HICKORY RUN BOULDER FIELD (1): NEW OBSERVATIONS FROM LOW-ALTITUDE AERIAL PHOTOGRAPHY

Hickory Run Boulder Field, Hickory Run State Park, Carbon County, PA is the largest (c. 18 vegetation-free acres), lowest gradient (1º W), and best displayed boulder field (or felsenmeer) in eastern USA. Declared a Registered National Landmark in 1967, this spectacular field has long been an attraction to the curious and an enigma to scientific interpretation. This latter is largely because, when standing on the field, individual boulders obscure larger, meaningful features of the field. Recently acquired low-altitude aerial photography allows ready identification of boulder patterns, but raises the following questions: what is a meaningful pattern and, conversely, what is the meaning of the pattern?

The field comprises sandstone and conglomeratic boulders derived from the Duncannon Member, Catskill Formation. Boulder rounding increases both downfield and with depth beneath the surface. Boulder details such as weathering differences between tops and bottoms of surface boulders, high polish of some boulder surfaces, variations in roundness, and reddish-brown (Fe) staining of below-surface boulders all have meaning. Traceable ‘streams' of similar lithologic composition occur and enter the field both obliquely and parallel to the field axis. Field surface micro-relief irregularity and boulder size variability make on-surface identification of boulder patterns very difficult. Recent observations include: (1) the field is rife with sorted stone rings (SSRs); (2) SSRs vary considerably in ring diameter; (3) SSR boulders vary in size, shape, and pattern within the SSR; (4) most SSRs are circular and complete, but some are partly destroyed by an adjacent SSR; (5) both straight and curved ‘linears' (long, narrow, and uniformly wide separations between adjacent boulders) occur; (6) upfield boulder streams are narrow, elongate, field centered, and comprise angular boulders that contrast markedly with marginal rounded boulders; (7) surface texture becomes finer and surface micro-relief decreases as the forest margin is approached; and (8) numerous, single-occurrence, non-SSR patterns are present.

Interpretations of the boulder patterns at Hickory Run are in the early stages, but will help elucidate the dynamic origin of the boulder field.