THE GEOGRAPHIC LIMITATIONS OF APPALACHIAN BOULDER FIELDS IN RESPECT TO LITHOLOGY

Appalachian Boulder fields have been considered as evidence for paleoclimatic reconstruction of the Pliestocene, as they are generally considered to be the result of intense freeze-thaw cycles. It has been suggested that the location of these features reflects the extent of ancient tree lines, which increased exposure of bedrock. The results of our previous study suggested that boulder fields: (1) occurred almost exclusively within the valley-and-ridge province, (2) rarely occurred above the latitude of maximum glacial extent during the Wisconsinan glacial maxima, and (3) displayed a clear inverse relationship of latitude with the average boulder field elevation that was also paralleled by the maximum elevation of the mountain range.

Using the same procedure, a new, high-resolution data base of boulder fields was constructed for Huntingdon County, Pennsylvania and surrounding areas. This search was located exclusively within the valley-and-ridge province. New results indicate that boulder fields in this region do not have a preferred aspect, or facing direction, discounting microclimate effects that may have led to boulder field formation. Boulder fields were, however, controlled primarily by lithology. In this region, they occurred predominantly within the Tuscarora Formation, a meta-quartz arenite. In fact, the density of boulder fields almost exactly paralleled the outcrop area of the Tuscarora Formation: more boulder fields were found in areas with more exposed Tuscarora. Furthermore, the northernmost limit of Tuscarora exposure coincides almost exactly with the line of maximum glacial extent, calling into doubt whether the limit of boulder field formation is controlled by past glaciation or by lithology alone. Although these features may have formed due to intense freeze-thaw cycles, our results suggest the locations of Appalachian boulder fields in central Pennsylvania are primarily controlled by lithology.