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<br />33 <br /> <br />action puffs up the surface materials and obliterates the rills. Con- <br /> <br />...... centrated runoff on hillslopes, during the first thunderstorms of the <br />W <br />00 summer, erodes material softened by the winter freeze-thaw action <br />t4>o <br />and forms rills. During field studies in the summer of 1980, the first <br /> <br />thunderstorm on the piedmont did not occur until August 15. Until <br /> <br />that storm, only partially obliterated and deeply incised rills were <br /> <br />found in the basins, whereas after the storm, the old rills were <br /> <br />rejuvenated, and a new set of rills had developed in the affected <br /> <br />basins. <br /> <br />New rills developed during the August 15 storm in Basins A, <br /> <br />N, P, and Q. The rills were most common in the convex segments <br /> <br />of valley cross section (Figs. 5.5a, 5. 5b). It might be suggested, <br /> <br />therefore, that erosion processes are most active in the upper por- <br /> <br />tions of basins, where the convex segments occur. However, hill- <br /> <br />slope studies by Schumm (1964), hillslope gradient measurements <br /> <br />(Table 5.2), along with field observations, suggest that all portions <br /> <br />of basins are undergoing active erosion. <br /> <br />Schumm observed that soil creep, primarily related to winter <br /> <br />freeze-thaw action, is an important erosion process in the area. His <br /> <br />surveys indicated that soil creep is most rapid on the steepest <br /> <br />portions of hillslopes with material accumulation occurring at the <br /> <br />bottoms of hillslopes. The hillslope gradient measurements in the <br /> <br />measured drainage basins indicated that all portions of basins have <br /> <br />steep hillslopes. <br /> <br />Field observations indicated that the residual soil layer is very <br /> <br />unstable after freeze-thaw activity has loosened it. On some very <br /> <br />steep slopes, large quantities of soil had slumped into the channels <br /> <br />I <br />',~.d <br />