DATING DIABASE ROCKFALLS WITH LICHENOMETRY: SOLEBURY TOWNSHIP, PENNSYLVANIA

A massive boulder field of Jurassic diabase covers more than 10,000 m² on a steep east-facing slope of the Delaware River, Bucks County, Pennsylvania. The bedrock slope ranges from nearly horizontal at the base, to 50º mid-slope, to sub-vertical near the bedrock exposures. To assess the number and age of mass-wasting events, we measured maximum diameters (D) of 257 crustose lichens at five 10-m distance increments from the base of the slope. Lichens were more numerous and 68% had larger diameters (by 21%) on the downslope (east-facing, D_E) than the upslope (west-facing, D_W) sides of the measured clasts. At all slope elevations, the size distribution is multi-modal, suggesting discrete rockfall events. The median diameter for the midslope position (15-20 m away from base) exceeded those at the top (30-50 m) and basal (<15 m) sections by 5.6% and 15.7%, respectively. This indicates that over time, more boulders accumulated midslope than continued to the base. For establishing a chronology of rockfall events, no lag time between substrate exposure and the onset of lichen growth, as well as a constant growth rate, were assumed. The growth curve was constructed using the diameters D_E and D_W maxima (n=31) of the same lichen species on diabase stonework within a structure of known age (AD 1929) at the nearby Washington Crossing Historic Park. The time-averaged growth rate is slightly higher for east-facing (0.89 mm/yr) than west-facing (0.83 mm/yr) surfaces. Linear extrapolation of the D_E growth curve for the rockfall yields at least five decadal intervals: AD 1853-1863, 1875-1885, 1911-1921, 1938-1945, and 1952-1962. Although the causes of temporal rockfall clusters have not been determined, several regional earthquakes documented within these time ranges may have served as triggers. A larger D_E dataset and complementary growth-curve constraints (additional structures with similar components, dendrochronology, and documentary record of boulder fall) will help refine the history of these prominent boulder fields. This study further emphasizes lichenometry as an effective, non-destructive means of dating large mass-wasting events, especially when other exposure-age chronometric techniques are cost-prohibitive or limited by rock composition (e.g., lack of quartz).