SURFICIAL GEOLOGY AND LAND USE IN VERMONT

The surficial geology in Vermont has strongly influenced where humans have settled and how they have used the land. Detailed surficial geologic mapping, under the auspices of the Vermont Geological Survey, has elucidated this connection with prehistoric and historic human development.

Archeological work has shown that the earliest recorded Native American settlements occur along the shoreline deposits of the Champlain Sea, a relatively long-lived, albeit slowly lowering, arm of the Atlantic Ocean. The distribution and thickness of both glaciolacustrine and glaciomarine sediments in the Champlain valley has provided the basis for assessing the potential for slope failures during earthquakes, groundwater remediation activities, and septic system suitability.

Working in central Vermont, surficial geologic mapping by Larsen (1972, 1987) documented a relatively simple time-transgressive stratigraphic architecture occurring in most of the major mountain valleys where coarse ice-contact sediments are overlain by a fining-up sequence of lacustrine sediments deposited in ice-dammed lakes. The lacustrine sediments frequently coarsen again as the lakes in the narrow mountain valleys filled with sediments. Subsequent stream erosion has removed parts of this section and mantled the underlying glacial stratigraphy with alluvium. Historically these ice-contact, lacustrine, and alluvial sediments have provided the state’s best farmland. Another consequence of this stratigraphy is that ice-contact gravels occurring in many valleys are confined by the overlying fine lacustrine sediments producing high-volume aquifers in the same areas where groundwater demands for both agricultural and concentrated human development are highest. The detailed varve stratigraphy in these mountain valleys has proven extremely useful for dating these sediments but has also been used to identify the failure plane of the large landslide in Jeffersonville in 1999.

Alluvial fans in Vermont have been active throughout the Holocene (Bierman et al., 1997) and surficial geologic mapping in the uppermost Ottauquechee River valley shows that most historic development occurs on large alluvial fans. During Tropical Storm Irene (August 2011) almost all of these fans were active spawning considerable damage to human infrastructure.