Peculiar Potholes at Lion's Head
|The photos on this page show a group of several potholes at Lion's
Head, on the eastern side of the Bruce Peninsula, Ontario, at latitude
45 degrees N. They form a cluster of inter-connected potholes in the side
of the cliff of the Niagara Escarpment overlooking Isthmus Bay.
The cluster of potholes is locally called the "Eagle's Nest", probably because some of them retain large rounded boulders, and viewed from a boat in the lake, they suggest collections of large eggs in a nest high in the cliffs. In the photo at right, boulders are present at two levels. These are the remnants of a gravel fill similar to that which is eroding out of the innermost potholes.
|A partial pothole is shown at right, with a large angular boulder at the bottom. This pothole opens into a much larger one that widens out below, and is shown more completely in the next photo. The boulder has trapped some gravel behind it, hosting a small shrub.|
|The photo at right shows a curved wall of the large partial pothole
below the previous pothole. The bottom part of the smaller pothole in the
previous photo is at the top.
Note the joint at the right, which probably controlled the location of these potholes. Large, well rounded boulders are shown at the bottom of this photo and the one below. These boulders rest partly on the gravel and boulder fill of yet another mostly filled pothole, and some are supported by a thin natural arch of rock, not visible in this photo, beneath the boulders. It forms a bridge between the sides of the pothole. This rock arch is probably an extremely delicate feature and seems to discredit the idea that such large boulders could have been introduced from outside, as conventional interpretations assume, because it could not have withstood the impact of big boulders.
There is active erosion here, and real danger that boulders could come crashing down the cliff at high velocity through the forest and into the lake below. It would be a great pity if this were to cause the destruction of the fragile and unique rock arch which is being exposed by erosion of the gravel in the upper section of the pothole.
partial pothole to the left of the previous photo is shown here.
Tops of trees on the scree slope at the bottom of the cliff, and water
of the lake are visible on the left.
One of the puzzling questions about the Eagle's Nest landmark is the origin of the boulders. They look like foreigners, since they are composed of silicate minerals, and the rock in which the potholes occur is a carbonate rock, the Silurian Amabel Formation, consisting of dolostone. The escarpment in this area is formed of the Wiarton/Colpoy Bay member [Kor 1992, p. 1]. One theory is that the boulders were carried across Georgian Bay by ice. Why would the ice sheet have placed them in the deep potholes high on a cliff?
Could the boulders have been tools for drilling the potholes? Many people assume they were, but they seem to be a bit too big to have been swirled around inside the potholes. Most of them are rounded and smooth. However, at least one of the boulders is not rounded. How could boulders being swirled around by currents have drilled the holes, since part of their walls are missing? Why would the boulders not have crashed down the cliff? According to one explanation, the missing wall of the potholes was supplied by the ice. However, ice would be a lot softer than rock, and would be melted by the heat generated by friction, so this seems unlikely to explain intersecting potholes.
Traditional theories advanced to explain partial potholes in the cliffs of the Niagara Escarpment involved subaerial flow at a time when ice stood alongside the escarpment, or solution processes. Ice would probably be far too soft for the drilling to work, and there is little evidence in support of solution as the cause of these potholes.
Erosion of potholes on both sides of Georgian Bay has been attributed to catastrophic currents of a subglacial meltwater flood [Kor et al, 1991; Kor, 1992; Kor and Cowell, 1998]. These currents are thought to have been pressurized by the weight of the ice sheet. However, the volume of water available in this hypothesis seems limited, as it is thought to have affected large segments of the Niagara Escarpment, causing erosion and streamlining effects from Manitoulan Island to Blue Mountain, eroding the indentations or re-entrants of Colpoys Bay, Owen Sound and Beaver Valley, and streamlining the regions beyond, west of the escarpment. Shoemaker referred to the inadequate supply of water as a major problem for the subglacial meltwater flood hypothesis [Shoemaker 1991, p. 1250]. Because it is believed to have extended over such a wide area, the catastrophic flow from a meltwater reservoir beneath an ice sheet would probably be insufficient for maintaining a steady flow, long enough for drilling deep potholes in hard rock. It would have to be a short-term event, lasting only days or weeks [Kor, 1992, p. 12].
