Potholes
A pothole, in geologic terms, is a hole that is worn into the bedrock of a stream in strong rapids or at the base of a waterfall. As the flowing water swirls into eddies, the force of the water spins rock fragments, sand and gravel scouring out a small indentation in the bedrock. These debris get caught in the eddies and essentially act as a drill, boring down into the rock. After years and years of constant drilling, the stones and sands carve a cylindrical hole.
Figure 1.
Glacial Potholes
Glacial potholes are common features throughout New England. They formed when the region was covered by the massive ice sheets of the Wisconsin glaciation between 11,000 and 30,000 years ago. The glacial potholes in this area were carved in the bedrock by glacial meltwater carrying large amounts of coarse sediment. Stones accumulated in the pothole were swirled around by the flowing water, mechanically eroding the solid rock. The sediment carried in the meltwater served as liquid sandpaper, further rounding and smoothing the cavity.
What are potholes doing on Caps Ridge?
We know that fast flowing water from glacial melting plays a critical role in the formation of potholes, and that these conditions usually occur in the bedrock of streams. If glacial potholes form in the bedrock of rivers, how were these potholes formed on a ridge at 3771 ft, far above the glacial outwash stream beds in the valley below.
A likely scenario is that meltwater flowing on top of the glacier broke through a crevasse 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. As the fragments swirled, they carved the bedrock, first making a small depression then carving deeper and larger. As the rock fragments were reduced to tiny particles, new material from the surface was continually washed into the hole, enabling the grinding process to continue.
Other characteristics
When two potholes form close together, they sometimes merged into one larger pothole. As the rocks and swirling water slowly increase their size the wall between them eventually wore away, forming a single pothole. Sometimes you may also see “half potholes” which occur after the pothole was carved, when countless freeze thaw cycles split apart a piece of the bedrock leaving only a semi-cylindrical cross-section.
Logging Requirements:
Please send me an email with your answers to the following questions:
1. Fill in the blanks. There are ___ potholes on ___ separate boulders.
2. Estimate the diameter and depth of the largest pothole (no need to carry up a tape measure, your best guess is fine).
3. Can you see any merged potholes? How many?
4. Can you see any half potholes? How many?
5. Optional: Weather permitting, enjoy the view. Post a picture if you want, but please no spoilers.
Sources: http://www.uwec.edu/jolhm/interstate2006/potholes.htm
http://www.thepoconoguide.com/archbald.htm