently accepted theory is that the Colorado River continued to cut through the Colorado Plateau while the land rose around it.

At some point around 5 million years ago something happened to cause the Colorado to change its course and exit via its present route down to the Gulf of California. The most likely cause for the change in its course was probably due to it being captured by another river, which was draining the western portion of the Colorado Plateau. This other river eroded northward along the San Andreas fault, then eastward and eventually entered the Grand Canyon and joined with the Colorado near present day Kanab Creek. The Colorado would then have abruptly changed its course and flowed out this newly formed exit.

Much of the eastern Grand Canyon was already formed by the time the river changed its course. Side canyons had formed along fault lines in the rock and these were eroded away and the rock within them carried down to the Colorado. The Colorado River took all of the rock that was put into it and carried it off to the Pacific Ocean. Over many more millions of years the erosion along the course of the Colorado continued to widen the Canyon to present the vistas that you see today. Before the Glen Canyon Dam was built the Colorado River used to carry three cubic miles of sediment into the Pacific Ocean every hundred years.


When did all this happen?

  • The Earth was formed approximately 5 billion years ago.
  • The roots of the ancient mountain range that now lies at the bottom of the Grand Canyon were formed about 1.7 billion years ago.
  • There is then an unconformity of about 450 million year in which the rocks are missing.
  • At 1.25 billion years ago the first sedimentary layer, the Bass Formation, was laid down. Ancient coastal dwelling colonies of algae known as Stromatolites are preserved within this layer and indicate that the area was coastal at that time.
  • At 1.2 billion years ago the sea retreated leaving mud flats behind which eventually became the Hakatai Shale.
  • At 1.19 billion years a similar layer was deposited which is known as the Dox Formation. This was again formed of mudstones and shales and contains ripple marks as well as other features that indicate that it was close to the coast.
  • Between 1.25 and 1.1 billion years ago there was also some volcanic activity with the region of the Grand Canyon and this is when the Cardenas Basalts were formed.
  • Between 1 billion and 825 million years ago additional coastal and shallow sea formations, which are now classified as the Chuar group, were deposited.
  • There is then another unconformity of about 250 million years in which new rock layers were probably laid down but were completely eroded away.
  • The Tapeats Sandstone was then deposited around 550 million years ago along long vanished coastline. There are places in the Canyon in which in which off shore islands have been found imbedded within this layer.
  • The Bright Angel Shale was deposited around 540 million years ago and indicates that the ocean was again advancing.
  • The Muav Limestone was deposited around 530 million years ago at the bottom of a shallow sea.
  • The thick layer of Redwall Limestone which began to deposited around 330 million years ago indicates that the land was submerged for a great deal of time.
  • The Supai Group which rests atop the Redwall is dated at 300 million years ago and indicates that it was formed in an above water and coastal environment.
  • The Hermit Shale which was deposited around 280 million years ago contains many plant fossils which indicate that it was also above water.
  • The Coconino Sandstone represents the remains of a vast sea of sand dunes which was blown down from the north around 270 million years ago.
  • The layers found within Toroweap Formation contains both sandstone and limestone, indicating that it was sometimes coastal and sometimes submerged. These layers date to around 260 million years.
  • The top layer of the Grand Canyon, the Kaibab Limestone, contains many marine fossils which indicate that it originated at the bottom of the sea. This layer is around 250 million years old.
  • Rock layers younger than 250 million years have been eroded away and no longer exist in the immediate vicinity of the Grand Canyon.
  • The Rocky Mountains begin to form 60-70 million years ago and at some point later the Colorado River is born.
  • At this point there are at least two popular theories which describe what happens next:
    • Around 20 million years ago the Colorado River begins to carve into the Grand Canyon at its eastern end, Marble Canyon, and probably exiting via Kanab Canyon.
    • At 17 million years ago the Colorado Plateau begins to uplift and causes the river to cut deeper.
    • Around 5 million years ago the uplift ceases and another river working its way northward along the San Andreas fault and eastward along the western Colorado Plateau captures the Colorado River.
  • OR
    • Around 35 million years ago the Kaibab Plateau begins to uplift and diverts the ancestral Colorado, which was already established on a course very similar to that of today, to the southeast. The cut-off western portion, now named the Hualapai Drainage System, contines to drain the western region.
    • About 12 million years ago the Colorado's path to the sea is blocked and a huge lake, Lake Bidahochi, is formed.
    • Eventually the Hualapai cuts back through the southern portion of the plateau and recaptures the Colorado. Lake Bidahochi is drained and becomes the Little Colorado River.

Formation of the Grand Canyon

Click here or on the image above for illustrations.


The Coconino Sandstone of Mount Hayden


Upper layers (Kaibab - Redwall) from Grandeur Point


Redwall cliffs and Supai butte of Horseshoe Mesa


Brachiopod fossil along Boucher Trail


Crinoid and coral fossils along Widforss Trail


Deer Creek slicing through the Tapeats Sandstone and emerging as a waterwall at the Colorado River


Thunder River emerges from the Muav Limestone


Arch in the Kaibab Limestone - Angel's Window


Eroded Kaibab Limestone - Duck-On-A-Rock


Tapeats Sandstone and Vishnu Schist of the inner gorge as seen from Pima Point


Good view of upper rock layers (Kaibab, Toroweap, Coconino, Hermit, Supai and Redwall) as seen from Mohave Point


Tapeats Sandstone and Vishnu Schist of the inner gorge as seen from the Tonto Platform


Cliffs of Redwall Limestone from South Kaibab Trail


Shell Fossil along Beamer Trail


Most of the primary layers are visible here. Can you find them?


