Pebbles in Tills of Minnesota
Till of a particular color and containing distinct rock types may indicate the direction from which the glacier advanced. In Minnesota, where the glacial history is complex, it is important to be able to determine where and when a glacier originated. One of the best ways to determine a glacial till deposit's place of origin (or its provenance) and to retrace the movement of a glacier, is by considering the geographic location and type of bedrock (or lithology) within the surrounding region (Fig. 1).
Doing so enables geologists to classify the various till deposits by provenance and, together with other geologic evidence, surmise the direction of ice-lobe movement across the landscape (Fig. 2). In Minnesota, four main till provenances are recognized. They are the Riding Mountain Provenance, Winnipeg Provenance, Rainy Provenance, and Superior Provenance (Fig. 2).
At the broadest level, the general source area of till in Minnesota can oftentimes be inferred by color alone (Fig. 3). Gray till, containing higher quantities of carbonate and shale bedrock, was most likely derived from a northwestern source area, while red till, containing higher quantities of crystalline bedrock, was most likely derived from a northeastern source area (Fig. 3).
The Till Provenances: As Shown By Pebbles
Pictured below are end member pebble examples of the four till provenances of Minnesota based on the type of bedrock they contain and the quantity of each. Note the strong contrast in color between the northwestern-sourced pebbles and the northeastern-sourced pebbles.
Riding Mountain Provenance
NORTHWESTERN SOURCE AREA
NORTHERN SOURCE AREA
NORTH-NORTHEASTERN SOURCE AREA
NORTHEASTERN SOURCE AREA