The park on the Internet

August 10, 2002

Archbald Pothole State Park has its own Web site that, among other things, describes when and how the pothole was formed. The site is at www. dcnr.state.pa.us/stateparks/ parks/archbald.htm.

Here is an excerpt from the explanation provided on the site:

"A pothole usually is a hole which is worn into the bedrock of a stream at the base of waterfalls or in strong rapids. The moving water spins sand, gravel and rock fragments in any small indentation in the bedrock. After enough time, the sand and stones carve out an elliptical hole. Potholes may also form under or near the edge of glaciers by the action of glacial meltwater.

"Archbald Pothole was formed during the Wisconsin Glacial Period between 30,000 and 11,000 years ago. A meltwater stream flowing on top of the glacier probably broke through a crevasse (a crack in the glacier) and fell to the bedrock hundreds of feet below. There was enough force generated by the falling water to begin a whirling motion of rock fragments in a small depression. As the rock fragments swirled and bumped each other, they carved the bedrock, making the depression deeper and larger. The rock fragments eventually were reduced to tiny particles, but new rock fragments continually tumbled into the hole, enabling the grinding process to continue. As the glacier moved, so did the crevasse and the waterfall. Sand, gravel and rounded stones filled in the pothole and the waterfall moved off to make a new pothole.

"At Archbald Pothole, the water first wore away the top layer of bedrock which is sandstone. Next the swirling water and rock carved through gray shale leaving a particularly smooth and polished surface which shows a typical, well-rounded, wave-like surface. This feature is especially noticeable in the lower half of the northern side of the pothole. The bottom layer of bedrock is black anthracite coal.

"The southern and western sides of the pothole are nearly vertical, while the other two sides are deeply terraced. This is evidence that the waterfall that formed the pothole moved in a northeast and southwest direction. It is unknown whether the pothole formed during an advance or retreat of the glacier.

"Preserved underground by nature for around 13,000 years, the pothole was uncovered in 1874 and has been exposed to weathering. The sides of the pothole are slowly eroding and are covered in ferns and lichens."

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