Agency Spotlight: Archbald Pothole
Archbald Pothole State Park is a 150-acre park in northeastern Pennsylvania. The park is named for Archbald Pothole, a geologic feature that formed during the Wisconsin Glacial Period, around 15,000 years ago. The pothole is 38 feet deep and has an elliptical shape. The diameter of the pothole decreases downward. The largest diameter is 42 feet by 24 feet. At the bottom it is 17 feet by 14 feet. The pothole has a volume of about 18,600 cubic feet, so could hold about 140,000 gallons. It would take 35 fire truck tankers to fill the pothole.
The geologic point of interest was discovered in 1884 by coal miner Patrick Mahon while extending a mine shaft. Mr. Mahon fired a blast of explosives and water and stones came rushing out. The miners fled fearing that the mountain was falling on them. Edward Jones, the manager of the mining company, investigated and ordered the area cleared of debris. About 800 to 1,000 tons of small rounded stones were removed and Mr. Jones realized that the vertical tunnel was a large pothole.
About 1,000 feet north of Archbald Pothole, another pothole was found, but it was thought to be larger than the first pothole and was not excavated because of the excessive cost.
Archbald Pothole was briefly used as a ventilation shaft for the mine. A large fire kept burning in the bottom made the pothole function like a chimney, drawing air out of the mine. In 1887, Colonel Hackley, the landowner, built a fence and retaining wall around the hole. Edward Jones gave many tours of the pothole to local citizens and to noted geologists. The pothole became a popular tourist attraction. In 1914, the widow of Colonel Hackley donated a one-acre deed, which included the pothole, to the reformed Lackawanna Historical Society.
With the addition of 150 acres, Archbald Pothole became a Lackawanna County park in 1940. The county deeded the property to the Commonwealth of Pennsylvania 1961, andafter improvements, Archbald Pothole State Park was dedicated in 1964.
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.
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