Bell curves and glacier mechanics

Fresh lodgement till in Iceland. There will be some pretty big spikes in the particle distribution or weight curves for a deposit like this. At a guess, I would say there will be 3 spikes -- one for the silt-clay fraction, one for the cobble fraction and one for the boulder fraction.....

In continuing with debate with Ned Pegler, I have pasted this onto his site:

Good to discuss this, Ned! I think your basis problem is that you have assumes the EVERYTHING IS ERODED, TRANSPORTED AND DUMPED TOGETHER when an area is overridden by glacier ice. That may be so in the grand scheme of things — for example, a gigantic glacial trough is eroded, leading one to conclude that all of the bedrock that was there before is not there any longer, and must therefore have been taken and dumped somewhere else. But actually when you look in detail you see that erosion, entrainment, transport and deposition are all actually very selective, depending on the mechanics involved. So if shearing is involved, you get huge blocks of rock caught up in the glacier, and if abrasion is involved you may get sand, silt and clay fractions carried away, with very little in the way of pebbles and boulders. That is why you get huge variations in the nature of glacial deposits. In some cases a fine-grained till with just a few entrained stones, in some cases “free boulders” or erratics, with virtually no matrix of finer sediments, and in other cases (if there has been flushing by water) virtually all sand and gravel with hardly any silt and clay and (at the other end of the scale) hardly any stones and boulders either. So while all of the rock in a glacial trough may have been removed, it is carried away in different ways, and “redistributed” into different parts of the deposition zone down-glacier. If you then try to make a bell curve of the sediments in any one area, you will end up with bimodal or even multimodal spikes.

I did this sort of work 50 years ago as a research student, and while techniques may have changed between now and then, the principles of sedimentology haven’t! OK?