VENTIFACTS IN DEVILS LAKE STATE PARK, WISCONSIN: IMPLICATIONS FOR PLEISTOCENE WIND DIRECTION

In April 2003, ventifacts of Baraboo Quartzite were observed along the Ice Age Trail east of Devils Lake near Baraboo, Wisconsin. Of the hundreds of ventifacts present in and around the study area, 53 were selected for measurement of rock orientation, size, keel orientation, and dip direction and angle of faceted sides. They were also photographed and mapped.

The ventifacts are quite varied in shape, ranging from one-sided to multi-sided, including specimens showing weak to very strong keel development as well as ones with some parts of their surfaces apparently not affected by abrasion. Abraded sides range in shape from concave to flat to convex with some sides exhibiting variable shapes. Sizes range from small cobbles to 2 m boulders. The degree of polish, pitting, and grooving increases at higher elevations in the landscape. A small glacial lake, located just northwest of the study area, was likely the source of the abrasives. Particle-size analyses of soils show a silt component of 45% at lower elevations and 68% in the highest part of the study area. Silt could more easily be blown to higher elevations than sand and was probably the abrasive agent responsible for more polishing at higher elevations.

Keel orientations with their interpreted wind direction were plotted on a map, and interpreted wind directions were plotted on a rose diagram. The patterns visually showed no preferred orientation, and statistical analysis of the directional data confirmed this. R=0.13 was statistically insignificant at every confidence level.

The study area is located within a small, unglaciated peninsula tucked into the surrounding end moraine. The area possibly was a “wind hole,” receiving strong, swirling katabatic winds flowing down from the surrounding glacier. These multidirectional winds could explain the varied shapes and the multi-sided ventifacts as well as the ventifacts oriented at different angles. Alternatively, if a paleowind direction was dominate during the formation of the ventifacts, then frost action and root wedging probably re-oriented many of the rocks into their present positions, destroying the evidence for a paleowind direction.