CHARACTERISTICS OF THE DES MOINES LOBE WASHBOARD MORAINES IN HARDIN COUNTY, IOWA

Washboard moraines -also referred to as minor moraines and corrugated moraines- are described as sub-parallel low relief (generally 3-6 meters high) ridges of glacial till (with occasional sands and gravels) that extend over large flat areas, but are frequently located adjacent to topographic highs and are oriented transverse to ice flow in the area. The washboard moraine complex located in Hardin County, Iowa can be found immediately to the west of the terminal moraine of the Wisconsinan age Des Moines Lobe. Aerial photo interpretation indicates that the individual ridges are oriented in a general northeast-southwest direction over a large portion of Hardin County. The washboard moraines in the extreme northwest portion of the county, however, are aligned north-south. Field work done in the west-central portion of the county indicates that the washboard moraines of this area are composed of mainly sand-rich till. The composition of this till does not vary greatly moving away from the axis of the Des Moines Lobe, but it does vary moving down the length of the glacier, becoming less sandy in the southern direction. This sediment size trend is exactly opposite that found in the washboard moraines associated with the Des Moines Lobe in south-central Minnesota, in that, the sediment size in Minnesota varies from the axis to the margin, but does not present a significant change down glacier. One possible explanation for the decrease in particle size down the length of the Des Moines Lobe in Hardin County is the addition of finer sediments (possibly loess) into the glacier and the comminution of larger sediments previously incorporated up glacier.