Folded Mountains - Laurel Hill Anticline EarthCache

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Hidden : 7/30/2013
Difficulty:
2.5 out of 5
Terrain:
2.5 out of 5

Size: Size: other (other)

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Geocache Description:

This is an EarthCache, so there is no cache container hidden at these coordinates. An EarthCache is meant to provide an earth science lesson by having you make observations and reporting these to the cache owner. For more information about EarthCaches, visit EarthCache.org

As you're taking a scenic drive through the mountains in our area, have you ever wondered how they formed, and what they look like inside? Of course you have!! Well, with some simple observations and measurements, you will discover those answers today.

Try this: put a sheet of paper flat on a table. Hold one end immobile and slowly slide the other end toward it. The paper will buckle upward as you move the ends together. Now picture doing this with a newspaper or magazine: all the pages buckle upward together. Now imagine that instead of paper, you have horizontal rock layers. As plate tectonics move the continents around the globe,two continents can collide and force the rock layers to buckle upward. Geologists call this process "folding" and it is this folding that created our mountains. The top of one of these folds is known as an "anticline" while the valley between two anticlines is a "syncline". The center of the anticline is also known as the "axis".


If a strong enough river is flowing through an area being folded upward, and the folding is slow enough, then the river can cut through the anticline as it is being formed, creating a steep-walled gorge. This is what we see along the Youghiogheny River from Confluence to Ohiopyle as the Yough cuts through the Laurel Hill anticline. The ancient river (or Proto-Yough) eroded material away as fast as it was uplifted by the folding. It is in steep gorges like this that many layers of rocks can be seen, and such a gorge is a treasure trove of geological information.

The rocks you will see today were formed in the Paleozoic Era, starting in the Devonian Period (420 - 360 million years ago, or "Ma"), and on into the Carboniferous Period ( 360 - 300 Ma). In America, the Carboniferous Period is divided into the Mississippian Period ( 360 - 325 Ma) and the Pennsylvanian Period ( 325 - 300 Ma). The collision between North America and Africa that created the Allegheny Mountains occurred over about 85 million years during the Pennsylvanian and Permian Periods. This may seem like a long time, but remember, the continents move at only an inch or two per year - about the rate your fingernails grow!
During the millions of years that an Era or Period can last, the types of rocks being formed in an area can change. Maybe the sea level rises and a coal-producing swamp gets buried by beach sand and limestone made from corals. When geologists started identifying rock layers, maybe they noticed that a particular brown sandstone always had a gray shale above it, and thick limestone below it. So a bookkeeping system was developed by geologists: they would group a related series of rock layers into "Formations". This allows you to identify rock layers in one area as being the same layers found in another exposure miles away, even if no rocks connect them. The rock formations you will see today and the times they were formed are described below.

Glenshaw Formation - Formed late in the Pennsylvanian Period, the Glenshaw Formation is a series of shales, coal, limestones and sandstones created by a fluctuating sea level. It is only present on the farthest east and west portions of Laurel Hill, but is common in the valleys on either side.
Allegheny Formation - These rocks also formed in the Pennsylvanian, and are a similar mix to the Glenshaw, but with many clay layers. It also appears only on the sides of Laurel Hill.
Pottsville Formation - This formation from the mid-Pennsylvanian is mostly a hard grayish sandstone with some limited conglomerates. It is found at the top of Laurel Hill and also creates Ohiopyle Falls. Finding such a layer at different elevations was an important clue to the folding process.
Mauch Chunk Formation - This layer is from the late-Mississipian, and is mostly shales and sandstones. Included in the Mauch Chink is the Loyalhanna Limestone, which is quarried in many places in these mountains. It is exposed at the surface on the western side of Laurel Hill, where erosion has removed the Pottsville layers.
Burgoon Sandstone - This early-Mississippian layer is a thick tannish-gray sandstone.
Sharon-Oswayo Formations - The layers formed during the late-Devonian to early-Mississippian are very confusing, even after years of study, and are usually considered all together. Sandstones and grayish shales predominate. There is very little surface exposure except down in the gorge.
Catskill Formation - This Devonian layer is mostly a grayish sandstone with some shales. It also is only exposed deep in the gorge.
Foreknobs Formation - These late-Devonian sandstones and shales are the oldest rocks exposed in the gorge (or anywhere else in Fayette County). Some conglomerate beds are found, as well as marine fossils.