Good view of the Coconino Sandstone or bath tub ring running around the north rim


Top layers as seen from Bright Angel Point on the north rim


Strangely eroded feature of Esplanade Sandstone along the Thunder River Trail


Surprise Valley and Inner Gorge as seen from rim of the Esplanade Sandstone


Palasides of the Desert - almost a clear cut from rim to river


Gorge of Tapeats Sandstone below Marble Canyon as seen from the Beamer Trail

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Grand Canyon Rock Layers


Grand Canyon rock layers

[ Grand Canyon Home ]

 

Copyright Bob Ribokas, 1994-2000, all rights reserved. This publication and its text and photos may not be copied for commercial use without the express written permission of Bob Ribokas.

 

 

 

 

 

Bright Angel Quadrangle 1:48,000 Geomorphic Province _____________________

 

The topography shown on this quadrangle is often called scarp (escarpment, cliff), slope, shelf. The scarp would be the edge of a resistant rock layer, the shelf would be the top of a resistant rock layer which would form a "shelf" (a small somewhat flat area) between a scarp and a slope. The slope would form on lesser resistant rock between scarps or between a shelf and a scarp.

 

1. Examine the diagrams showing the rock layers of the Grand Canyon on the diagram above and on Fig. 2 on the back of the Bright Angel quadrangle. What type of geologic structure is shown above the Great Unconformity? Horizontal

 

2. a. What other type of geologic structure could you justify to characterize the geology below the Great Unconformity? Complex

b. Why could you justify this term? Presence of metamorphic rock.

c. Which type is probably the more appropriate to use to characterize the area below the Great Unconformity? Complex, because of large amount of schist with granite intrusions and lesser amounts of the formerly horizontal layers.

d. Draw arrows on the diagram above to indicate the direction of movement along the

fault below the Great Unconformity.

 

2. a. Which family of rocks is found above the Great Unconformity? Sedimentary

 

b. What specific members of this family do you find? Limestone, Sandstone, Shale

 

3. What specific types of rock do you find below the Great Unconformity? Vishu Schist, Zordaster Granite, Limestone Shcle, Quartzite, & Extrusive Lavas

 

4. What is the width of the canyon between the Bench Mark just west of El Tovar Hotel (SC) and the bench mark at Tiyo Point (NC)? 8.6 Miles

 

3. What is the elevation of Tiyo Point? 7,765 ft. What is the elevation of the Bench Mark near the El Tovar Hotel? 6,866 ft.

 

4. What is the direction of regional slope between Tiyo Point and the El Tovar Hotel? South

 

5. a. Why do the streams from the Kaibab Plateau flow toward the Colorado River and those on the Coconino Plateau flow away from the Colorado River? Regional slope is from north to south.

 

b. Which is wider, the canyon north of the Colorado River or south of the river? North

 

c. How does your answer to b relate to your answer to a? More water flows toward the Colorado River from the north rather than from the south, therefore, the canyon should be wider on the north side of the river and narrower on the south side of the river.

 

6. a. What is the depth of the Grand Canyon between Tiyo Point and the Bench Mark at the river a short distance to the east from the line between Tiyo Point and the El Tovar Hotel? Tiyo Point 7765 ft. Bench Mark at River 2436 ft. =5,329 ft. of elevation between Tiyo Point and the river.

 

b. How close is the depth of the Grand Canyon to being one mile? Very close, 5,280 feet in a mile, so the Grand Canyon at this point is a bit more than a mile deep.

 

7. The average annual rainfall at the Ranger Station on the Kaibab Plateau is 25". This qualifies the area to be arid to semi-arid. Given the relatively sparse rainfall, which members of the sedimentary family would you expect to be resistant and which would you expect to be non-resistant. Sandstone and Limestone would be expected to be resistant and Shale would be expected to be non-resistant.

 

8. Examine the diagram above and Fig. 2 on the back of the quadrangle and describe the type of slopes you find on the sandstone, limestone, and shale. Sandstone and Limestone form cliffs and shale forms steep slopes.

 

9. a. How are the sandstone and limestone cliffs portrayed on the topographic map? Solid brown lines.

 

b. How are the steep shale slopes portrayed on the topographic map? Closely spaced regular contour lines with index contours at 250 foot intervals.

 

c. What is the minimum number of sedimentary rock layers you would expect to see if you hiked the Bright Angel Trail from the El Tovar Hotel to Pipe Creek? At least 5. What rocks would you expect to see when you reached the Colorado River? Visnu Schist

 

Trail Description : Bright Angel Trail

If you never been hiking in the Grand Canyon before then this is the place to start. The Bright Angel Trail is one of the two superhighways of the Grand Canyon, the other being the South Kaibab Trail. Both of these trails are well maintained and offer some spectacular views of the Canyon. The Bright Angel Trail has the advantage of offering a considerable amount of shade (depending on the time of day) of which the South Kaibab Trail offers virtually none. Water is also available on the Bright Angel Trail at the One-and-a-Half-Mile and Three-Mile Resthouses and again at Indian Garden (4.6 miles from the rim) and the River Resthouse (7.7 miles from the rim). Water is available at the upper two resthouses Spring through Fall only. Water is available at Indian Garden and the River Resthouse all year. Do not drink water from springs or creeks anywhere in the Canyon without treating it first. Toilets are available just beyond the One-and-a-Half-Mile resthouse, at Indian Garden and at Bright Angel Campground.

