PLUNGE INTO THE PAST OR GO WITH THE FLOW: MULTIPLE HYPOTHESES FOR THE ORIGIN OF THE ARCHBALD POTHOLE, LACKAWANNA COUNTY, PENNSYLVANIA

The Archbald Pothole, 38 feet deep and 42 feet in maximum diameter, lies in a small valley on a hillside in northeastern Pennsylvania. Discovered by anthracite miners in 1884, it is cut (from top down) into sandstone, shale, and coal of the Pennsylvanian-age Llewellyn Formation. The pothole became a local tourist sensation as soon as it was cleared of its gravel plug. A year later, miners encountered another pothole of similar size about 1000 feet to the northeast—but it was never cleaned out. In 1961 Archbald Pothole State Park was established at the site of the first pothole.

Two main hypotheses for the origin of the Archbald Pothole surfaced soon after its discovery. The explanation of the first on-site investigator of the pothole, John C. Branner, was that it had formed as a plunge pool at the base of an inclined moulin waterfall. Two years later, Charles Ashburner put forward the idea that the pothole had been carved mainly by subglacial meltwater. Recent restudy of the exposed pothole and the surrounding terrain provides a few new insights.

The following observations tend to support the moulin hypothesis: 1) A great disparity exists between the amount of erosion in the two potholes and in the valley between them. 2) The two giant potholes are physically isolated with no other evidence of well-formed potholes in the vicinity. 3) The long axis of the exposed pothole is oriented N60E, while the axis of the valley is N20E. 4) Pre-strip-mining topographic maps show no continuous valley upstream of the still buried pothole. 5) Studies of the time required for pothole formation suggest that erosion of a large pothole could take place in the relatively short time that a stagnant-ice moulin would exist.

Favoring the subglacial erosion hypothesis are the following: 1) A subglacial stream could exist for a much longer time than an eroding moulin in stagnant ice, allowing drilling of the potholes to take place over hundreds, or even thousands of years, rather than years—or, at most, tens of years. 2) A deep notch in the mountain ridge northwest of the two potholes could have funneled basal meltwater from continual pressure-induced melting through the site; the subglacial flow would have been pressurized, high-velocity pipe flow that would have been particularly effective in eroding the potholes.