GLACIATED PEBBLES are small stones whose shapes have been altered by the
grinding action of a glacier. Such pebbles commonly have at least one
flattened side that shows scratches (called striae). The striae were
produced when the pebbles were pushed over bedrock or ground against
other pieces of rock.
Glaciers tore fragments from the bedrock over which they moved and the
fragments accumulated in, on, and under the mass of ice. The rock
fragments were transported, some of them far from their source, and were
deposited as the glacier moved along or when the ice melted.
Soft rocks like limestone and dolomite are easily scratched, but soft
rocks cannot make grooves in hard igneous and metamorphic rocks.
Therefore soft rocks have more striae.
Glaciated pebbles can be found in deposits in many parts of the state,
especially in northeastern and east-central Illinois. However, many
deposits near the surface have been weathered and striae have been
destroyed. An especially good place to look for striated pebbles is in
quarries and strip mines where glacial drift overburden has been
removed. Pebbles found in such deposits show good striae for they are
but little weathered.