The Bright Angel Trail was originally an indian trail used by the Havasupai indians to commute between the rim and Indian Garden. The trail was improved by prospectors in the late 1800's. One of the miners, Ralph Cameron, realizing that the tourist trade was more profitable than the mining trade, bought out his partners and took control of the trail. He extended the trail from Indian Garden to the river and began to charge a toll of $1 for its use. The Park Service constructed the South Kaibab Trail shortly thereafter to provide tourist with a free access path to the river. In 1928, after a long ownership battle with the Mr. Cameron, ownership of the Bright Angel Trail was finally transferred to the National Park Service.

Camping along this trail is in designated campgrounds only, those being at Indian Garden and at Bright Angel Campground. You need a Backcountry Reservation for a site.

The trailhead for the Bright Angel Trail is located a few hundred feet to the west of the Bright Angel Lodge, next to the mule corral.

Mileages are as follows (one-way):

Elevations:

  • Rim - 6860'
  • Indian Garden - 3800', 3060' below rim
  • Colorado River - 2400', 4460' below rim

The upper section of trail looks easy enough on the way down. Nice wide trail, long switchbacks and some incredible views. This section of the trail can be a killer on the way back up, depending on how far you went down. If you're hiking up from the river, or anywhere from Indian Garden or beyond, it seems to take forever to get from one switch back to the next and it feels like you'll never get back to the top. There are some petroglyphs along the top stretch of the trail near the first tunnel.

The Bright Angel Fault is also quite obvious along this section of trail as you will discover by examining the rock layers on either side of this side canyon. This is not as obvious at the top of the trail as you descend through the Kaibab and Toroweap formations, but is very obvious at the contact between the Toroweap and Coconino. Take a look around after passing through the second tunnel on the way down the trail and locate the top of the Coconino layer on both sides of the canyon. You will notice that geologic contact between the Supai and Coconino formations is much higher, almost 200 feet higher, on the west side than it is on the east.

Once you pass beyond the first resthouse the switchbacks come a little closer as the canyon narrows. The section of trail between the first and second resthouses is very scenic. There are some more petroglyphs carved into the rock above you at around the two mile mark, at a spot called Two Mile Corner.

The Three-Mile resthouse makes for a good day hike for those wanting to see the inner canyon but not wanting to exert themselves too much. There is a very nice spot for viewing just beyond the resthouse. Follow the trail that leads up past it to the right. Here there are also the remains of the old cable car system that was used to bring supplies down to Indian Garden.

Beyond Three-Mile Resthouse the trail becomes a little steeper, as the trail descends through a break in, first, the Redwall limestone, and then through the Muav formation. The switchbacks at Jacobs Ladder will seems to go on forever on the climb out. Once you are beyond this the trail levels out for the remaining mile or so to Indian Garden.

As you approach Indian Garden you will be walking across a formation known as Bright Angel Shale, which forms a wide bench about 2/3 of the way down the canyon, known as the Tonto Platform.

Indian Garden makes another very fine day hike and a great place for a picnic. From here you can also take the trail out to Plateau Point, 1.5 miles each way, for an awesome view of the Inner Gorge and the Colorado River. To head to Plateau Point take the fork in the trail to the left just beyond Indian Garden. The fork to the right will keep you on the Bright Angel Trail and take you to the river. If you are heading for Plateau Point, watch for another fork in the trail and this time keep to the right. The left fork will put you on the Tonto Trail heading west, and although it is very scenic you won't get to see the river for a long time. You will also not find anything along the Tonto Trail to make you stop and turn around as you could walk for days on this trail and not get to the end.

If you are heading for the river, you will still have another mile or so before the trail really starts to head down again. The slope is very gradual as you descend through the Bright Angel Shale and the top of the Tapeats Sandstone formations, until you get to the Devil's Corkscrew where it begins a rather abrupt descent through the Vishnu Schist. Beware of this area in the summer time as the temperature can easily reach 130 degrees. Beyond the Devil's Corkscrew the trail levels out again for maybe another mile that will bring you to the Colorado River. This technically marks the end of the Bright Angel Trail though some people consider the River Trail that takes you to Bright Angel Campground, 2 miles beyond, to still actually be part of it.

Hiking along the River Trail is not quite as easy as one might expect. The trail makes a couple of fairly steep ascents and descents along the way and walking across some of the dune sections with a full pack can be difficult. The trail along the river runs for 1.7 miles before it comes to the Silver Suspension Bridge. To get to Bright Angel Campground continue over the bright for approximately 1/3 of a mile. To reach the South Kaibab Trail or the Black Suspension Bridge continue east along the river trail for approximately 1 mile more. At the Black Suspension Bridge marks the other end of the River Trail. From here you can head south up the South Kaibab Trail to Yaki Point or across the bridge and north along the North Kaibab Trail to Bright Angel Campground, Phantom Ranch and the North Rim (14 miles away).

