Sea Level Changes EarthCache

This cache has been archived.

egroeg: The extensive repairs to the trail and stream bed make doing this cache problematic without a major rewrite, so I'm archiving and will consider redoing.

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Hidden : 9/11/2013
Difficulty:
3 out of 5
Terrain:
2 out of 5

Size: Size: not chosen (not chosen)

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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

The park is only open from dawn to dusk.

Okay, I know what you're thinking: "egroeg has finally lost all touch with reality. How can we do an EarthCache about sea levels when the closest beach to Pittsburgh is over 300 miles away." Well, the answer can be found by examining a topic that is beloved by all geologists - facies. (And no, I don't need spellcheck.)

We've all driven past roadside cuts that display many different rock layers. Have you ever wondered why there are so many layers, or why they suddenly change from one type to another? Each of these layers was deposited in a different environment - whether a beach, a riverbed, a desert, swamp, ocean bottom, delta, lake or whatever. Each of these depositional environments leads to particular types of rock layers. Wiki defines facies as "a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment". So the facies includes not only the rock type but the environment where the rock formed. In summary, a vertical stack of rock layers (with the oldest rocks at the bottom and younger ones progressively higher) records a series of changes in the environment that existed when the rocks formed.

Yes, we're getting closer to actually discussing sea level changes in Pittsburgh, but first we have to know about transgressions and regressions. Average sea level has changed many times in the Earth's history, whether because continents slowly rose or sank, or water was locked up in ice caps and glaciers, or plate tectonics created or eliminated an ocean. Whenever sea levels rise and the ocean slowly creeps up and covers the land, a transgression is occurring. (More specifically, becasue it is the ocean it is a marine transgression.) As water levels rise, a coastal plain may become a swamp, then a beach, a lagoon and then ocean bottom. Each of these changes represents a new facies, and different rocks are deposited.

When sea levels fall and the ocean recedes, it is a marine regression. The facies changes are usually the opposite of a transgression, and a new set of rock layers are deposited on top of the layers formed during the transgression. If a vertical column of these layers is exposed which shows this sort of cyclic facies change, you have evidence of a sea level change long ago.

A transgression-regression can cover a very large area if the land is flat. Some of the layers you will see today are also found in Ohio and West Virginia. The short horizontal distance you will walk on the trail does not reveal the facies changes. You will be walking uphill on the trail, so you are passing up through the different layers in the vertical stack.

Now you know how and why we can discuss sea level changes in our area: such changes are recorded in the facies of the rocks formed during the Late Pennsylvanian Period, about 300 million years ago. Western Pennsylvania was a large basin, slowly being filled in by sediments being washed down from the Appalachian Mountains. The periodic transgression-regression cycles also added their own sediment layers. We usually think this was a time of hot, steamy swamps that produced all of our area's coal, but the rest of the globe was actually undergoing a long period of extensive glaciation. It is the cyclic rise and fall of great ice sheets that caused the changes in sea levels recorded in the rocks. These changes in ice coverage are now thought to be caused by long-term cyclic eccentricities in the Earth's orbit.
Eventually, the basin filled in, and our area looked like the Great Plains do today. But our restless Earth wasn't done shaping our area. Small streams were cutting channels into the plains. The small streams became bigger streams, which made bigger channels. Over millions of years, streams became rivers, and our area was sculpted into the hills and valleys we see today. And the rock layers deposited 300 million years ago are again exposed.

When geologists talk about some of these rock layers, they might name them for the location where they were first studied. (The example most of us are familiar with is Marcellus Shale, first studied near Marcellus, NY.) Geologists also combine related rock layers together as Groups and Formations. The rocks we will examine for this EarthCache are all part of the Glenshaw Formation, which formed over a period of about 8 million years. During this time, up to 9 distinct transgression-regression (T-R) cycles are thought to have occurred. The two marine limestones we will see are the Brush Creek Limestone and the Pine Creek Limestone.

EARTHCACHE The coordinates given above are for the parking lot of Fall Run Park in Shaler Township. Fall Run Park is only open from dawn to dusk. To complete this EarthCache, you will follow the walking trail alongside Fall Run for about a mile roundtrip. Keep track of how many bridges you cross - you will mostly use them to determine where to stop, but some coordinates will also be given. No special tools are required, but you might want to bring a magnifying lens for a closer look at the rocks and fossils you will see. The park also has a playground and picnic shelter, and responsible dog owners are welcome.

