BH250-145
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Subject
Description
Minor Minerals: chlorite, zircon
The dropstone at Caron Park is a classic example of ice-rafted debris preserved within glacial sediments, and it occurs near exposures of Platteville Limestone rather than finely laminated varved deposits. The surrounding bedrock consists of carbonate units, making the presence of a large, compositionally distinct clast especially striking.
The dropstone itself is granitic in composition, composed primarily of quartz, feldspar (both plagioclase and potassium feldspar), and minor mafic minerals. This mineral assemblage clearly indicates that the clast did not originate locally from the surrounding carbonate rocks. Instead, it was transported from a distant source region, most likely the Canadian Shield, where granitic rocks are widespread. This strong contrast between the granitic dropstone and the local limestone provides clear evidence for long-distance glacial transport.
Notably, this dropstone is exceptionally large, approximately 8 by 10 meters, and is split into two major blocks. Its size indicates transport by substantial ice masses capable of carrying very large clasts over long distances. The splitting of the boulder likely occurred after emplacement, unloading, or freeze–thaw weathering.
A fragment of granitic rock was incorporated into glacial ice, transported southward, and eventually released as the ice melted. As it fell into soft sediment, likely a mix of fine-grained glacial or glaciolacustrine deposits, it deformed the underlying material, producing localized disturbance beneath the clast. Even without classic varves, this deformation supports deposition from above rather than transport along the ground.
This site also presents excellent opportunities for student research projects. One approach would be to determine when the boulder fractured using cosmogenic exposure dating, which could constrain its post-depositional history. Another valuable direction would be to analyze the geochemical composition of the granite and compare it with known granitic terranes to the north, including those near St. Cloud and farther into the Canadian Shield. By correlating major and trace element chemistry, students could potentially identify the provenance of the dropstone and reconstruct its glacial transport pathway.
Overall, this dropstone illustrates key geological concepts, including glacial transport, provenance, depositional processes, and post-depositional modification. Its granitic composition links it to distant Precambrian source regions, while its occurrence within carbonate terrain highlights the powerful ability of glacial systems to mix materials from vastly different origins.
Coverage
GPS Coordinates: 44°21'5.26"N, 93° 9'50.46"W