If you are planning to hike both the Bright Angel and South Kaibab Trails you are advised to come down the South Kaibab Trail and go out on the Bright Angel. The reason being that the hike out on the South Kaibab Trail can be quite hot and dry during any period other than the winter months. There is no water available anywhere along the trail and almost nothing in the way of shade.

Tilted Strata

 

Southern Indiana and the Mammoth Cave Quadrangle will be used to illustrate tilted strata. The large scale topographic features on tilted strata will be cuestas and lowlands. The rocks will all be dipping in this type of geologic structure and the cuestas and lowlands will form along the sides of large anticlines or the around large domes.

 

Physiographic Divisions of Indiana


Map of Indiana showing physiographic divisions.

 

Bedrock Geology of Indiana


Introduction

Indiana is a large anticline that plunges to the northwest. Consequently, the age and type of rocks in Indiana are governed by this large structural feature. The youngest rocks are in the northeastern and southwestern corners of the state, and the oldest are in the southeastern corner. The oldest rocks are primarily limestone, dolostones, and shales, whereas the youngest rocks are mostly sandstones and shales with minor amounts of limestone and coal. The distribution of rock types is the major control on the physiographic provinces in the south-central part of the state. Let's look more closely at the geology of the bedrock surface.

Getting to the rocks

In Indiana, it is not that easy to see bedrock and the bedrock surface. Two-thirds of the state is covered with glacial material (see Freeze Frame). In the northwestern corner of the state, these sediments are as much as 350 feet thick. Only in the south-central part of the state are rocks exposed at the earth's surface. It is here that we can look at large exposures of rock along stream and river valleys, road cuts, and excavations (quarries and surface mines). Commonly, geologists use chips and cores brought up from holes drilled into the earth as their only source of information.

Types

There is a large variety of rocks that are found in Indiana. All the rocks that are exposed at the bedrock surface, however, are sedimentary rocks. Most consist of sandstone, shale, siltstone, limestone, and dolostone. Other rock types are coal, conglomerate, gypsum, claystone, and chert. Deep wells and exploratory test drillings have encountered granite, gabbro, basalt, andesite, and metasedimentary rocks at depths of 3,500 feet to about 3 miles below the bedrock surface.

Groupings

Sedimentary rocks occur as parallel or nearly parallel layers, or beds. Beds vary in thickness (<1 to 10s of feet) and spatial distribution (<1 to 10s of miles). Beds that commonly occur together or have similar characteristics and distribution are lumped together by geologists and called "formations." Formations are a shorthand way to describe a collection of rocks that are similar. Typically, formations are tens to hundreds of feet thick, and they can be traced for tens to hundreds of miles. A collection of formations can be lumped together into "groups." Groups are often hundreds to thousands of feet thick and can be traced hundreds of miles. The map to the right shows groups in Indiana. Groups are commonly shown on statewide maps, because formations, and especially beds, would be too thin to draw without making a very large map. Like formations, there is nothing special about the group names. They are useful when you want to talk about large collections of rocks--which is what we are going to do.

Distribution

In the southeastern corner of the state, most of the rocks consist of gray, greenish-gray, and brown shales with a minor amount of shaly limestone. These rocks are part of the Maquoketa Group, and they were deposited during the Upper Ordovician (~440-446 millions of years ago [mya]). They are some of the the oldest exposed rocks along the axis of Indiana's anticline. Here, the anticline is often called the Kankakee or Cincinnati Arch. Throughout the Paleozoic Era, this area was a locus of uplift and erosion.

North and northwest of the Maquoketa Group are rocks attributed to the Silurian Period (~440-410 mya). These rocks are described by a wide variety of names at the group (Salina, Bainbridge), formation (Cataract, Sexton Creek, Wabash, etc.), and member levels across the state. Most consist of limestones and dolostones with varying amounts of fossils and argillaceous material. Of particular importance is the occurrence of bound-together skeletal material interpreted as ancient reefs. Two prominent occurrences of these deposits occur as bands from Fort Wayne to Lake Michigan and Terre Haute to Spencer County.

Rimming the Silurian rocks are Devonian (~ 410-360 mya) carbonates and evaporites of the Muscatatuck Group. These rocks are often used to define that margins of the Michigan Basin to the north and the Illinois Basin to the southwest (see Tectonic Features of Indiana). Consisting of mostly dolomite, the Muscatatuck Group contain granular and fibrous anhydrite and gypsum in the Detroit River Formation.

Overlying the Muscatatuck Group is a thick sequence of Devonian and Mississippian (~360-320 mya) shale known as the New Albany Shale. The New Albany Shale is a formation that is from 100 to 340 feet thick. Parts of the New Albany are brown to black shales that are rich in organic materials. Recently, these black shales have been drilled for possible recovery of natural gas. The New Albany shale occurs across a wide area of Indiana in the northern tier of counties. Here, the shale may be overlain by hundreds of feet of glacial material.

A swath of siltstone, shale, sandstone, and minor amounts of limestone extends north and northwestward from the Ohio River at Floyd County to Benton County on the Illinois border with Indiana. These rocks are called the Borden Group. The erosion of these rocks provide the scenic views in Brown County.

