This EarthCache is part of a series of four EarthCaches, following the Falls Trail and Highland Trail, in a 5 mile hiking loop. This one can be done on its own since this is the only section under a 4 terrain.
Ricketts Glen State Park is located on the Red Rock and North Mountains of the Allegheny Front. The Allegheny Front is an escarpment forming the boundary between two of Pennsylvania's main geologic regions, the Allegheny Plateau and the Ridge and Valley region. To the north the Allegheny Plateau consists almost entirely of horizontal layers of sedimentary rock. To the south of the escarpment, many tilted layers of sedimentary rock form the Ridge and Valley region. The Allegheny Front is capped by hard sandstone, in contrast to the less resistant shales and sandstones found in the Ridge and Valley below.
All of the exposed rock in the park was originally deposited as unconsolidated gravels, sands, and muds by ancient rivers that flowed across the region from about 370 to 340 million years ago, when what is now Pennsylvania, was located south of the equator. These sediments accumulated on flood-plains near sea level, and as the region subsided they became buried by other sediments. Several thousand feet down in the earth's crust these sediments were consolidated into solid rock by pressure and high temperatures. Then about 250 million years ago a collision between the North American continental plate and Africa deformed the rocks into the large tight folds of the Ridge and Valley and the gentler folds of the Allegheny Plateau. Continued erosion beginning at this uplift has removed over 5,000 feet of overlying rock to date.
Equally important in the formation of the park today were the glaciers. In the last million years at least three continental glaciers have buried northeastern Pennsylvania under thousands of feet of ice. The last glacial event happened about 20,000 years ago and left extensive evidence of it's advance. This glacier picked up large blocks of sandstone from the ledges, scoured up loose soil and rock, cut striations into the bedrock of the Pocono Formation, and finalized changes in the drainage pattern that were started by earlier ice sheets.
The Midway Crevasse is a narrow passageway, along the Highland Trail, between large blocks of Pocono sandstone and conglomerate that have been split apart along natural fractures formed by the stress placed on these rocks when they were buried under other sedimentary layers. The blocks have split off of this ledge and gradually moved downhill to their present location. The last glacier through the area plucked away exposed rocks leaving behind a sandstone ledge. The constant assault of water entering cracks, freezing, and thawing is responsible for these blocks being removed from the ledge. During the brief summers of the early post-glacial period, the ground thawed out under these blocks to a shallow depth. This thawed material had a high water content and it slowly carried these large blocks, down the slope from the ledge, to their current position. Tree roots that penetrate the blocks along with frost-wedging have continued to break down these large blocks.
The Highland Trail is a 1.2 mile hike with easy terrain.
There are a few ledges where you may want to keep an eye on children.
Be sure to pick up a park map at the visitor center.
To claim this as a find:
1. Go to Example 1 to see a rock being split and tell me what you observe here.
2. Go to Example 2 to see rocks split off of the parent ledge from above and take a photo of yourself here. (This requirement is now optional)
3. Take a photo of yourself and your GPS at the wooden Midway Crevasse sign. (This requirement is now optional)
4. Estimate the height of the larger of the two rocks where the trail passes through the crevasse.
5. Find the distance the rocks have moved from the parent ledge to the north.
E-mail the answers to numbers 1,4,5 to me within a few days of your log. Failure to comply with these requirements will result in log deletion.
Good luck and good caching.
- Rev Mike