Stratigraphy and Sedimentology of a Paraglacial Fan Near Hancock, Vermont

The Bowen Fan is an anomalously large alluvial fan in the Third Branch of the White River Valley. This project involved investigation of the sedimentology, size, and physical characteristics of the Bowen Fan in an attempt to determine the processes responsible for its formation, and its anomalous size. The internal stratigraphy of the Bowen Fan was studied in a large borrow pit excavated in the fan by the Bowen family. Interbedded coarse debris flow and finer fluvial deposits indicate that the fan formed through a combination of sedimentary mechanisms. Particle size distributions were determined for each major horizon through dry sieving (>0.063 mm). A Coulter counter was used to determine the particle size distribution for finer fractions. Age control for fan formation was provided by age estimates from the oldest living tree on the fan surface and physical setting. Based on historical reports, the presence of trees on the fan surface, and the extent of incision of the modern stream, there has been no activity on the fan for at least 60 years. The working hypothesis is that the Bowen Fan is a paraglacial fan that formed soon after retreat of the Laurentide Ice Sheet from Vermont. At this time, large volumes of unconsolidated glacial sediment would have been available for transport from the unvegetated slopes in the fan catchment. This scenario explains the discordant size and volume of the fan relative to other fans in the White River Valley. To further evaluate this theory, a GIS analysis of fan volume and drainage basin area was completed for fourteen alluvial fans along the White River Valley, north and south of the Bowen Fan. This analysis revealed that within the total population of fans in the valley, there is a subset of paraglacial fans (including the Bowen Fan) that have extremely large volumes relative to their drainage basin area. These fans contrast strongly with the subset of smaller fans, many of which have been active during the post-settlement period.