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Rocks In My Head
Older glaciations in Minnesota either eroded away or are buried under later deposits. It is hard to figure out what happened but a few conclusions have been drawn. One is that the ice must have been very thick (at least a thousand meters). Another is that, based on the number of wind-polished and faceted stone surfaces, dust bowl conditions worse than those of the 1930's once took place.
Limestone and shale deposits from the Cretaceous lie under the younger drifts. Some contain plant fragments carbon-dated to 40,000 years ago. In some cases the sands and soils were subjected to intense chemical weathering so that even the toughest rocks became soft clay. How long this takes to happen is debatable.
Since I was blogging about Minnesota geology and glaciers, I thought it might be appropriate to link to the website of my uncle the glaciologist. His site has some amazing, and alarming, information that we all ought to be aware of. In particular, this statement: " The statement “glaciers are sensitive to the climate” was made countless times in published articles and the presentations we made throughout the 1960s-1970s, but little did we know just how sensitive they were. The now impending demise of many of them suggests glaciers are much more sensitive to the earth’s climate than are humans. We should have heeded their warning signals long ago. "
The glacial theory was first proposed by Swiss naturalist and teacher Louis Agassiz in 1837. His idea that the earth had experienced very cold climates and an expansion of glacial ice explained erratic boulders and striated bedrock across Europe. He came to the U.S. to teach at Harvard and found further evidence for his theories. Earlier conclusions about the distribution of boulders attributed the phenomenon to the great flood. A detailed reading of glaciation in Minnesota was started by N.H. Winchell, head of the newly created Minnesota Geological Survey, in 1872. He was assisted by New England glacial geologist Warren Upham. Within 10 years they had completed a great deal of fieldwork mapping moraines and presenting an accurate view of the ice border in North America.
Glaciers shape the landscape by means of erosion, which manifests itself as abrasion and deposition. Ice loaded with rocks and minerals slides along like coarse sandpaper. Evidence on bedrock outcrops appear as scratches, or striations. On a large scale the process is called quarrying or plucking. Often tapered, blunt nosed hills called whalebacks are formed. Another result is the excavation of basins which become lakes. Terms associated with deposition are till (unsorted debris), moraines (distinctive landforms), drumlins (streamlined hills with long axes parallel to the ice flow), kames (conical hills), and kettles (collapse pits formed when buried ice melts). Belts of lakes also mark the extent of former glaciers.
It's one of those long winter nights when Minnesotans ought to catch up on projects. One of mine, long neglected, is blogging about MN geological history. I couldn't remember where I left off so I looked it up. I found the last topic I covered was the Cenozoic which brings us up to the Quaternary, 2 million years ago to the present.
It's interesting. The first few paragraphs describe erratics but do not mention them by name. It talks about a 20 ton fine grained green-gray rock different than the pink granite bedrock upon which it rests on the floodplain of the Chippewa River. Near the quartzite bedrock of Rock County are large granite fragments the Indians called the Three Maidens. In Dakota County a granite boulder was honored by a continuous wash of red ochre, probably because it was obviously different than the bedrock. This was long before European settlement. These rocks were all carried by glaciers. They are called erratics, which was the subject of my acrylic painting (of Finnish erratics) of which I am so proud to say was purchased by an employee of the Guggenheim and is displayed in her private gallery.
My reading in the subject area of rocks, minerals, and gems has been, for nearly a year, in the area of birthstones. It's a subject that says a lot about human history. But to my way of thinking is somewhat less interesting than geology. So I have gone back to studying the book Minnesota's Geology by Richard W. Ojakangas and Charles L. Matsch. The subject is glaciation. Many terms describe what was left behind when glaciers moved across Minnesota during the Quaternary (2 million years ago to the present). One interesting term is erratics. These stones can be any size from a boulder to a pebble. They were transported from one area to another by the movement of glaciers. They differ from the local bedrock in color and composition, sometimes so radically they have been mistaken for meteors. I take an interest in these because the glaciers dragged in so much material to this local area. Also, I who am not a seasoned artist sold a painting of Finnish erratics to an employee of the Guggenheim, which was a thrill and honor. Since then I've read about erratics at length. Technically, if a rock was moved due to human activity, it's an erratic. By that definition, rockhounds have created a lot of these!