Mystery of Pothole Origins

Eagle's Nest Potholes Along the top of the spectacular cliffs of the Niagara Escarpment in Ontario there are several locations where potholes occur. On the Bruce Peninsula, a large pothole is exposed in the face of the cliff near Lion's Head. It is locally known as the "Eagles' Nest" because of several rounded boulders that remain in it, that are visible from boats on the lake. Its location is in the center of the cliff in the photo at right.
	These two photos show the location of the potholes depicted on this page in a high cliff overlooking the lake. The 'Eagle's Nest' is a pothole in the cliff face behind the trees, overlooking Isthmus Bay near Lion's Head. It is associated with several smaller potholes.
The map at right shows the location of the village of Lion's Head, at latitude 45^o North on the eastern shore of the Bruce Peninsula. Potholes are found on the cliffs of the Niagara Escarpment nearby.
	According to the conventional uniformitarian explanation of potholes found in many geology text books, potholes like these were caused by vortices in former streams that rotated and vibrated "grinding stones" over long ages of time, the process gradually wearing down a deep, cylindical hole into the rock. In this case, however, the circumstances seem to discredit this explanation. Of course it is unlikely vortices could have existed high on the face of a steep cliff, and rotation could not be maintained in a hole if one side of it was missing.
	The picture at left shows portions of two potholes that intersect, at the top of the cliff near the site of the previous pothole. It is common to find partial potholes, where only a portion of a vertical cylindical hole remains in the side of a gorge or cliff. The theory of vortices whirling those grinding stones appears absurd when one tries to imagine how the stones could be down in the holes whirling around and around where one wall of the pothole was missing. And with part of the wall of a pothole missing, how could the currents have continued to circulate? What would keep the "grinding stones" from falling out, and tumbling down the cliff?
	The image at left is a view looking down into one of the potholes from above. Some large rounded boulders are present in the pothole.
	The boulders of the "Eagle's Nest" can be seen in the potholes in this photo. If there were a waterfall tumbling down the cliff here, no doubt many would assume the water had eroded the potholes. But in this location, no waterfalls are likely to have existed long enough to erode them. Rivers may have removed partially unconsolidated material, and sand, gravel, or drift, from potholes, and are often still doing so today. My research indicates the rivers had nothing to do with forming the potholes, but merely exposed them. The conditions in which the potholes formed were unlike those of the present. Potholes in some rocks probably formed during compaction, as joints formed in the rock. Many of the intersecting potholes are aligned along joints.
	Another pothole near the top of the escarpment which opens into the larger Eagle's Nest pothole. A small tree is growing inside the pothole.
	View from the top of the cliff at the Eagle's Nest, looking down. Note the boulders and gravel in the pothole.
The dolomite rock of the Niagara Escarpment where these potholes occur is part of the Amabel formation. The rock is deeply fissured, and because of this, there are no surface streams in the area that could have eroded potholes. The distribution of the potholes does not suggest the action of streams, as they are scattered over a wide area above the escarpment. Many potholes are situated in locations that seem unlikely to have ever been the sites of former streams.
	The view along the rugged and scenic cliffs of the Niagara Escarpment, looking west towards Lion's Head harbor in the distance.

Niagara Glen Potholes

The pothole at left is about 3 feet in diameter. An agile person could climb up through it to the top of the crag in which it occurs. The pothole is in an isolated crag in the gorge of the Niagara River at Niagara Glen, a park on the Canadian side of the Niagara River. There is a distinct spiral shape to the hole, a feature commonly seen in potholes, that has not been very well explained in conventional geologic theories of pothole origin.

Also at Niagara Glen a pair of potholes in a sloping slab of rock presents a puzzle: in the conventional interpretation, how would the hypothetical current vortices remain in their places, and drill vertical cylindrical potholes such as these? How were those grinding stones able to remain in position, when the holes were initiated?

An Oversize Boulder Inside a Pothole

The photo at left shows a pothole with an oversize boulder in it. This pothole occurs in granitic rocks of the Adirondack region of New York. Oversized boulders in potholes can be explained by an in situ disintegration theory of pothole formation, but the presence of oversize boulders in potholes would be difficult to explain by the conventional hypothesis of erosion by rotation of vortices and vibration of "grinding stones" inside the pothole by currents.

Hilton Falls Pothole

A pothole at Hilton Falls, near Milton, Ontario, is shown in the photo at right. There are much larger ones in the area that have not been excavated. A plaque erected by the Niagara Escarpment Commission states the pothole was drilled into the rock by water currents when the glaciers melted, and mentions "loose stones being spun around by the force of water flowing over the Escarpment at this point," but this seems to be incorrect.

Half a Pothole

The smaller pothole in the photo at right has formed along the right side of a joint, but not on the left side. If potholes formed by vortices rotating pebbles and boulders around, how could that process drill half a pothole like this? The potholes must have formed some other way. These potholes are found beside the Ausable River, NY.

Potholes in Basalt

These potholes in basalt boulders at the bottom of the South Esk Gorge, near Launceston, Tasmania, were exposed by the erosion of their contents by the currents of the river. Because potholes are often found in stream or river beds, many people associate them with erosion by currents or vortices, but most potholes probably formed by a pressure related disintegration process and were later exposed by erosion.

For More Information, See:

Natural Arch, Boulders & Potholes at Lion's Head
Peculiar Potholes at Lion's Head
On the Interpretation of Potholes
Pillars, Polystrate Formations, and Potholes

Intersecting Potholes

Discussions of the problem of their origin on the newsgroup talk.origins.