The Ice Ages
At least four ice advances, often called ice ages, have moved south into Pennsylvania. They are the Nebraskan, Kansan, Illinoian and the Wisconsin. Most of the evidence of glaciers in Pennsylvania is from the most recent advance, the Wisconsin Glacial Period. About 30,000 years ago, North America had a similar climate to modern time. Familiar animals lived here, but so did unique animals like saber-toothed tigers, giant ground sloths, beavers the size of wolves, deer the size of horses and other prehistoric animals.
For unknown reasons, the climate of the earth began to cool. Ice sheets in the arctic regions began slowly spreading in all directions. In the Northern Hemisphere, this glacier was called the Laurentide Continental Glacier and was several miles thick in the center while the edges were around 500 feet thick. The glacier moved very slowly, sometimes only several feet a year. The edge of the glacier often advanced in the winter, only to partially melt back in the summer. Like a giant bulldozer, the glacier scraped the land, removing vegetation and soil, and flattening hills and ridges. On the top, edges and underneath the glacier, ice melted and flowed in rivers carrying sand, pebbles and boulders.
Near the glacier, the climate was very cold. Winter lasted for six months and the annual temperature was 20 to 25 F cooler than the current climate.
About 13,000 years ago, the Laurentide Continental Glacier was at its greatest size, and covered two-thirds of North America, including the northeast and northwest corners of Pennsylvania.
Again the climate changed, becoming warmer, and the giant blanket of ice quickly melted and retreated. Animals and plants slowly repopulated the warming lands, but many of the prehistoric animals became extinct.
Formation of the Pothole
A pothole usually is a hole that 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 Archbald Pothole and the waterfall moved off to make new potholes.
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 that shows a typical, well-rounded, wavelike 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 1884 and has been exposed to weathering. The sides of the pothole are slowly eroding and are covered in ferns and lichens.
Please help preserve this signature of Pennsylvanias glacial history. If you observe someone vandalizing park property, please contact a park official.