Why Is That Rock Smooth on One Side and Ragged on the Other? A Glacier Passed
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A rock outcrop polished and scratched on one flank and torn into a broken cliff on the other records the direction a vanished ice sheet was travelling.
What the feature looks like
The form is a knob of hard bedrock, from a few metres to several hundred across, with two strikingly different faces. One side rises in a smooth gentle slope, polished and marked with parallel scratches running in a consistent direction. The other side is steep, rough and irregular, with blocks broken away and a jagged edge. The contrast is unmistakable in the field and the transition between the two occurs at the crest, which makes the whole outcrop read as an arrow pointing one way.
The two processes involved
Ice acts on the rock differently on each side:
- •On the upstream side, ice presses down and slides over
- •Embedded stones grind and polish the surface and scratch it
- •Pressure on that side keeps meltwater from freezing
- •On the downstream side, pressure drops as ice passes the crest
- •Meltwater there refreezes into cracks and grips the rock
- •The moving ice then tears blocks away, leaving a broken face
How geologists read them
The asymmetry makes these outcrops one of the most reliable direction indicators in glacial geology. The smooth polished side always faces the direction the ice came from and the broken side always faces where it was going, so a single example establishes flow direction unambiguously. The scratches confirm it by running parallel to the flow. Mapping many of them across a region reconstructs the shape of an ice sheet that disappeared thousands of years ago, including where it divided around high ground and how flow directions changed as it grew and shrank.
The neighbouring glacial forms
Several landforms record the same passage of ice and distinguishing them is the basis of reading a glaciated landscape. Long parallel scratches cut into bare rock indicate direction on their own but not which way along it. Streamlined hills of deposited material rather than bedrock are shaped like an egg, blunt at the upstream end and tapering downstream, which is the opposite asymmetry to the rock form here and a common source of confusion. Deep gouges and crescent-shaped chips mark where a single large stone was dragged. A whole region may carry all of these together.
The name and the argument about it
The French term was introduced by Horace-Benedict de Saussure in 1786 and its meaning has been disputed ever since. The usual translation refers to a sheep, and de Saussure is generally understood to have been comparing a field of these rounded outcrops to a flock of sheep lying down at a distance. An alternative reading connects the term to a wig style of the period, fixed with mutton fat, which the rounded shapes were said to resemble. The dispute cannot be settled from what he wrote, and both explanations appear in textbooks.
The takeaway
Ice sliding over a rock knob polishes and scratches the upstream side, while pressure release beyond the crest lets meltwater refreeze into cracks and tear blocks from the downstream side, leaving a smooth face and a broken one. The outcrop therefore points the way the ice travelled, and mapping many of them reconstructs vanished ice sheets.