HIGH-RESOLUTION TOPOGRAPHIC ANALYSIS OF X-TREME GEOMORPHIC FEATURES ON LATE GLACIAL MAXIMUM AGE STRATH TERRACES, SUSQUEHANNA RIVER, PENNSYLVANIA

Along the steep (~1 m/km) Holtwood Gorge (HG) of the unglaciated lower Susquehanna River, macroturbulent erosional features associated with two strath terraces include thousands of potholes and six elongate spoon-shaped deeps carved in highly jointed schist. From 10Be cosmogenic surface exposure dating of vein quartz (Reusser et al., 2004, 2006), terraces 1 and 2 at ~0.4 and 3.3 m above the modern bedrock channel were dated at 14.41.2 and 18.70.9 ka, respectively, coinciding with the Last Glacial Maximum (LGM). The cause of relatively rapid incision (~0.5 m/ky) from ~36 to 14 ka remains unclear, although catastrophic flooding has been invoked. Deeps up to ~1219 m long by 60-90 m wide and 30 m below the riverbed (Mathews, 1917) along the eastern margin of HG indicate that processes in addition to higher floodwaters merit consideration. In this study we investigate cold climate conditions and processes including frazil ice and hanging ice dams, frost-cracking, aufeis, abrasion, and block quarrying as mechanisms for bedrock erosion.

Uncrewed aerial system (UAS) imagery collected for photogrammetry during low flow conditions in 2021 provided high resolution (~7 cm) topographic data to analyze erosional and depositional features on the 1st and 2nd strath terraces. Potholes mapped on UAS-generated orthoimagery and joints mapped with edge recognition software were field-checked. Pothole diameters range from 0.04–1.18 m. Preliminary findings reveal a correlation between fractures and pothole locations. For sparse patches of cobbles and boulders, some sourced at least 35 km upstream, particle sizes were measured in ArcGIS from a fishnet grid overlay on orthoimagery (procedure developed by J. Carr, PSU) and compared to field measurements. Similar median values with both methods are 6.2 cm for cobbles on the 1st terrace and 82.8 cm for boulders on the 2nd terrace. From critical shear stress calculations, estimated flow depths range from 1–2.9 m (cobbles) and 13.3–39.6 m (boulders). A high-flow event during Hurricane Ida in September, 2021, had water depths of 4.5 m, but did not move any of these clasts. The range of scales and types of macroturbulent features indicate a variety of erosional processes possibly associated with frost action (e.g., segregation ice) for brecciation and hanging ice dams for the formation of deeps.