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• 11 • <br />Most weathering horizons reflect periods of land surface stability or <br />steady-state equilibrium (Nikiforoff, 1949), ranging from a few thousand <br />years to on the order of 10,000. <br />Dominant clay minerals are in the smectite group (montmorillonites), <br />thus rendering soils rich in divalent cation storage sites and highly <br />susceptible to shrink and swell as water is seasonally absorbed and lost <br />from the clay lattices. The shallower or more recently deposited soils <br />units are calcareous throughout, reflecting the glacial rock-flour origins <br />of most of soil minerals. The clays themselves may be in part derived <br />from bedrock but also reflect in situ illuvial weathering during earlier <br />times of stable soils~ollowed by eolian erosion and transport. <br />Soil-Plant Interrelationships <br />In essence, the northern high plains comprise a mosaic of seral ecosystems <br />that are limited in development by degree of soil formation. Climatic <br />change and possible grazing pressure by native high plains herbivores can <br />be envisioned as triggering degradational soil intervals in which a com- <br />bination of wind deflation, sparce vegetation, and rising salts interact <br />synergistically to allow both fluvial and eolian erosion and transport of <br />accumulated soils. Further climatic change with increase in summer rain- <br />fall or reduction in grazing pressure through population dynamics, then <br />allows increases in plant cover which in turn trap wind-blown silts. <br />