Ames Limestone EarthCache

This cache has been archived.

egroeg: Alas, two of the exposures are no longer readily available for examination. I will revisit the Ames Limestone if I locate other, more suitable outcrops.

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Hidden : 11/10/2010
Difficulty:
2.5 out of 5
Terrain:
3.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:


Every time we drive somewhere in western Pennsylvania, there's an opportunity to see the fascinating and complex geological history of our region. Because we like our roads as level and smoothly curved as possible, they have been cut into and through our many hills. The layers, or strata, of different rock so exposed are the pages of the Earth's history book that geologists read. Along the Interstates and Route 28 are excellent examples of this, but even the small exposures along secondary roads can be informative.

The rock strata in a hillside can reveal a lot about the changes in ancient climate and geography as they show transitions back and forth between sandstones, shales, and coals, but each exposure can only give a very local picture. An ancient stream channel may leave behind a sandstone layer that is only a few meters wide - but which direction did it flow? What was the land around it like? Is it the same stream channel as the one revealed in a neighboring hillside?


The branch of geology that answers these questions is known as stratigraphy. In order to piece together the Earth's changes over time, stratigraphers study the composition of the rock layers, how they were formed, and how they relate to the layers above and below. Early stratigraphers found that keeping track of some prominent strata was made easier by naming them, usually after the location in which they were first studied. Thus, we have the Pittsburgh coal stratum and the Morgantown sandstone. Stratigraphers noticed that where both these strata were found, the Pittsburgh coal was always higher on the hill than the Morgantown sandstone. They weren't always separated by the same vertical distance due to variations in the strata between them, but the coal was always on top. (The mining industry was very interested in the work of stratigraphers!) As more observations were made of the relationships between the positions of strata, geological charts were constructed showing these relationships. Related strata were combined into Groups, Formations, and Members as a bookkeeping device. An example for Allegheny County is shown below. The chart can be considered a side view of a giant slice through a hillside. Not all of these strata are found in all parts of the county, but they are always in this vertical arrangement. For an excellent review of the history of these charts, read "The Map That Changed The World" by Simon Winchester.


Ref: General Geology Report G59, Geology of the Pittsburgh Area, Wagner et al, Pennsylvania Geological Survey, 1970.

Even these wider pictures of geological history could be hard to link up over still larger areas. Would you be able to match the strata of lower-lying Allegheny County with the strata of hillier Fayette County? Can you correlate western Pennsylvania to the rest of the state?

Stratigraphers recognized the importance of some of these strata as marker strata, also known as index strata or key strata. A key stratum would be found almost continuously over a large area and remain nearly unchanged in appearance so that it is easily identifiable. If you now create geological charts of different areas and you find the same key stratum in both areas, you can create a picture of the whole region's history. If shale and coal are found right above a key stratum in the northern part of the region, and sandstone is found there in the southern part, you might conclude that you had a large, swampy area (coal and shale) next to an ocean (sandstone) much like the Everglades of today. You have a window onto the climate and geography of the ancient world.



The Tully limestone and Marcellus shale would be key strata in these regions.

Ref: http://drakescience.org/?p=42

One such key stratum in our area is the Ames Limestone. During the Late Pennsylvanian Period (about 300,000,000 million years ago, or 300 Mya), an arm of a vanished ocean extended into Ohio, Pennsylvania, and West Virginia. Over a period of 400,000 years, this shallow bay left behind a series of marine strata, collectively known as the Ames Member, with one stratum being the Ames limestone (first studied in Ames Twp., OH). This limestone is a hard, clay-containing, fossiliferous layer of 30 - 150 cm thickness. and can be gray, greenish, or buff-colored. Typical fossils found in the Ames limestone are brachiopods like Crurithyis, horn corals such as Lophophyllidium, and crinoids.



Approximate extent of Ames Member strata.
Ref: "Gradient and Recurrence Analyses of Four Marine Zones in the Glenshaw Formation (Upper Pennsylvanian, Appalachian Basin)", Joseph G. Lebold, Ph. D. dissertation, WVU, 2005.