To the southwest of the Borden Group is a sequence of carbonate rocks that is 250 to nearly 500 feet thick and has significant amounts of gypsum, anhydrite, shale, chert, and calcareous sandstone. These rocks make up the Sanders and Blue River Groups. Within the Sanders Group is a formation known by geologists as the Salem Limestone and by architects as "Indiana Limestone." This thickly bedded limestone is quarried for a variety of architectural purposes and is known as one of the premier dimension stones in the world.

Sandwiched between the Blue River Group and overlying Pennsylvanian rocks are 140 to 350 feet of sandstone, limestone, and shale, split into more than 20 formations. These rocks are part of the Buffalo Wallow, Stephensport, and West Baden Groups. Erosion by southwestward-flowing rivers (much like today) at the end of the Mississippian and during the early Pennsylvanian Period has dissected many of these units. The missing rocks between these two periods are known as an unconformity. The Mississippian-Pennsylvanian Unconformity (3-8 my long) is a undulatory surface dissected by V-shaped valleys of up to 125 feet deep.

In the southwestern part of the state the rocks of the Raccoon Creek Group overlie the Mississippian-Pennsylvanian Unconformity. This group consists of mostly sandstone and shale with minor amounts of coal, limestone, clay, and chert. The Raccoon Creek Group is the first of the coal-bearing units in Indiana. Ten named coals occur in this interval.

The outcrop belt of the Carbondale Group extends from Warrick County northward to Vermillion County. The Carbondale Group is similar in composition to the Raccoon Creek Group and includes some laterally persistent limestones and four of Indiana's commerciallyimportant coals. This unit averages about 300 feet in thickness, although it thins northwestward.

Rocks that outcrop in the southwestern corner of Indiana comprise the McLeansboro Group. This group can be as thick as 770 feet and consists of mostly sandstone and shale with discontinous beds of coal and limestone throughout the sequence.

1. What is the dominant structure that is oriented NW-SE from SE to NW Indiana? Anticline

2. Where are the oldest and youngest rocks in the State found? Oldest: Southeast corner Youngest: NE and SW corners

3. What specific sedimentary rocks are found in Indiana? Limestone, dolostones, shale, sandstone, coal.

4. The average rainfall in Indiana is over 30" per year which qualifies it to be humid climatic area. Which of the rocks you identified in question 3 would you expect to be resistant and which would you expect to be non-resistant?

Resistance: Sandstone would be resistant, dolostone would normally be semi-resistant and would be expected to form cuestas. Limestone and shale would be non-resistant and would form the lowlands. Coal would be non-resistant, but is not found in sufficient thickness to significantly impact the development of large topographic features like cuestas and lowlands.

5. Examine the Physiographic Divisions of Indiana map and answer the following. The Physiographic Divisions across the southern part of the state are dipping west/southwest off the anticline (Cincinnati Arch). Name the Physiographic Divisions with a north/south directional orientation from east to west beginning with the Muscatatuck Plateau Scottsburg Lowland, Norman Upland, Mitchell Plateau (Plain), Crawford Upland, Wabash Lowland.

6. In which provinces would you expect to find the sandstones? Muscatatuck Pleatau, Norman Upland, Crawford Upland.

7. In which provinces would you expect to find limestone and shale? Scottsburg Lowland, Mitchell Plateau (Plain), Wabash Lowland.

8. a. Which direction do the rocks in 6 and 7 dip? Westward.

b. Which direction would the steep edge of the cuesta face? East

9. Draw your best estimate of a geologic cross section from the Muscatatuck Pleatau to the Wabash Lowland which shows the provinces and the rock layers that produce them. Label the provinces and indicate the type of rock which could be found in each.

Mammoth Cave Quadrangle and General Geology Smiths Grove and Rhoda, Ky Diagram

Physiographic Region: ______________________________

The techniques for showing topography on this map are the contour line and plastic shading, which gives a 3 Dimensional effect by casting a light source from the northwest. This places the south and southeast slopes in a shadow which gives the 3-D effect. The advantage is the 3-D effect. The disadvantage is that the contour lines cannot be read in the shadowed area.

This area has the same basic geology that you saw in the Physiographic Divisions of Indiana. Here the rock layers are dipping from south to north. Park City is located at the contact point between the lowland and the cuesta (upland). The edge of the cuesta (escarpment) is called the Dripping Springs Escarpment.

1. Examine the General Geology diagram.

a. What is the significance of the Big Clifty, Hardinsburg and Caseyville sandstone? The sandstone layers produce the cuesta because they are resistant.

b. What is the significance of the St. Louis, St. Genevieve, Girkin, Haney, Glen Dean limestones? If they were the only rocks in the area then the entire area would be a lowland.

c. Which geologic formation dominates the lowland? St. Louis. You will hear more about this formation when we do the chapter on karst.

