Friday, December 24, 2010

Paul Karrow

Department of Earth and Environmental Sciences, University of Waterloo
 
A previous account in Wat on Earth in 1995 described kettle holes as ice block depressions formed during glacial retreat, where stranded blocks of glacial ice became partly or completely buried by meltwater deposits (outwash sand and gravel).  When the ice block later melted, a depression was left in the ground surface, forming a kettle hole.
 
A second kind of surface depression is found in solid bedrock, and appears as circular/cylindrical erosional forms caused by rapidly flowing water (Figures 1 and 2). These are called “potholes”.  Where there are (or were) eddies where fast water vortices entrained cobbles or small boulders of hard rock, such as fast-moving streams generated from melting glaciers, the sediment load acts as a large abrasive drill and can bore into the rock surface.  Some well rounded clasts may be found in the bottom of a pothole, possibly left behind when stream flow conditions changed and the pothole was abandoned.  The wall of a pothole may bear grooves or flutes, usually spiraling downward and formed by the swirling sediment-laden water.  Also sometimes seen is a low central hump on the floor of the pothole, around which the water swirled.
 
Maurice Dusseault on the central island remaining within a large pothole.

Figure 1. Earth and Environmental Sciences professor Maurice Dusseault is seen on the “central island” remaining within a large pothole developed on banded gneisses of the Precambrian Grenville Province at Montmorency Falls, Quebec.

 
 
As with some other technical terms, “pothole” may be applied by the layperson in various non-technical and unrelated ways.  Thus the term “pothole” is sometimes used by farmers in referring to kettle holes in their fields.  City-dwellers are also familiar with the word as applied to splash-excavated holes in pavement.  In fact, the cities of Kitchener and Waterloo have specific phone numbers to call to report road pothole problems!
 
Potholes can form in any solid rock and are widely distributed in Ontario and elsewhere.  They may occur singly, or in numbers of several tens or even hundreds.  Publications describing occurrences in Ontario date from the late nineteenth to late twentieth centuries, with the earliest lacking any illustrations.
 
McKellar (1890) described a group of 50 potholes inland of Lake Superior in “dark green hornblende rock” considered to be Huronian (Precambrian) schist.  Some 20 of the potholes are individually described, with diameters ranging from over 1 m to 18 m, with depths commonly unknown because of partial infill by sediment and vegetation, but exposed to at least 6 m.
 
Stanley (1934) described 49 potholes in Huronian  quartzite of the La Cloche Mountains, north of Little Current on Manitoulin Island.  Several maps and photographs are included.  These potholes range from 0.5 m to 4.5 m in diameter with visible depths up to 6 m.  He also noted several potholes in similar quartzite at Whitefish Falls, about 15 km east of the larger group. 
 

2.4m Devil's Open PotholeFigure 2. 2.4 metre Devil’s Oven Pothole at Niagara Glen.

 
Kor et al. (1991) make general reference to potholes as part of a suite of streamlined erosional forms in northeastern Georgian Bay, centered near the French River.  Bedrock of the area spans the Grenville Front and includes highly metamorphosed gneissic to granitic rock.  Kor et al. (1991) attribute the suite of erosional forms to large releases of sub-glacial meltwater from the north.  Kor and Cowell (1998) made similar reference to potholes in the Bruce Peninsula, developed in Silurian dolostone, with comparable origin in large subglacial floods.  A few potholes are shown in photographs and some occur on the cliff of the Niagara Escarpment.
 
The earliest description of potholes was that of Panton (1888) for a few of a large group at Rockwood (east of Guelph) in Silurian dolostone associated with small caves.  The area of potholes has been included in a park by the Grand River Conservation Authority.  Here, as well as in other pothole groups, some are composite or overlapping, suggesting sequential phases of pothole development, with later ones superimposed on older ones.  The Rockwood potholes are in more populated southern Ontario, readily accessible to the public.  The highest in elevation and largest – and presumably oldest pothole at Rockwood – is locally known as “the Devil’s well” and had its “15 minutes of fame” in 1983 when it was cited as “the world’s largest glacial pothole”  by the Guiness Book of World Records, but mislocated at Bloomingdale (north of Kitchener) where there is no bedrock exposure (McKenzie 1994).  The record size claim is dubious (6.4 m wide and 13 m deep) as McKellar (1890) describes larger ones, and a world record needs fuller documentation.  This pothole is additionally interesting because excavation at its bottom has revealed an apparent till, suggesting it was formed before the last ice advance.  A more detailed, well illustrated, and recent description of the Devils Well was provided by A.V. Morgan in Wat'on'Earth in 2001. Most recently, Kunert and Coniglio (2002) consider alternate origins for the Rockwood features and conclude that they are true potholes, formed during late.glacial retreat by meltwaters.
 
Other accessible places to see potholes include the Niagara Gorge area, at Warsaw caves near Peterborough, and at Chippewa Falls north of Sault Ste. Marie.

References

 
Kor, P.S.G., and Cowell, D.W., 1998.  Evidence for catastrophic subglacial meltwater sheetflood events on the Bruce Peninsula, Ontario.  Canadian Journal of Earth Sciences 35, 1180.1202.
 
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.
 
Kunert, M. and Coniglio, M. 2002. Origin of vertical shafts in bedrock along the Eramosa River valley near Guelph, southern Ontario. Canadian Journal of Earth Sciences, v. 39, p. 43.52.
 
McKellar, P., 1890.  On pot.holes north of Lake Superior unconnected with existing streams.  Bulletin Geological Society of America 1, 568.570.
 
McKenzie, D.I., 1994.  Quaternary geology of the Waterloo area, Ontario.  Geological Association of Canada Field Trip A6 Guidebook, 46p.
 
Panton, J.H., 1888.  The caves and potholes at Rockwood, Ontario.  Proceedings of the Canadian Institute, 3rd series, 6, 244.253.
 
Stanley, G.M., 1934. Pleistocene potholes in the Cloche Mountains of Ontario.  Papers, Michigan Academy of Science, Arts, and Letters 19, 401.416.
 
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