EXAMINATION OF SCULPTED EROSION FORMS IN BEDROCK USING AIR- AND GROUND-BASED LIDAR DATA, NORTH FRANKLIN, CONNECTICUT

This study examines water-sculpted erosional forms that occur in Devonian Schist along a ≈0.5 km reach of Bailey’s Brook (BB), a small and relatively steep first order stream that drains a ≈1.7 km² upland catchment near North Franklin, CT. Similar forms on other upland bedrock surfaces, including a site within 2 km of BB, have been interpreted as artifacts of meltwater spillways or moulins. We utilize 1m² bare earth Lidar data, previously published maps, ground-based terrestrial laser scanning (TLS) point clouds (≈0.1 m spacing), total station profiles, and geologic field observations to: (1) characterize bedrock control on the landscape, (2) evaluate the likelihood of a meltwater-origin for sculpted forms, and (3) develop geometric measures of sculpted forms for comparison with similar features elsewhere in CT. Lidar models provide excellent visualization of the heavily forested landscape revealing a series of small (1-3m high) bedrock escarpments that trend obliquely across the valley, but are generally parallel to the strike of nearby geologic contacts. Likely joint-controlled, these lineations create a stair step long-profile for BB with knick points at locations where the stream is incised through escarpments. Commonly sculpted forms are largest (to ≈3.5 m tall) near knick points. Manning-derived discharge estimates for present-day bank full flow and water level with the tops of sculpted forms at 9 surveyed cross sections suggest that paleo-discharge averaged ≈550 times greater than present bank full flow. The misfit size of BB with a small drainage basin incapable of delivering sufficient runoff to sustain large paleo-flows argues for a meltwater origin for the sculpted forms. The geometry of ≈9 forms along a 40 m knick point-reach was measured with TLS. These data were cleaned, meshed, and textured with georeferenced images to identify form boundaries. Sub-clouds were then extracted along form boundaries and framed using form centroids and best-fit planes. Form meshes created from these sub-clouds were sliced to determine volumes and planar and slant surface areas at 5% depth increments for hypsometric analysis. Form analysis continues, but preliminary results suggest that differing hypsometric curves may distinguish isolated forms from conjugate forms that result when two forms overlap.