BIRTH AND GROWTH PROCESSES OF THE MILLER BROOK GULLY, NORTHERN VERMONT

Many surface processes in New England, such as lansliding and gully formation, are influenced by glacial sediments. Such sediments and the associated slope instability are commonly located in river valleys, such as the Miller Brook valley in northern Vermont. Here, glaciers are responsible for deposition of (from bedrock up) 1) till 2) esker or pro-glacial runoff deposits (sand and pebble gravel) 3) glaciolacustrine sediments (fine sand to clay) and 4) capping sands and gravels. Such common stratigraphies and the associated slope instabilities have major impacts on private and public land use in New England. Aerial photographs suggest the Miller Brook gully first became active in the late 1960s or early 1970s. The gully is presently ~50 m x 8 m x 2.5 m. Eroded sediments are deposited as a small alluvial fan on an adjacent terrace. An adjacent gully, ~30 m south, is stable as indicated by the presence of gully bottom till, large trees on the gully slopes, and the well vegetated, undersized alluvial fan at the base of the gully. Gully initiation processes at Miller Brook result from the differences in hydraulic conductivity of the esker/pro-glacial runoff deposits and the overlying lacustrine deposits. The lacustrine deposits are a low permeability cap above the higher permeable sands and gravels. Groundwater was thus focused in the sand and gravel layer which caused pipe erosion. Positive feedback between pipe void space and focused groundwater flow increased pipe erosion leading to roof collapse. Present retreat of gully walls is dominated by slumping and toppling of the near vertical walls where groundwater gradients are high; and by slumping of gentler sloping walls where the local groundwater table is lowered by the adjacent gully. First order estimates of erosion rates of the gully suggest decades more of gully erosion before gully walls stabilize and the gully bottoms on till. Such long-term erosion impacts the land owner who cannot utilize this land for farming purposes.