Killey, Mryna M.

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C C s }L.f. Cr-5 G-e.o5 C.. 2 ILLINOIS GEOLOGICAL SURVEY LIBRARY ILLINOIS STATE GEOLOGICAL SURVEY Urbana, Illinois 61801 ANCIENT DUST STORMS IN ILLINOIS Myrna M. Killey GEOGRAM 5 October 19 7 5 Fierce dust storms whirled ·across Illinois long before human beings were here to record them. Where did all the dust come from? Geologists have carefully put together clues from the earth itself to get the story. As the glaciers of the Great Ice Age scraped and scoured their way southward across the landscape from Canada, they moved colossal amounts of rock and earth. Much of the rock ground from the surface was kneaded into the ice and carried along, often for hundreds of miles. The glaciers acted as giant grist mills, grinding much of the rock and earth to "flour"-very fine dust-sized particles. During the warm seasons, water from the melting ice poured from the gla-cier front, laden with this rock flour, called silt. In the cold months the melt-water stopped flowing and the silt was left along the channels the water had fol-lowed, where it dried out and became dust. Strong winds picked up the dust, swept it from the floodplains, and carried it to adjacent uplands. There the forests along the river valleys trapped the dust, which became part of the moist forest soil. With each storm more material accumulated until the high bluffs adjacent to major rivers were formed. The dust deposits are thicker along the eastern sides of the valleys than they are on the western sides, a fact from which geologists deduce that th_e prevailing winds of that time blew from west to east, the same direction as those of today. From such clues geologists conclude that the geo-logic processes of the past were much like those of today. The deposits of windblown silt are called loess (rhymes with "bus"). Loess is found not only in the areas once covered by the glaciers but has been b lawn in to the nonglaciated areas. The glaciers, therefore, influenced the pres-ent land surface well beyond the line of their farthest advance. Loess has several interesting characteristics. Its texture is so fine and uniform that it can easily be identified in roadcuts-and because it blankets such a vast area many roads are cut through it. Even more noticeable is its ten-dency to stand in vertical walls. These steep walls develop as the loess drains and becomes tough, compact, and massive, much like a rock. Sometimes cracks de-velop in the loess, just as they do in massive limestones and sandstones. Loess makes good highway banks if it is cut vertically. A vertical cut permits maximum drainage because little surface is exposed to rain, and rainwater tends to drain straight down through it to the rock underneath. If the bank is cut at an angle more water soaks in, which causes the loess to slump down. Along Illinois roads the difference between a loess roadcut and one in ordinary glacial till is obvi-ous. The loess has a very uniform texture, while the till is composed of a ran-dom mixture of rock debris, from clay and silt through cobbles and boulders. Many loess deposits are worth a close look. Through a 1O-power hand lens separate grains can be seen, among them many clear, glassy, quartz grains. Some loess deposits contain numerous rounded, lumpy stones called concretions. Their formation began when water percolating through the loess dissolved tiny - More than 300 inches ~ 160 - 300 inches fi}ti 60 -160 inches R Up to 60 inches D Little or no loess ,..-....., Boundary of last glacial advance in Illinois <=Ee320c::::EE--i"34,0 Miles O 20 40 Kllomtltn c::=sa limestone grains. Some of the dissolved minerals later became solid again, gathering around a tiny nucleus or along roots to form the lumpy masses. A few such concretions are shaped roughly like small dolls and, from this resem-blance, are called "loess kindchen, t1 a German term meaning "loess children. t1 They may be partly hollow and contain smaller lumps that make them rattle when shaken. Fossil snails can be found in some loess deposits. The snails lived on the river bluffs while the loess was being deposited and were buried by the dust. When they are abundant, they are used to determine how old the loess is. The age is found by measuring the amount of radioactive carbon in the calcium car-bonate of their shells. Some of the early loess deposits were covered by new layers of loess following later glacial invasions. Many thousands of years passed between the major glacial periods, during which time the climate was as warm as that of today. During the warm inte;rvals, the surface of the loess and other glacial deposits was exposed to weather. Soils developed on most of the terrain, al-tering the composition, color, and tex-ture of the glacial material. During later advances of the ice, some of these soils were destroyed, but in many places they are preserved under the younger sediments. Such ancient buried soils can be used to determine when the materials above and below them were laid down by the ice and what changes in climate took place. The blank.et of loess deposited by the ancient dust storms forms the par-ent material of the rich, deep soils that today are. basie to the state's agricul-ture. A soil made of loess crumbles easily and has great moisture-holding capaci-ty. It also is free from rocks that might complicate cultivation. Those great dust storms that swirled over the land many thousands of years ago thus endowed Illinois with one of its greatest resources, its highly productive soil. C 

Ancient dust storms in Illinois

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Ancient dust storms in Illinois

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