KEEWATIN POTHOLES & GLACIAL STRIATIONS
GPS Co-ordinates: N5512671, E387579 located 1 block south
of Highway 17 at 6th St. in Keewatin, Ontario
These round, cylindrical holes in the outcrop appear to be man-made but they
were formed by the action of running water during glaciation. Though the sign
says they are "rockholes", the proper geological term is potholes. These holes
are thought to have formed from water-spun rock fragments that have slowly
eroded holes in the bedrock. Potholes occur throughout the Kenora-Keewatin area
and may be as large as 2 or 3 m deep and 2 m in diameter.
A small outcrop at the entrance to the boardwalk
displays glacial striations. The rocks in this outcrop have been polished and
scratched by the movement of a glacier. The long grooves and scratches on the
outcrop surface are glacial striations. These were formed by fragments of rock,
embedded in the ice at the base of the glacier, which scratched and gouged the
outcrop surface as the ice moved over the land.
About 18000 years ago, an extensive sheet of thick continental glacial ice,
referred to as the Laurentide Ice Sheet, covered 90 percent of Canada including Lake of the Woods. The
erosive action of glacial ice and meltwater carved and sculpted the bedrock
surface of the land and deposited surface materials such as sand and
gravel. The Keewatin rockholes are an example of water sculpted erosional
forms left behind by the glacier.
The rockholes are round rimmed shafts, deeper than they are wide with
internal spiral grooves. The rockholes were sculpted into the hard bedrock
surface by the action of swift jet-like streams of swirling glacial meltwater
that carried a considerable amount of gravel and fine sedement. The high
velocity, sediment-laden water eroded circular depressions into the rock by
abrasion and grinding. The four holes in the rock range in diameter from
30 inches to several feet with a depth range from four to seven feet.
The Keewatin rockholes provide evidence for
glaciation in the Lake of the Woods area and demonstrate that sediment-laden,
high-velocity water can perform major and unusual feats of erosion.