The colossal scratches left behind on bedrock by the movement of massive ice sheets are called glacial striations. These distinct marks are direct evidence of the immense scale and direction of past ice movement. The marks are created as the glacier’s base acts like a giant, slow-moving file, scouring the underlying rock surface. By studying these features, scientists can reconstruct the paths of ancient ice sheets.
Glacial Striations: The Defining Marks
Glacial striations, or striae, are scratches etched into the bedrock. They are the result of smaller rock fragments, like sand grains, embedded in the bottom of the ice. The consistency of the marks being straight and parallel reflects the steady, unidirectional flow of the glacier over the terrain.
A distinction is made between these finer striations and the much larger glacial grooves. Grooves are deeper, wider channels cut into the rock, often created by larger pieces of coarse gravel or boulders frozen into the ice. Impressive glacial grooves can be found at Kelleys Island in Ohio.
The abrasive process that creates the scratches often leaves the surrounding bedrock with a high sheen, a feature known as glacial polish. This polishing is caused by the grinding action of the finest sediments, or rock flour, at the base of the glacier. The combination of polish and parallel marks is a classic indicator of a once-glaciated landscape.
The Mechanics of Abrasion
The scratches are not made by the ice itself, as ice is relatively soft. Instead, the actual cutting tools are pieces of rock debris, ranging from silt to large boulders, that have become frozen into the base of the glacier. This process is known as glacial abrasion.
The immense weight of the glacier exerts downward pressure on these embedded rock fragments. This pressure forces the fragments against the bedrock surface with a strength that allows them to scour and wear away the rock. The moving ice then drags these rock tools forward, creating the linear gouges.
The presence of meltwater at the bottom of the glacier facilitates this action. This basal meltwater acts as a lubricant, enabling the glacier to slide more easily over the bedrock, a process called basal sliding. The meltwater also helps flush away the fine, pulverized rock material, known as rock flour. The rate of abrasion is influenced by the concentration of debris, the hardness of the underlying rock, and the velocity of the glacier’s movement.
Reading the Direction of Ancient Ice Flow
Glacial striations serve as a record of the movement of ancient ice sheets. Since the scratches are aligned parallel to the direction of the glacier’s flow, their orientation acts like a fossil compass. By measuring the direction of the striae, scientists can determine the exact path the ice followed.
Mapping multiple striations across a region allows geologists to reconstruct the flow history of entire ice masses. In some areas, different sets of striations that cross each other indicate shifts in the ice flow direction over time. This data is essential for understanding the extent of past glaciations and for reconstructing the center and margins of former ice sheets.