When geologists explored the Yough Gorge, they found many isolated outcrops of rock - most of the formations were hidden under the soil and trees, just like today. But when they plotted the coordinates and elevations where they found the various formations, and then connected the dots, they created a remarkable picture. (View is to the south, west is to the right.)


Instead of finding the expected horizontal rock layers, the layers in the gorge were gently arcing. (Note that the axis is slightly west of the highest part of the ridge - erosion is faster on the west side of the mountain.) Older rocks were exposed near the river, and the younger rocks were missing. Clearly, some massive event had occurred here, and at other locations throughout the Alleghenies where similar structures were found. From evidence such as this, the concept of mountain building through folding was born.

For this Earthcache, you will follow the bike trail along the river from Ohiopyle to the parking lot at Ramcat Run, a trip of about 8 miles one way. You can do the stops in the reverse order, but the descriptions might be more confusing. You will be asked to measure the tilt of several rock layers along the way, so plan for this. (I searched for "homemade inclinometer" and found several easy to make designs. There are also free apps for your phone or tablet that will do this.) You might want a printout of at least the material after this, since cell phone reception is spotty. If you go off the bike trail, be wary of the poisonous snakes that inhabit this gorge. Take a camera - the trip is gorgeous!!


As you arrive at the given parking coordinates, you should see this sandstone wall to your right. If I asked you to measure the slope of this rock bed, you might wonder exactly where is a good place to measure. In fact, there really isn't a good place here. There are plenty of cracks but they are random, instead of following the plane of bedding. ("Bedding planes" are what make rocks look like a slice of layer cake. They would be fairly straight, separating two rock layers, and extend for long distances, as opposed to random cracks.)
If you turn around, you will see this large sandstone block across the lot close to the trail, and here there are several places more appropriate for measuring tilt. You can use the top of the block, especially to the right of the tree, or you can measure the layer that's about a foot off the ground. It's best to take a couple of measurements and average them. Always measure slope parallel to the bike trail, and report it by specifing the angle you measure and by saying which side is lower. To avoid needing a compass, just say "West" if the lower end is toward Ohiopyle and "East" if the lower end is toward Ramcat Run. A typical response might be "10° West". Geologists refer to this measurement as the dip of the layer.

Task 1: The rocks here in the parking lot might be either the top of the Pottsville Formation or the bottom of the Allegheny Formation. Study these rocks a bit, maybe get a photo, because a later task is for you to compare with other exposures further down the trail. Think of color, texture, amount of cracks, etc. Report the dip of this exposure in degrees and orientation. Now, Get on your bikes and ride!

Task 2: This exposure is definitely of the Pottsville Formation. You can get a dip reading from the bottom of the overhang about 2 feet off the ground (angle and direction!). Compare these rocks to those in the parking lot, but don't make a decision yet, since there are other spots to examine.

You will pass through quite a stretch of trail that has few outcrops of rock. About the only thing you see are some large boulders sitting in the woods near the trail. Boulders like these are not useful to our study, since any orientation is lost when the rocks fell. All of the Mauch Chunk Formation is unexposed through here.

Task 3: This sandstone exposure beginning here is the Burgoon Sandstone. Note the different color and appearance. Just a few more yards down the trail is a larger exposure, and you can get a slope measurement here.

You pass some exposures of the Shenango-Oswayo Formation starting at N39 50.712 and W79 27.961 but they are too high to measure. Do you still see the same dip?

