GC6T5K2 A Dissected Plateau (Earthcache) in Pennsylvania, United States created by egroeg

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GC6T5K2 ▼

A cache by egroeg Message this owner

Hidden : 9/17/2016

Difficulty:

Terrain:

Size: Size: other (other)

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Geocache Description:

This is an EarthCache, so THERE IS NO CONTAINER AT THIS LOCATION. For an EarthCache, you make observations as part of an earth science lesson and report your findings to the cache owner. Failure to send them may result in the deletion of your log. For more information about EarthCaches, visit EarthCache.org

There are no hills in Allegheny County.
I'll bet that got your attention. Anyone who has cached in this county has stories about burning-thigh-hikes up and down our area. And we even have place names like North Hills, South Hills, Penn Hills, etc. Sewickley is so convoluted they have both Sewickley Hills and Sewickley Heights. I'm sure you're ready to dispute my statement so I'm not even going to try to convince you. Instead, I'll ask you to do this EarthCache and let you convince yourself!

The state of Pennsylvania has been divided into various areas, or sections, based upon the landforms. Most of western Pennsylvania is in the Appalachian Plateaus Province. (A province is a collection of similar sections.) This province extends into western New York, eastern Ohio, and the part of West Virginia near the Ohio River. This large area is also known as the Allegheny Plateau. The map to the left shows the various sections in western PA, with the boundaries of the Allegheny Plateau given by the darker lines. Allegheny County lies in the Pittsburgh Low Plateau Section.
The Pennsylvania DCNR describes the Pittsburgh Low Plateau Section as "...a smooth undulating upland surface cut by numerous, narrow, relatively shallow valleys." Where there are rivers or larger streams, the valleys can be deeper and wider. Sound familiar?

The Allegheny Plateau formed around the same time as the Allegheny Mountains were being created, about 300 million years ago. The low, undulating landscape would have resembled present day Kansas or Iowa, and the average elevation was a few hundred feet higher than it is today. In the years since then, sluggish rivers and streams wove their way back and forth across the plateau, slowly lowering the overall elevation, but not causing any major changes in the landscape.

Unlike today, these rivers flowed north to Lake Erie. When the Ice Age began, and the glaciers blocked the northward flow, the rivers had to change direction. This change in direction caused the streams and rivers to cut new channels, and a more rapid erosion of the plateau began.

The increased erosion caused the rivers to cut their way more deeply into the plateau, and there was no more of the side-to-side wandering of the rivers. Smaller streams formed on the sides of the river valleys, and began sculpting the valleys. All of this led to the topography we see today in the county.

When a normally flat plateau is eroded in such a way, the area is then known as a "dissected plateau". The highest points of a dissected plateau are the original plateau surface, and these are separated by valleys cut by erosion. Many of these high points are long ridges that parallel the streams. But how can we know that this is the case in Allegheny County?

One way to do this is to construct an elevation profile. Draw an imaginary line from west to east across Allegheny County, starting in Findlay Township, cross the Ohio River, the North Side, the Allegheny River and on to Plum Boro. Plot the elevations along the way in a graph like the one on the left. You can see that most of the high spots are 1100-1300 feet in elevation. The low spots are numbered on the graph and are: 1. McClaren Run, 2. Montour Run, 3. Moon Run, 4. Ohio River, 5. Allegheny River, 6. Washington Boulevard, 7. Sandy Creek, 8. Thompson Run, 9. Abers Creek.

Now do the same for a line running north to south from Pine Township, past the Point and ending in South Park. Once again, the peaks are in the 1100-1300 feet range, and this time the low points are: 1. PA Turnpike, 2. North Park Lake and Pine Creek, 3. Little Pine Creek, 4. Girtys Run, 5. Ohio River, 6. Saw Mill Run (at RT 51), 7. Saw Mill Run (at RT 88), 8. Catfish Run, 9. Piney Fork.

If you were to draw such a line anywhere across Allegheny County you would see a similar profile: high points of 1100-1300 feet divided by the many streams in the county. Some areas are a little higher than 1300 feet, including the highest point in the county, which is just over 1400 feet, across the Mon River from New Eagle. (The highest point is NOT in Robinson Township, or Bethel Park as I have heard several people say.)

So, what we have in Allegheny County are not a bunch of hills - what we have is the remnant of a plateau that has been cut by numerous creeks, streams, and rivers. We should not be talking about the hills of the county, but the valleys. Still not convinced? Okay, it's time to go out and see for yourself!

EARTHCACHE
This EarthCache involves some driving through South Park, with short, easy walks at several stops. CACHE IS ONLY AVAILABLE DURING DAYLIGHT HOURS. You will need a compass, or some other means to determine directions. Record your responses to the following questions and send them to me when you log the cache. The answers may differ depending upon what time of year you visit, as leaves drop and open other vistas.
Travel to the given coordinates in South Park, near the BMX track. There are some gravel turnouts along the road near the coords, or park at the track.

STOP 1: Find the highest point near the coords (about 1210 feet high). Look in all directions. What is the approximate direction to the highest point you see? (I'll accept a compass bearing, or a general direction like N, SW, etc.) Does this point appear to be a lot higher than where you are standing?

STOP 2: From Stop 1, drive down East Park Drive. (Note: You are following the path that the stream to your right has cut into the plateau.) Turn left on Stone Manse Drive, go to the coords for STOP 2 (elevation 1190 feet). What is the approximate direction to the highest point you see? Does this point appear to be a lot higher than where you are standing?

STOP 3: Continue down Stone Manse Drive (stream coming in from the left), turn left on Maple Springs, right on Brownsville Road, then a quick left on Ridge RD to the coords for STOP 3 (elevation 1160 feet). What is the approximate direction to the highest point you see? Does this point appear to be a lot higher than where you are standing?
Note: This road may be closed in the winter, so you'll have to do a little walking.

STOP 4: Return to Brownsville RD and turn left, then take the first right for the final stop (elevation 1100 feet). To your south is a ridge that parallels Catfish Run, the main stream through the park (just like in the description of a dissected plateau). There are several high points along this ridge. What is the approximate direction to the highest point you see? Are the other high points about the same height?

Convinced yet? How about if I direct you to N 40 18.992 and W 080 00.900 in the parking lot for the wave pool (elevation 1220 feet) and then have you look at a bearing of 115 degrees (ESE)? On a clear day, I saw Chestnut Ridge from here, which is about 40 miles away. Think you can do that if there are a lot of hills in the way?
Or maybe you should look at the background picture for this cache. This was taken near GC2J5WX at 2200 feet on Chestnut Ridge. No "hills" there, just a picture of a plateau dissected by many streams. Or you could just go to any nearby "hilltop" in the county where the trees have been cut down to provide a view, and look around. You will see a dissected plateau.

Well, I think I proved my case, but whether you agree with me or not, I hope you enjoyed this EarthCache. As always, cache safely and have fun!!

References:
PA province map is an older version of http://www.dcnr.state.pa.us/topogeo/field/map13/index.htm
Erosion diagram adapted from https://media1.britannica.com/eb-media/55/7755-004-A569A5BA.jpg
Elevation profiles from http://www.gis.dcnr.state.pa.us/geology/index.html

Additional Hints (No hints available.)