STOP 1: This spot is actually before the parking lot, so walk back to Waypoint 1 where you can see some large sandstone blocks forming the creekbed. (No need to go down over the hill here, you're just looking.) This sandstone is the top of the Mahoning Sandstone formed in a river delta, which means it was very close to sea level. Return to the parking lot and head up the trail.
STOP 2: A little after you cross the first bridge, you can look across the stream to the right and see some thin layers of shale. The river delta has moved upstream and there has been a facies change. Silt is now being deposited instead of sand, and this area was just off-shore.
STOP 3: After you cross the second bridge, look to the left across the stream. You will pass more layers of shale. Just before you reach the third bridge, there is a nice shale exposure to the right that you can examine.
- - - TASK 1: What color are the pieces of shale that have fallen off this exposure? Rocks formed in tidal flats are gray, brown, or tan, while rocks formed in a lagoon are dark gray to black. Based on the color of the shale, what is the facies of this shale? (That means "Where was it formed?")

STOP 4: After crossing the third bridge, continue for about 100 yards. In the stream to the right is a small waterfall about 8 feet high. It is flowing over a different looking rock, called siltstone. Siltstone is similar to shale, but the particles making up the rock are smaller. Siltstone does not form nice layers like shale.
- - - TASK 2: What color is this siltstone, and what do you think is its facies? Comparing the facies of the siltstone to the facies of the shale at STOP 3, do you think the sea is transgressing or regressing? (Remember, the older rocks are below the younger rocks. If a deep-water facies is on top of a shallow-water facies, then the sea level is rising, meaning it is a transgression.)

STOP 5: Cross the fourth bridge and continue about 100 feet to Waypoint 2. In the streambed are some large slabs of darker rock, which is the Brush Creek Limestone, formed in off-shore waters. Some broken bits of seashells can be found if you look closely.
- - - TASK 3: What is the approximate thickness of this marine limestone?

STOP 6: Continue up the trail over the fifth and sixth bridges. Just past the sixth bridge there is a massive sandstone boulder that has fallen into the creek to your left. To your right at this location is a large shale outcrop. You know what I'm going to ask...
- - - TASK 4: What is the facies of this shale (from the color) and is the sea transgressing or regressing? (Remember, you're comparing this facies to the one below it.)

STOP 7: Just past the seventh bridge you can see the waterfall that gave this stream its name. The falls are about 35 feet high and seem to be flowing over shale layers. At the top, though, is the Buffalo Sandstone. This delta sandstone is very hard and resists weathering, which is why it forms the lip of the falls.
STOP 8: Continue up the steps around the falls. Shortly thereafter is a side trail toward the stream - take this trail and follow upstream. You will pass a point where the streambed is shale, and some shale layers are exposed in the sides of the streambed.
Continue to Waypoint 3, where several different layers are exposed, including the Pine Creek Limestone. This marine limestone is best examined in the fallen slabs at the upstream end of the cliff. Some small fossils can be seen in these slabs. In the photograph, the Pine Creek Limestone is just above the bottom. Also, near the center of the photo, there is an oval-shaped bit of sandstone, with another one up and to the right. - - - TASK 5: Based on the color changes you see here, do these rocks record a transgression or regression? Toughest question so far: What may have caused the sandstones found here?


This is the farthest point required for the EarthCache, but the trail continues past several more rock exposures if you wish to test your newfound abilities at identifying facies. Before the trail ends there is at least one more T-R event.

When you log this cache, please send me an email with the answers to the questions. Feel free to post photos in your log.
Please note that during this whole discussion, I did not comment on the current "global warming/ sea level change" debate, nor is this cache meant to support either side of the debate. This EarthCache conerns only the events of 300 million years ago, and I do not wish to entertain any arguments about the political nature of the current debate. If you are still Geocaching 300 million years from now, you may add a note to the log with your "I told you so" comments, but only if you show me the facies.

As always, cache safely and have fun!!

References: Transgression-regression diagrams from Marshak, S., 2001, Earth: Portrait of a Planet, W. W. Norton & Company, New York.
Description of Fall Run Park from Harper, J. and Neelan, J.,"Almost (but not Quite) Heaven - FALL RUN PARK",Pennsylvania Geology, Vol. 27, No. 1, 1996.

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