DOCUMENTATION OF PREFERENTIAL ORIENTATIONS OF CRACKS IN BOULDER FIELDS OF TEMPERATE CLIMATES: FURTHER EVIDENCE FOR THE INFLUENCE OF DIRECTIONAL INSOLATION IN PHYSICAL WEATHERING

Recent studies addressing the role of insolation in crack initiation and propagation hypothesized that rock cracks are genetically linked to thermal stress produced by recurrent directional diurnal heating and cooling. McFadden et al., 2005 and Eppes et al., 2010 tested this hypothesis by collecting data from 310 and 242 clasts, respectively. Both studies found that clasts in semi-arid to arid environments exhibited cracks with preferred north-south orientations, thus supporting the hypothesis that directional insolation plays an important role in physical weathering.

Here we test the hypothesis that cracks will exhibit preferred orientations due to the influence of directional insolation in environments outside of deserts by collecting field data in Quaternary aged boulder fields located the humid, temperate climate of the Eastern continental United States. We collected crack and clast data comparable to earlier studies from a total of 310 clasts in Crowders Mountain State Park, North Carolina and Ringing Rocks County Park, Pennsylvania. We divided our field sites into two categories, those with vegetation (forested) and those without (open). The clasts at our NC forested site are an aluminous quartzite, a Neoproterozoic metasedimentary rock deposited by a debris flow that showed evidence of geologic stability, such as lichen and moss growth and a dark manganese varnish. The clasts at our PA forested site are a diabase from an Early Jurassic sill and were likely deposited by periglacial creep and solifluction during the Late Pleistocene.

Cracks were predominately observed in the Northern hemisphere of clasts. After removing cracks that were parallel to rock heterogeneities, the NC forested data set exhibited preferred multi-modal trend with N/S and E/W orientations (mean vector = 65.22 ± 63.24; n= 266) with heavily favored dip directions to the South and West. The PA forested data set exhibited preferred NE/SW orientations (mean vector = 43.78 ± 62.14; n=610) with no favored dip when cracks parallel to rock heterogeneities were removed. Overall, our data demonstrates that rocks in surficial deposits located in temperate climates exhibit preferred orientations similar to those of deserts, suggesting that insolation plays a key role in physical weathering in these environments.