Enigma: South Dakota Granite Boulders

The figure above has his foot on a very large granite boulder which are occasionally found in western South Dakota. Since it is an igneous rock it could not have a local origin and had to be transported to its current location. These rocks have been transported hundreds if not thousands of miles from their origin in the Canadian Shield. The question and the puzzle is how did it arrive at this location? Large boulders such as this are called “erratics” and are very common in areas that have been glaciated during the ice ages in eastern South Dakota. However, these rocks are found beyond the western limit of the glaciations. They are far too large to have been transported by water (flowing streams). There is also a tendency for them to be found at roughly the same elevation. There are no clear cut explanations for how these rocks came to be where they are.

One explanation for these enigmatic rocks requires a review of the geologic history of South Dakota. The Rivers in Western South Dakota used to flow north to Hudson Bay. During the last Ice Age these rivers flowing to the north were blocked by the glacial Ice sheets which had their origins in Canada. This “damming” of the rivers resulted in diversion of the flow and the ultimate formation of what is now the Missouri River at the margin of the ice sheet. However, most likely there would have been a period of time where the ancient river valleys west of the current Missouri River backed up and lakes formed. These lakes would have persisted until the divides between the streams were overtopped and the modern Missouri River could assume its present day course which marks the extent of the ice sheet.As is shown below:

(Courtesy of South Dakota Geological Survey)

Contrary to popular opinion – glaciers don’t “push” material. As illustrated below – there is a zone of accumulation of snow which transforms to ice under pressure as succeeding snow falls bury earlier snows. Under the pressure of hundreds or even thousands of feet of ice the base of the glacier acts like a plastic rather than a solid ice body and begins to “flow”. Under this tremendous pressure rocks are “plucked” and incorporated into the plastic flow of the body of the glacier to be discharged at the “terminus” of the glacier.

(Courtesy of South Dakota Northern State University)

If the terminus of the glacier is the sea or a lake – “icebergs” filled with rocks and dirt break off and float away from the glacier. In the case of a lake the ice berg is likely to ground on the shore and melt – releasing its “load” of rock and dirt. If the lake persists for a period of time then a grouping of rocks at approximately the same elevation would mark the former shoreline. This supports the hypothesis that the igneous rocks found a large distance from their origin represent ice rafted erratics from the ice sheets.