Linda J. Brewer
CSS 205 Soils: Sustainable Ecosystems
Lab Paper 1: Glacial Influence on Soil Parent Materials
The Missoula Floods, product of the Pleistocene Ice Age, have strongly influenced the soils of the Willamette Valley. These floods occurred when many thousands of cubic miles of glacial melt water were released repeatedly as the glaciers covering Central Canada melted. Roaring down the Columbia River to the Pacific, these waters stripped away the soils of Eastern Washington. The water backed up into the Willamette Valley and suspended solids settled out of the water column. The larger particles were deposited in the north valley, near the source of the flood, and the finer silts and clays settled out in the southern valley. This alluvium became the parent materials of the Willamette Valley soils, like the Woodburn series, that we value today. What other types of parent materials, resulting from the Pleistocene, affect North American agriculture?
The spread of glaciers in North America was strongly influenced by the topography of the underlying landforms. The Rocky and Appalachian Mountain ranges were able to block the spread of ice to their west and east, respectively. Most of Washington and Oregon, for example, did not directly experience glaciation. One exception is the Puget Sound area. By contrast, broad valleys like the Mississippi Valley, did not offer topographic resistance to the flow of ice. The geologic record indicates that glaciers extended much further south in the Mid-West than they did in the Far West. In areas where glaciers did cover the land, soil parent materials are generally glacial till. Till consists of ground and broken rock in a vast range of sizes, left behind when the glaciers melted. The soils developing from till are quite variable, and their value to agriculture is dependent upon both the nutritient value of the parent materials and the size and percentage of rock fragments in the resulting soils.
Yet a third type of parent material, loess, exists. In many places loess overlies till. The rock fragments that became loess flowed into river valleys during summer seasons, while glaciers were actively melting. Somewhat similar to the Woodburn soils, these fragments settled out of the water, but were completely unaggregated. With the return of winter and the end of glacial melt, these materials dried out. The particles fine enough to be carried away by wind settled in some other location. Loessial deposits became important parent material for the soils of the central Mid-West and lower Mississippi valley. Again, broader valleys tended to promote the deposition of loessial materials.
Soils from the Pleistocene parent materials are young soils, about 12,000 years, and generally of high fertility. The North American "bread basket" includes soils of glacial till and loessial types. These soils can be contrasted with other, older soils. One example is the Jory series. The Jory is an Ultisol, at least 200,000 years old, and highly leached and weathered. Soils developing from alluvium, from till or from loess are quite variable, due to the chemical composition of the parent material, and the climate under which they develop.
Reference
Brady, N.C. 1984. The Nature and Property of Soils. 9th Ed. Macmillan Publishing Co., New York.