CompuServe Thread

Dean, Your notion that wind turbulence (and some physical object to create the turbulence) is necessary to get sediment both air borne and deposited on land 400 feet higher up is a reasonable stab at explaining loess deposits. But here's my big question: what's the role of climate and vegetation in the formation of loess deposits? One would think grasses would trap silt much closer to the sediment source. Late-Pleistocene Iowa must have had a dry, windy climate with stunted grasses that were sparse and not efficient at trapping silt. Or, get this: maybe grass itself functioned as a turbulance-making object; that the grass blades create eddies needed to stir-up the "dead air" layer — that 1/32-inch of air just above ground surface that a normal wind never disturbs. You envision — and please excuse me if I'm in error — a large structure, perhaps 100-feet high that would turn high speed winds into a drawn-out cascade of eddys. Sediment would then be scourred from the lee side and suspended in a high velocity air stream and carried the great distances we now find loess deposits relative to their sediment source. Certainly the edge of an ice sheet could be this "object." But steep stream banks would also do the trick, and also happen to be right where you need 'em. Your idea of objects and turbulence has merit in my opinion. In any event, the only difference between your concept and mine is scale. I see loess deposits being formed more slowly and under lower energy conditions. Here's my scenario: A rock, ground-up inside or underneath a moving glacier, is rendered into pebbles, sand, silt and clay. Melt water then carries this sediment mixture into a river, where it finds its way atop the flood plain and dries out. Wind comes along and the finest grains (fine sand, silt and clay) are blown beyond the flood plain. The sand and pebbles stay and are later washed further down stream when the river rises. Most silt that makes it out of the river's flood plain, will be free to find its way further up onto the land, but some will be blown back into the river, or washed back by rain. Sediment now moves across the land a few 100 yds one day, a 1/2 mile the next day, and not at all for the next two years, all depending upon wind speed, dryness, vegetation type, and other factors: but it's a slow, steady progression of silt migrating away from the river. I think vegetation is important since I'm describing a relatively low-energy environment where the slightest change, like a change in species of grass, can either promote further silt migration or halt it. If the ground is bare enough, I think something like a 30 MPH wind is all that is needed to move a lot of silt a fair distance. The problem, though, as you point out, is how to initially get the silt stirred up. I'm looking at a map of N. Amer. glaciation in a general geology text co-written by Richard Flint, (Yale; glacial geology expert). It shows that all of Iowa has been glaciated by one or more advances. The last ice sheet (Wisconsin) came down through the center of the state and stopped about even with Des Moines, or about 2/3s of the way south. Here's the best part: neither the Missouri nor the Mississippi river valleys ("west" and "east" coasts, as you call them) were glaciated during the Wisconsin. I find that fascinating that the ice sheet didn't follow these major valley systems, and instead, appeared to have almost avoided them. Answers to this puzzle might be 1.) that the valleys' topographic relief was not great enough to influence a thick ice sheet, or 2.) that the current Missouri and the Mississippi valleys formed for the first time, at this latitude, on either side of this glacier "lobe" that decended into Iowa. I'm open to suggestions (or the "truth" if anybody knows it). Flint says that loess has a maximum thickness of about 90-100 feet (as developed along edge of sediment source). Typical thicknesses range between 20 and 100 feet near river and drop off to under 10 feet within maybe 20 miles. I think your 400 feet of loess is most likely a combined total of underlying non-loess sediment and a loess cap. Tell me if you know this is wrong, because that would be interesting. Incidentally, loess is not run-of-the-mill silt. Under a magnifying lens, the grains have angular shapes, instead of rounded shapes as is found in water-transported silt. This angularity means that loess is "young" and strongly suggests air transport. (I'm wondering if angularity actually improves a grain's ability to stay aloft.??) So, what do you think? Incidentally, you sure seem to know your Midwestern topography. Travelled a lot? Regards, Phil