Glacially Erratic

A Glacial Erratic is a bit of stone, generally LARGE, that shouldn’t be there. They are rocks picked up by glaciers and transported, sometimes for hundreds of kilometers, away from their place of provenance to some other geologic neighborhood. Once there, they tend to stick out like sore petrologic thumbs – in many cases, they’re too big to lug away (remember, a one cubic meter of rock weighs about three tons). The largest erratic in the United States (found at Lake Stevens, Washington) is 10.36m tall and 23.77m in length.

In some localities, the solution of “What to Do with Them” has been to incorporate them into a local public square or garden designed around them, such as the erratic stone in this photo from Geneva. In fact, Ryan Thompson, an artist combining the geologic within his creations, has documented many erratics into an exhibit of stereo-paired photos that seem, well, to “wriggle” uncomfortably within their present setting (see them dance at: http://tinyurl.com/o9qzovx). In a way, this is exactly what erratics look like to passing geologists – rocks that are not at all comfortable in the geologic setting where a glacier has dumped them.

Here in Greece, our glacial erratics are blamed as being off-target stones thrown by the titans from the top of Mount Ossa at their enemies, the Olympian gods. Since the type of rock in the erratic can be traced back to its point of origin, and since so many glacial erratics in Greece do seem to trace back to the Ossa-Olympos formations, then the myth must be true, right?

Either that or those erratics are doing some amazing dancing on their own when under the influence of glaciers…


Photo courtesy of Ryan Thompson (Glacial Erratic Monuments: Geneva, 2010)

http://departmentofnaturalhistory.com/projects/glacial-erratic-monuments-geneva/
http://departmentofnaturalhistory.com/about/
http://nwgeology.wordpress.com/the-fieldtrips/glacial-erratic-field-trips/