LICHENOMETRY OF TRIASSIC BEDROCK REACHES ALONG THE NESHAMINY CREEK, LOWER BUCKS COUNTY, PENNSYLVANIA

Sandstone outcrops (Stockton Fm, Upper Triassic) along Neshaminy Creek and Porter Run, (Tyler State Park, Bucks, County, Pennsylvania) offer an opportunity to examine the influence of bluff elevation and exposure direction on lichen size distribution. This approach is designed to establish a local lichenometric framework for determining the rates of landscape change on exposures with varying characteristics (elevation, azimuth of exposure, and lithology). Thallus diameter of crustose lichens was greater on north-facing (2.7-110.0 mm; mean: 12.95 mm) than south-facing (3.0-70.0 mm; mean: 10.45 mm) slopes, with implications for lichenometry of historical structures (e.g., early Pennsylvania farm dwellings, with date stones of AD 1775 and subsequent 20^th century structures). Structures of known age within the park yielded a mean growth rate of 0.578 mm/yr. Therefore, natural exposures range in age from 18-23, with maxima of 120-200 years. Likewise, average diameters of north-exposed lichens on the large boulder varied from 15.94 mm (27.58 yrs) on S-exposed face to 62.16 mm (107.54 yrs) on the north. Overall, crustose lichens were more abundant and larger in size near the bluff tops. Along elevation intervals of 10-15 m, 5-10 m, and 1-5 m, average exposure ages decrease 3-4-fold, from 165 yrs to 44 yrs and 52 yrs, respectively. This distribution is likely controlled by abrasion and undercutting during flood events, as supported by erosional niches and rockfall patterns. Our study highlights the use of lichenometry for assessing the rates of slope processes and fluvial erosion along bedrock reaches of small streams, with potential extension to other lithologies (mudstones) offering a means to examine the role of substrate composition on lichen growth.