NEOICHNOLOGY OF LARGE MAMMAL BURROWS ALONG STEEP FORESTED STREAM BANKS, LOWER BUCKS COUNTY, PENNSYLVANIA

Large mammalian burrows, many originally excavated by groundhogs (Marmota monax), occur along the banks of the Delaware River and its tributaries and are common biogenic structures in lower Bucks County, Pennsylvania. Their large size, depth, and complexity present a challenge in studying the details of their morphology and associated features. Recent studies of groundhog burrows in context of their effect on slope stability suggest they play an important role through altering vegetation patterns, soil properties, and hydrogeological processes. Whereas geophysical (georadar) imaging provides information about gross burrow geometry, plaster casting remains one of the most effective methods in burrow neoichnology. A cast of a large burrow located 0.8 m from the 3-m-high bank of Neshaminy Creek shows that it dips at ~30º away from the stream and extends for ~ 2 m, narrowing downward from 26x30 cm to <20 cm and becoming dome-shaped. Near its entrance, a small horizontal burrow (22 cm; diameter: 5 cm) was likely produced by a small commensal rodent. Floor cast of the outer tunnel segments contains 4-10-cm-long grooves (possible bioglyphs) and 0.5-0.8-mm-wide cylindrical indentations consistent with insect burrows. Cast fragments contained a variety of foliage, twig fragments, and nuts. Large tree roots and siltstone cobbles revealed by excavation likely acted as roof-stabilizing agents. Similar burrows along the creek banks will be ultimately exposed by erosion, appearing at progressively lower elevations. The resulting potential for misinterpretation highlights the need for comparative neoichnological studies of mammalian, reptilian (aquatic turtles), avian, and fish burrows along stream banks.