VENTIFACTS on THE STONE LINE OF THE IOWAN SURFACE OF NORTHEAST IOWA

Ventifacts are rocks that are abraded and faceted by windblown particles such as sand and snow. Ventifacts are commonly found on the Iowan Erosion Surface of Northeast Iowa along the stone line that lies atop pre-Illinoian glacial till. Although the actual timing of the production of the ventifacts on the stone line is not known with certainty, it is likely they were produced during an episode of severe erosion in a former periglacial environment that existed in this area between 21,000 and 16,500 years BP. This research involved field observations of the stone line at several sites in Black Hawk County. Excavations were made to uncover the stone line and collect ventifacts as well as to sample the sand at the till/stone line interface. Characteristics of the ventifacts allow inferences to be made regarding various aspects of the former periglacial environment. Particle size and composition of the abrasion particles (sand overlying the stone line) was determined using standard lab procedures. Sand grain surface textures were analyzed microscopically to determine amount of abrasion the particles undertook. Multiple facets on ventifacts indicate rotation of stones during the sandblasting process, especially for the smaller stones. Larger ventifacts with multiple facets suggest changing wind directions, although related landforms such as NW-SE oriented sand stringers on the Iowan Surface imply that prevailing winds were mostly from a northwesterly direction. Although it is clear that wind erosion must have been severe with little to no vegetation present, additional fieldwork is needed to provide a better understanding of the formation of the Iowan Surface ventifacts.