2. Find Pilot Knob on the topographic map and the General Geology diagram. It is a hill that is more representative of the topography found in the cuesta and the topography that surrounds it. A knob (hill) like this is called an outlier. It gets detached from the escarpment by streams cutting around it and then stands as an "outlier" of one type of geology that is surrounded by another type of geology.

a. What two rock formations form Pilot Knob? St. Genevieve and Girkin.

b. What specific rock is found in the formations named in a? Limestone

c. Which formation and specific rock type lies below the St. Genevieve formation? St. Louis limestone.

d. Which formations used to be found above the Girkin limestone that is currently at the top of Pilot Knob? Big Clifty (ss), Haney (ls), Hardinsburg (ss), Glen Dean (ls) and Caseyville (ss).

e. Is there currently a resistant rock layer protecting Pilot Knob? No

f. What will the area occupied by Pilot Knob become when the Girkin and St. Genevieve formations are eroded away. Lowland

g. Locate Little Knob to the west of Pilot Knob. What topographic feature does it represent? outlier

h. Where would you look for features like this to appear in the future? Along the edge of the Dripping Springs Escarpment.

i. Where was the Dripping Springs Escarpment in the geologic past? South of its present position.

j. Where would you expect to find the Dripping Springs Escarpment in the geologic future? Farther to the north.

Harrisburg, Pa. Topographic Quadrangle and 3-D Plastic map (upstairs hallway).

1. Which Physiographic Province is shown in the central part of the map? Ridge and Valley

2. This is a classic area of folded structure. What two geologic structures do you expect to find in a folded structural area? Anticlines and Synclines.

3. What structure do you think Peters, Third, Second, and Blue Mountains represent? Anticlines

4. What structures do you think the valleys with Clark Creek, Stony Creek, and Fishing Creek represent? Synclines

5. Anticlines and synclines can be eroded by running water to produce a complex of six different erosional features. List them. Anticlinal ridges and valleys, Synclinal ridges and valleys, Homoclinal ridges and valleys.

6. Examine the Folded Appalachians diagram. Source: Atlas of American Geology, C.S. Hammond & Co., 1960

 

 

 

 

 

 

 

 

 

 

 

 

 

a. What type of structure is found between Peters and Blue Mountain? Synclinal

b. What type of erosional features do Peters, Second and Blue Mountain represent? Homoclinal ridges.

c. What type of erosional feature is found between Peters and Third, Third and Second, and Second and Blue Mountain? Homoclinal valleys

d. What type of erosional feature does Third Mountain represent? Synclinal ridge

e. What does this example illustrate with respect to basing an answer on only one source of information, the topographic map in this case, for an answer to a geologic question? One source of information cannot necessarily provide all the information that is needed to answer a question.

Loysville, Pa. Quadrangle Physiographic Province _____________________

1. a. What is your best estimate of the structure of Tuscarora and Conococheague Mountains in the northwestern portion of the map? Anticlinal

b. What is the justification for your answer? 1. They are mountains (ridges) and the first assumption is that a ridge is an anticlinal ridge. 2. They are very symmetrical and this would be the expected shape of an anticline. A homoclinal ridge would be expected to have one side steeper than the other. A synclinal ridge would be expected to have steeper slopes facing outward in both directions from the center of the structure.

2. Examine the southern third of the map from Bowers Mountain to the eastern edge of the map.

a. What types of structures appear to dominate this area? Anticlines and Synclines

b. What types of topographic features can be carved into these structures by the force of running water? Anticlinal ridges & valleys, Synclinal ridges and valleys, Homoclinal ridges and valleys.

c. What is your best estimate of the geologic structure of Bowers Mountain? Anticlinal

d. What is the most likely topographic feature represented by Bowers Mountain? Anticlinal ridge.

e. What is your best estimate of the geologic structure of Sheaffer Valley? Synclinal

f. What is the most likely topographic feature represented by Sheaffer valley? Synclinal valley.

g. Locate the boundary line between Perry County and Cumberland County on the topographic map. The boundary line is reproduced on the Figure below. Label the most likely structures that are identified by the dashed lines. Anticline, Syncline, Anticline, Syncline.

 

 

 

 

 

 

 

 

 

 

 

 

h. Indicate with arrows the direction of strike where you see the letter S. Indicate with arrows the direction of dip where you see the letter D.

i. What is the most likely topographic feature represented by Blue Mountain? Homoclinal ridge.

j. What is the most likely topographic feature in the valley occupied by Gap Creek? Anticlinal valley

k. What is the most likely topographic feature occupied by Trout Run, McCabe Run, and Green Valley? Synclinal valleys

l. Locate the closely spaced contours immediately south of the word Tyrone and Barkley Ridge. These tapered features represent the noses of plunging (pitching) anticlines. Put an arrow indicating the direction of plunge (pitch) on the Figure where you see the letters P. Plunge (pitch) is toward the northeast. The noses of plunging (pitching) anticlines can be perceived as being stabbed into the ground like the point of a spear. They disappear underground beyond this point.

m. How does the direction of plunge (pitch) on the anticlines compare with the direction of strike? They are the same.

n. Locate the area immediately southeast of the words Lower Mifflin and focus on the county line between the Beacon and Flat Rock. Locate the ridge where the line labeled Approximate (between Upper Frankford and Lower Frankford) joins the Perry County/Cumberland County line. These ridges represent the noses of plunging (pitching synclines). Put an arrow indicating the direction of plunge (pitch) on the Figure where you see the letters P. Plunging (pitching) synclines are in a sense being "pried out" of the ground at this these points. The "prying out" produces the wider and more rounded shapes.

o. How does the direction of plunge (pitch) on the synclines compare with the direction of strike? They are the same. Everything is plunging (pitching) to the northeast.

p. Complete the "origami" project as directed in class. This activity will demonstrate how the rock layers in plunging (pitching) anticlines and synclines produce a zig-zag pattern on the landscape and enhance the ridge and valley topographic expression of areas with folded structures.

