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).

The locations and types of bedrock in and around Minnesota.
Figure 1. The locations and types of bedrock in and around Minnesota. The relief map shown is land-surface topography.

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).

The four main till provenances and the various ice lobes mapped in Minnesota. Arrows indicate the inferred direction of ice lobe movement.
Figure 2. The four main till provenances and the various ice lobes mapped in Minnesota. Arrows indicate the inferred direction of ice-lobe movement.

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).

Gray versus red till in Minnesota and their general location and direction of movement from their source areas.
Figure 3. Gray versus red till in Minnesota and their general location and direction of movement from their source areas.

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
End member example of Riding Mountain provenance.
End member example of Riding Mountain provenance. From left to right: Cretaceous shale, Paleozoic carbonate, felsic intrusive and high-grade metamorphic, dark metasedimentary and metavolcanic, and from top down: reddish volcanic, ironstone, and quartzite. Sample source: 8-16 mm fraction from site N4 of Thorleifson et al. (2007).

Winnipeg Provenance

NORTHERN SOURCE AREA
End member example of Winnipeg provenance.
End member example of Winnipeg provenance. From left to right: Cretaceous shale, Paleozoic carbonate, felsic intrusive and high-grade metamorphic, above mafic intrusive and high-grade metamorphic, and from top down: dark metasedimentary and metavolcanic, quartzite, reddish volcanic, and ironstone. Sample source: 8-16 mm fraction from site U13 of Thorleifson et al. (2007).

Rainy Provenance

NORTH-NORTHEASTERN SOURCE AREA
End member example of Rainy provenance.
End member example of Rainy provenance. From left to right: felsic intrusive and high-grade metamorphic, dark metasedimentary and metavolcanic, and from top down: sandstone and quartz; elsewhere, felsics may dominate. Sample source: 8-16 mm fraction from site S10 of Thorleifson et al. (2007).

Superior Provenance

NORTHEASTERN SOURCE AREA
End member example of Superior provenance.
End member example of Superior provenance. From left to right: felsic intrusive and high-grade metamorphic above ironstone, dark metasedimentary and metavolcanic, and from top down: mafic intrusive and high-grade metamorphic, quartzite, and sandstone and finally, reddish volcanic rocks. Sample source: 8-16 mm fraction from site O11 of Thorleifson et al. (2007).

 

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