Task 4: Here you are in the midst of a series of exposures of sandstone from the Catskill Formation. (You should be an expert at telling the differences in these sandstones by now!) Find a spot along here to get a dip reading. Also, at N39 50.488 and W79 27.574 is an example of tafoni weathering - look for the honeycomb-like structures overhead. (Check out GC3T94K for more details on tafoni.) This area is prone to landslides, and you should be able to see remnants of some recent slides.

Right after these big exposures of the Catskill Formation, you pass into the zone of the Foreknobs Formation. Again, however, there is little to see of this Formation. On the right side of the trail at about N39 51.138 W79 26.239 is a shoe box-sized block of Foreknobs conglomerate. Look for the round, white pebbles in the sandstone. Just east of the "Railroading" display is the axis of the Laurel Hill anticline, but... not much to see. The rocks collected in the wall behind the display are worth a look, since many of the different formations are represented. The icon for the cache is where I calculated the axis of the anticline to be.

Once again you pass through a long stretch of few exposures, just random boulders. The geologists who studied the gorge had to make their way up the hill to find more complete information on the geology of the gorge.

Task 5:This exposure is of the Mauch Chunk Formation, and specifically, it is the Loyalhanna Limestone. The thin lines shown in the photo are not bedding planes, they are artifacts of how the sand was deposited and give a lot of info to a geologist. (These are festoon cross-beds if you want to read more about them.) At the top of the photo is a layer that can be measured, but not here. Follow that layer until it reaches head-high or even waist high, and get a dip reading (angle and direction). The Mauch Chunk disappears underground soon after this.

Task 6: Here you pass through an exposure of the Pottsville Formation on both sides of the trail. (Does it resemble what you saw in the parking lot?) There is a place to get a dip reading about 2 feet off the ground, if you go to the far eastern end of the exposure and look to the right.

At about N39 49.636 and W79 23.022 is one of the few places where you can see contact between two formations. (GPSr readings were unsteady around here - just keep an eye on the hill to the right when you get near.) There is a layer of flaky shale with a layer of tannish sandstone on top. The shale is the top of the Pottsville and the sandstone is the bottom of the Allegheny Formation. The different rocks represent different ways that rocks can be created, and such a change is a good place to divide Formations.

Task 7: Just before you reach the Ramcat Run parking lot, there is an exposure of the Allegheny Formation. You might have to look a bit, but there is a place to measure dip. Remember the sandstone in the parking lot at the start of this quest? Now that you've seen both Pottsville and Allegheny sandstones, which do you think the parking lot sandstone resembles more?

YOU"RE FINISHED!! Well, except for the return trip. (Sometimes when I'm feeling lazy, I'll go with another rider. We park one car here, then take the bikes to Ohiopyle, make the ride, and then just drive the bikes back.)
I hope you've gotten an appreciation for the work that went into mapping the geology of the Yough Gorge - and think about this: the same amount of mapping has occurred in almost every part of the US, and most of the world. It is through such mapping that we have come to understand more about what shaped our Earth and how it is still changing.

Please email me the 7 dip readings you measured, and give me your opinion on what Formation is found in the Ohiopyle parking lot, with your reasoning.
If you're driving through the park, be sure to stop at Baughman Rock Overlook. Not only is the view of the gorge superb, but you'll be standing on the Pottsville Formation that's almost 800 feet higher than the falls. Hmmm, I wonder what the dip is here?

If you'd like further information on some of the topics discussed here, Wiki is good for "Formation (stratigraphy)", "Geologic Time" and "Paleogeography". The article about "Plate Tectonics" is pretty complex, but there are many other sources with good explanations. All illustrations and photos are my own, except for the folding diagram, which is modified from the Antecedent Stream diagram at http://bc.outcrop.org/GEOL_B10L/lab12_13s.html and the time scale, which is excerpted from the Geological Society of America Geologic Time Scale v 4.0

As always, have fun and cache safely!!

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