 

 

 

 

 

 

 

 

 

 

 

 

Sideling Hill Brochure

Read the brochure and answer the questions that follow the text.

Coastal & Estuarine Geology Program

Pamphlet Series

GEOLOGY OF THE SIDELING HILL ROAD CUT

contact: Dale Shelton (dshelton@mgs.md.gov)

by David K. Brezinski

1989 (revised 1994)

FIGURE 1. Location map of the road cut through Sideling Hill.

One of the best rock exposures in Maryland and indeed in the entire northeastern United States is located approximately 6 miles west of Hancock in Washington County, where Interstate 68 cuts through Sideling Hill (Figure 1). Almost 810 feet of strata in a tightly folded syncline are exposed in this road cut. Although other exposures may surpass Sideling Hill in either thickness of exposed strata or in quality of geologic structure, few can equal its combination of both. This exposure is an excellent outdoor classroom where students of geology can observe and examine various sedimentary rock types, structural features, and geomorphic relationships.

Sideling Hill lies in the Valley and Ridge Physiographic Province of eastern North America, a region characterized by tightly folded strata. Erosion of these folds has produced a series of subparallel ridges and valleys, in which the ridges are capped by erosion-resistant sandstones, and the intervening valleys are underlain by soluble limestones and easily eroded shales. At first, Sideling Hill may appear to be a somewhat unusual feature, inasmuch as the downfold, or syncline, exposed in the road cut would seem to be more likely to form a valley, rather than a ridge. However, the youngest rocks, or those highest in the stratigraphic section, are the resistant sandstones and conglomerates of the Purslane Formation, which occur in the center of the fold and cap the ridge.

The valleys on either side are underlain by more easily eroded rocks of the Rockwell and Hampshire Formations. This topographic inversion, in which the structural low becomes a topographic high, is also seen at Town Hill, the next major ridge to the west and a structural twin to Sideling Hill. Between these two ridges the intervening lower area is composed predominantly of Devonian age shales and siltstones.

FIGURE 2. Geologic cross-sections of the north (top) and south (bottom) sides of the I-68 road cut through Sideling Hill.

The Rockwell and Purslane Formations were deposited during the early Mississippian, about 330 to 345 million years ago. At the road cut, approximately 450 feet of the Rockwell Formation are exposed and consist of interbedded, tan and gray-green, clay rich sandstones, gray-green to dark-gray, silty shales, and gray to dark-gray, sandy siltstones with several intervals of red-brown claystone near the top. In places, thin shaly coals and coaly shales are interbedded with shales and siltstones. These coals are interesting in that coal is typically not common in Lower Mississippian strata. An even rarer and indeed unusual lithology, termed diamictite, is present approximately 70 feet above the base of the section (A of Figure 2). A diamictite is a very poorly sorted to unsorted rock composed of clay, silt, sand, and pebbles or cobbles. The larger pebbles and cobbles consist of a multitude of lithologies including granite, graywacke, chert, and quartzite. The origin of such diamictites is highly debatable and no generally accepted theory has yet been proposed.

Fossils are moderately common in the Rockwell Formation, but almost all are plant fragments and imprints. Marine fossils are present within the black silty shale 165 to 178 feet above the lowest exposed strata (B in Figure 2).

 

The fossils are generally rare within these intervals and consist of brachiopods and bivalves. A similar marine unit has been recognized in correlative rocks in central Pennsylvania, where it has been termed the Riddlesburg Shale. The Sideling Hill exposure is the first recognition of this zone in Maryland.

The Rockwell Formation in the area of Sideling Hill was probably deposited in an alluvial plain environment near sea-level. The Riddlesburg Shale records a major shift of the shoreline which submerged an area from eastern Ohio to western Maryland (Figure 3A).

FIGURE 3. Sequence of development of the rocks exposed at Sideling Hill.
A, Shallow marine waters and adjacent shoreline swamps of the Riddlesburg sea.
B, River systems of the Purslane.
C, Folding during the Alleghenian mountain-building episode.
D, Post mountain-building erosion to the ridges and valleys seen today.

Overlying the Rockwell Formation is the Purslane Formation, typified by gray-green, tan, and white, cross-bedded sandstones and quartz-pebble conglomerates with interbedded gray siltstones, shales, and coaly shales. Only about 350 feet of the formation occur on Sideling Hill, the remainder, an unknown thickness, having been removed by erosion. Individual sandstone units range in thickness from 25 to 75 feet.

Near the top of the exposure are 45 feet of dark-gray siltstones and shales in which numerous thin shaly coal beds are present. Analysis of one of these coals shows it to be semianthracite in rank. This same shaly sequence may be observed more closely at the sharp turn in old U.S. 40 (now Scenic 40) as it crosses the crest of Sideling Hill, 1.5 miles south of this road cut. The only fossils found in the Purslane Formation are plant remains common in the lower part of many of the thick sandstone units, and in the upper coaly sequence.

The prominence of thick sandstone units with quartz-pebble conglomerates plus the lack of marine fossils also suggest an alluvial plain environment of deposition for the Purslane Formation in the area of Sideling Hill. The sandstones and conglomerates represent channel deposits of sand and gravel laid down by rivers. The coal beds may have formed in swamps on flood plains adjacent to the fluvial channels (Figure 3B).

