Thursday, June 08, 2006

Glaciers don't make rock piles

Last weekend when I took a neighbor to see a rock pile site, when we first parked there was a small site with minor rock piles, just next to the parking lot at the wetland's edge. It was here, while I was taking some pictures, that the neighbor was saying: "couldn't people who were building stone walls nearby have made these?" and then trying again with "couldn't these just be where the glacier left the rocks?" I would like to show pictures from the site but it is almost more important to address these questions a bit. So let's look at some pictures and discuss this.
The first question is kind of bogus. The sizes of rocks here have nothing to do with the sizes of rocks used in stone walls and there are no stone walls around, so why even ask about a connection?
The second question is considerably more interesting. What my neighbor actually said was more to the effect that his understanding of the glacier was that it contained rocks of all sizes and simply dropped them willy-nilly into the positions that they currently occupy. To this there are a number of answers. One is that there have been way too many more recent agitations of the land surface, much more weathering, and much more forest "violence", which need to be accounted for. In fact the rocks are not currently in the position they were left in by the glacier.
And here is how I know this. Go most anywhere and these types of small rocks lying on top of larger rocks simply do not occur. It is not a matter of random events occurring at a certain frequency. If rocks did start out piled up on top of each other after the glacer, forces since then have brought down those piles. So when you see a large number of violations of this probability, and all occurring within a few feet of each other (and in particular surrounded by areas of rock with no occurrances) this is not something which could have been done by the glacier.
It is also worth mentioning that in many of these pictures there are non-glacial rocks with sharp edges. So no, these could not have been dropped by the glacier in this way.

Now that I think about it, I missed a trick. I should have looked to see if there was organic matter under some of the upper rocks. So much for the discussion. I should finish up by pointing out that there are minor piles (effigies), rock-on-rock, and split-wedged-rocks in these pictures. This is a common combination of types found near springs and at the edge of wetlands. I call these "brookside" sites.

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