To complete this Earthcache, you must visit three outcroppings of the Ames limestone spread around the county, and make observations about its appearance and the reasons it was suitable as a key stratum for our area. Since I couldn't have you parking along the Interstates, I had to find some out-of-the-way locations. These Ames outcrops are not as dramatic as a roadcut, but are small and nearly hidden. That's why the directions are more detailed. Bring along a whisk broom or stiff paint brush - some outcrops are in hillsides and can get covered by dirt at times. If it's been raining, a small garden trowel is a good idea. You'll also need a camera, a tape measure (1 meter should be plenty), an accurate ruler, and a magnifying glass.

At each location, you must:
1. Measure the thickness of the limestone layer
2. Describe the layer: color, surface appearance (rough, smooth, etc.)
3. Describe any layers above and below the limestone, if visible (I don't expect a scientific description, with names, just your impressions of what types of rocks might help you locate the Ames layer at other locations.)
4. Describe the dominant type of fossil: color, size, shape, amount, etc. Use the links above for examples of what you might see.
5. Determine the altitude with your GPSr

Oh, come on!!! It's not as much work as it sounds like. It'll be fun and you'll get a feel for what field geologists do. There are also other caches near each location, so you can do this cache in more than one trip.

Location 1. Frick Park (Terrain 2) Park at N40º 26.223 W079º 53.814

The limestone is most easily found at this location, so it is a good place to learn how to recognize the stone. On the hillside just behind the tennis courts is a discontinuous series of limestone blocks running approximately east-west. The only block in its original position is at N40º 26.215 W079º 53.901, so you should use this for questions 1, 3, and 5. Fossils are found in all the blocks. The furthest east block, at N40º 26.209 W079º 53.874 is in the roots of a tree.
Park hours are 6AM - 11PM.

Location 2. Deadman's Hollow (Terrain 3.5) Park at N40º 18.640 W079º 49.880

Walk down the bike trail to the entrance to Deadman's Hollow, a land conservation area owned and managed by the Allegheny Land Trust. Follow the main trail until about N40º 18.970 W079º 50.660 where an intermittent stream comes in from the left. This becomes a narrow ravine that gets choked with deadfall, so you might want to get higher on the slope and follow the ravine in that way. (I went in on the southwest side of the slope. I passed a tree stand on the way, so exercise due caution in season.) Continue to N40º 18.786 W079º 50.575 where a gulley enters the ravine. There are numerous fallen blocks of the limestone here. Go up the gulley about 30 meters to N40º 18.796 W079º 50.575 where you can find a small intact exposure on the right hand side of the gulley, about a meter down from a sapling. If you look down the gulley, up the opposite side of the ravine, and a little to the left, you can see another small outcrop at the same elevation.

Location 3. Montour Run Road (Terrain 2.5) Park at N40º 27.421 W080º 11.011

Montour Run Road is very busy at times, so please be careful crossing it. DO NOT park right along the road, use the parking area for the Montour Trail. There used to be a large exposure of the Ames limestone here but most of it has been buried as the hillside above it eroded. The only remaining exposure is at N40º 27.436 W080º 10.987 where the vegetation ends (watch out for jaggers!). A fallen block is at N40º 27.447 W080º 11.031 and is probably better for examining the fossils.

To complete this Earthcache, please e-mail me the answers to the questions for each location. You must also post in your log a photo from at least one of the locations.
ONE FINAL QUESTION: Which of the 5 characteristics you examined would be most useful to you in identifying other outcrops of the Ames limestone?

After completing this Earthcache, you might recognize other outcrops of the Ames limestone while caching our area. Let me know where they are, so I can use them as alternate locations for this cache. Outcrops are reported south of Glassport, above Homestead, along McNight RD, and throughout the Turtle Creek valley. There are also some near Wellsburg and Morgantown WV (and Ames, OH of course) but I think that's a might far for one smiley! I hope you enjoy completing this cache as much as I enjoyed scouting locations.

Additional Hints (No hints available.)



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