FIGURE 4. Movement of water producing the seepage
within the Purslane sandstones.

Movement of water producing the seepage

The Sideling Hill road cut exposes a section through the axis of a tightly folded syncline. A syncline is a fold in which the strata on either side dip inward toward the axis. Such folding resulted from the enormous compressional stresses developed in the Earth's crust by the collision of the North American and African continents. This episode of mountain-building is termed the Alleghenian Orogeny and reached its maximum during the late Permian or early Triassic, approximately 230 to 240 million years ago (Figure 3C).

Moreover, these same stresses produced differential slippage between the strong or highly competent sandstones and the weak or less competent carbonaceous siltstones and shales. Such slippage resulted in the development of two types of fractures -- faults and cleavage. Cleavage can form where two competent units (e.g., sandstone) surround a relatively thin incompetent shale. The result is an abundance of small subparallel fractures within the shale. Numerous small faults can be observed in the shaly sequence near the top of the Purslane Formation. Here compressional stresses near the axis of the syncline have caused offsets along fractures in several of the siltstone beds (C of Figure 2).

During the spring, summer, and fall, water can be observed seeping out from along fractures in the rock along the axis of the syncline. This water has its origin as rain which infiltrates the permeable and porous sandstone and conglomerate, and runs down through the rock until it reaches a barrier such an impermeable layer of clay. When it reaches this barrier it runs down the dip of the beds to the axis of the fold and then is emitted at the exposed rock face (Figure 4). During the winter such places of outflow of water serve as points from which layers of ice originate and grow to cover much of the exposed face.


For more information on Maryland geology and geography check out our Earth Science Books Online page.

Suggested Readings:

Bjerstedt, T.W., 1986. Regional stratigraphy and sedimentology of the Lower Mississippian Rockwell Formation and Purslane Sandstone based on the new Sideling Hill road cut, Maryland: Southeastern Geology, v. 27, p. 69-94.

Brezinski, D.K., 1989. The Mississippian System in Maryland: Maryland Geol. Survey Report of Investigation 52, 75 p.

Pelletier, B.R., 1958. Pocono paleocurrents in Pennsylvania and Maryland: Geological Society of America Bulletin, v. 69, p. 1033-1064.

Read,C.B., 1955. Floras of the Pocono Formation and Price Sandstone in parts of Pennsylvania, Maryland, West Virginia, and Virginia: U.S. Geological Survey Professional Paper 263, 32 p.

Reger, D.B., 1927. Pocono stratigraphy in the Broadtop Basin of Pennsylvania: Geological Society of America Bulletin, v. 56, p. 397-410.

Stose, G.W. and Swartz, C.K., 1912. Paw Paw-Hancock Folio. Maryland-West Virginia-Pennsylvania: U.S. Geological Survey Folio 179, 24 p.

Girty, G.H., 1927. The Pocono fauna of the Broad Top coal field, Pennsylvania: United States Geological Survey Professional Paper 150 E, p. 111-123.


This electronic version of "The Geology of Sideling Hill" was prepared by R.D. Conkwright, Division of Coastal and Estuarine Geology, Maryland Geological Survey.

Please send comments on this page to Emery T. Cleaves, Director, at: ecleaves@mgs.md.gov

State of Maryland
Department of Natural Resources

prepared by:
Maryland Geological Survey
2300 St Paul St.
Baltimore, MD 21218

(410) 554-5505


updated 2/28/00


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1. How much folded strata is exposed in the roadcut for I-68? 810 feet.

 

2. What kind of geologic structure does Sideling Hill represent? Syncline

 

3. What kind of topographic feature does Sideling Hill represent? Synclinal ridge

 

4. Why is Sideling hill a ridge instead of a valley? Resistant sandstones and conglomerates are at the top of the structure.

 

5. What is a topographic inversion? When a structurally high area (anticline) has a valley as a topographic expression of when a structurally low area (syncline) has a ridge as a topographic expression.

 

6. What are the main rocks that are exposed in Sideling Hill? Conglomerate, Sandstone, Siltstone, Shale, Coal

 

7. a. Which rock in Sideling Hill is highly debated as to its origin? Diamictite

 

b. What are the characteristics of this rock? Poorly sorted to unsorted rock composed of clay, silt, sand, and pebbles or cobbles. The pebbles and cobbles include granite, greywacke, chert, and quartzite.

 

c. What would be a more common, or generic, name could possibly be used instead of diamictite? Conglomerate

 

8. What are the sequence of events that are suggested for the formation of Sideling Hill? A. Shallow marine waters and adjacent shoreline swamps of the Riddlesburg Sea producing marine sedimentary layers. B. River systems producing the Purslane Formation composed of sandstones and conglomerates and the Rockwell Formation that probably were deposited as an alluvial plain. C. Folding during the Alleghenian mountain building episode when the North American and African places collided. D. Post mountain-building differential erosion by streams to produce the ridges and valleys we see today.

 

9. a. What can you see on the sides of the Sideling Hill road cut during the winter? Ice

 

b. Where does the water come from? Rain infiltrates into the permeable sandstones and conglomerates and seeps out along the contact between the permeable sandstones and conglomerates and the impermeable shales and claystones where it can be seen wetting the surface during the spring, summer, and fall.