The presence of potholes and large erratic boulders high on the cliff seems anomalous. Because of the instability of large boulders in a high cliff face, they tend to fall from their precarious locations. From the point of view of conventional pothole interpretations, their location in a high cliff, and the accumulation of boulders and emplacement in such positions seems unnatural and strange.
|The photo at right is a wide angle view of the intersecting potholes in the previous photos. The natural rock arch or bridge which supports some large boulders in a precarious perch above the cliff and retains the gravel in the main pothole is visible at the very bottom of the picture. Gravel erodes from the pothole where it is exposed below the arch, and the area below this site is probably hazardous, as there is a risk of falling stones and gravel from the potholes, especially after rainfall.|
|The photo at left shows the curved surface of the west wall of the outer pothole, and the top of the narrow rock arch or bridge on the left of the picture, with several boulders resting on it. Since this photo was taken there has been erosion of the loose material shown here. A hole has appeared left of the arch which will probably accelerate erosion of the gravel and boulder fill from the inner pothole which is located to the left of the picture.|
|The photo at right is a more detailed view of the partial pothole
shown in one of the previous pictures. In this location high in a cliff
face, how likely is it that the large boulder at the bottom of the
pothole could have been swirled around by currents and drilled a deep hole
in the side of a cliff? Would the second hole be able to form by
this process, overlapping the first?
Because potholes are often exposed in beds of streams, many people suppose
that stream erosion caused the potholes, but there is little real evidence
for it. Streams were often the means that potholes were exposed, as drift
was washed out. The stream erosion hypothesis is severely strained when
applied to the potholes of Lion's Head.
|The photo at right shows the natural rock arch of the Eagle's Nest, which retains the gravel and boulders, as it appears from the lake. The location of the arch is indicated by an arrow. Boulders beneath are part of the fill of the pothole behind, and some of them have collected in the bottom of the partial pothole on the near side of the arch. The abundance of big boulders in the gravel fill of the potholes is a difficult problem to explain in a theory that claims they were introduced. There are far too many boulders here, for them to have all been swirled around by currents within the pothole when it was formed, as supposed in conventional interpretions. And, because the boulders are obviously unstable in this location, they must not have been there for an immense period of time.|
Theory of OriginThe traditional theory of pothole erosion is that they were produced by eddys of streams rotating within them. Scottish geologist A. Geikie wrote [Geikie, 1865, p. 28]:
In not a few places (in stream beds), too, we may notice cylindrical cavities, called pot-holes, in the bottom of each of which lie a few well-rounded and worn pebbles and boulders. These cavities are due to the circular movements of loose stones that have been caught in eddies, and have been kept whirling there till by their friction they have gradually worked they way downward into the solid rock.This would not explain potholes which intersect others, or potholes in the sides of high cliffs, so some alternative explanation is needed. The explanation should account for the rounded cylindrical form, variety, and distribution of potholes, and also explain the origin of the boulders which occur in them.
In the disintegration theory the boulders formed in place, and the contents of these potholes, now eroding out, is probably the original disintegration product. The disintegration occurred in conditions very different from the present, an environment of catastrophic erosion, when the rock was being lithified, and as the sediment composition rapidly adjusted to the changed conditions. It is assumed that impurities such as silicate particles and any silica in solution would be separated from the sediment which formed limestone in those conditions.
The disintegration theory says catastrophic currents swept over the region, excavating the Great Lakes and streamlining the landscape. As overburden was removed there was disintegration in the vicinity of joints where the potholes formed. The theory says that chemical disintegration processes involving crystallization, precipitation, and replacement formed drift sand and gravel, including a few boulders, many of them having concretionary features such as concentric structure. This was a rapid process, and because an existing joint would be a conduit for the compaction fluids escaping from the rock, these became sites for pothole development. Potholes also provided conduits for silica-rich fluids rising from the underlying shale, which were a possible source for the minerals in the erratic boulders.
As the rock was lithified, magnesium must have precipitated throughout the limestone, forming the mineral dolomite, and silica was segregated into nodules or concretions in areas of lower pressure. These became boulders as the matrix around them was disintegrated into sand.
The disintegration process tended to form smoothly curved concave surfaces in the intact rock. This was probably due to a pressure-related effect rather than abrasion.
The fragile rock arch or bridge, which retains some of the boulders
that represent the "eggs" in the cliff at the Eagle's Nest, is evidence
against the idea that the boulders were introduced, since a fall
of only a few feet would probably demolish it. If the boulders did
not fall in, how could they have been introduced? If they were not imported,
they must have formed in place. The formation of drift gravel and boulders
in place is the essence of the disintegration theory. The observations
indicate boulders and drift in these potholes are original, and are being
eroded away in present conditions.
Geikie, A., 1865. The Scenery of Scotland. First edition, Macmillan, London. 360 pp.
Kor, P.S.G., Shaw, J., and Sharpe, D.R., 1991. Erosion of bedrock by subglacial meltwater, Georgian Bay, Ontario: a regional view; Canadian Journal of Earth Sciences, 28: 623-642.