The Geological Story of Glacial Striations
What Is A Glacial Striation?
A glacial striation is a long, narrow scratch that appears on the surface of a rock. The scratch was cut into the rock by the movement of an ice sheet, more commonly called a glacier. Glacial striations are also named glacial striae.
In Ontario, most, if not all, glacial striations formed during the last great ice age, when glacial ice sheets covered almost all of Canada up to about 15,000 years ago. Often the glacial striations occur as several parallel scratches (Figure 1).
How Were Glacial Striations Made?
Glacial striations were abraded into the basement surface when hard rock debris, or pieces of rock, were dragged along the rock surface at the bottom of the glacier. The rock fragments acted like sandpaper and they abraded the basement rock as the glacier pushed and pulled them along. As the debris was dragged over the rock basement, a scratch, or glacial striae, was created. Sometimes the pieces of rock, which were the abrasion tools, also become striated! The glacial striae are exposed when the glacier melts and exposes the striated bedrock.
What Is The Shape Of Glacial Striations?
Most glacial striations are long, narrow, and straight. They can be several meters in length. They are generally a centimeters or less in width and about the same in depth (Figure 2).
How Do Geologists Use Glacial Grooves?
Glacial striations give geologists information about the direction the ancient glacier moved. The path of the glacier’s movement is parallel to the long direction of the glacial striation. When several parallel striations occur, with different orientations, they tell geologists that one or more glaciers flowed in more than one direction.
Additional photos of glacial grooves
Pakenham / Mississippi Mills area, Ontario, Canada
Andy Fyon, Nov 6/18.