U.S. Geological Survey
Earth Surface Processes

Grain Size Results

So far we have particle (grain)-size results from core 3, which has the oldest sediments of the three Bear Lake cores. Our radiocarbon dates for core 3 indicate that the top of the core is about 12,000 years old (in calendar years), while two meters up from the bottom of the core it's about 22,000 years old. The ice coverage during the most recent ice age reached its maximum extent between 19,000-25,000 radiocarbon years ago, so we expect this core to cover that period and show the sequence of melting afterwards.

The particle-size results (see figure below) show that Bear Lake's sediments are mostly silt and clay. Silt and sand values increase at the same times, probably indicating high rates of watershed erosion at the end of the ice age. Clay is the most abundant particle size during the ice age, probably formed as glacial flour.

Glacial flour is created as glacier ice grinds the underlying bedrock into very fine particles. In the winter, when the particles are dry enough to be carried by wind, this fine material is transported for thousands of miles and is called loess. Deep deposits of loess form the bread-belt soils of North America, in Iowa, Minnesota, Illinois and Missouri. During the spring and summer melt period, the glacial flour is carried downstream and forms thick clay deposits in nearby lakes.

The high clay values during the ice age indicate glaciers were present and actively eroding the Bear Lake drainage. The periodic decreases in clay amounts may represent times when glaciers were retreating or were frozen to the bedrock. Steadily decreasing clay values after 15,000 years make sense if the glaciers were in gradual retreat and are not eroding as much bedrock.

Particle Plot

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