Surficial Geology

Hinesburg’s forested ridgelines and low-lying valley were shaped by events more recent than the collisions of the continental plates that occurred millions of years ago. Beginning as far back as 80,000 ago, during the last period of glaciation in North America, Vermont lay under an ice sheet over a mile thick. The steady movement of this glacier over the landscape was a powerful force that depressed the land, eroding craggy mountain peaks and widening valleys, and plucking up rocks in its slow movement forward. These rocks were heavily compacted and churned over time, and then deposited back on the landscape as the climate warmed and the glaciers melted and retreated. The resulting mix of different sized stones left on the land is called glacial till, a legacy familiar to Hinesburg farmers and gardeners who have had to work with rocky soils.

Glacial till is Vermont’s most common form of surficial geology, a term referring to the loose materials deposited above the bedrock by wind, water or glaciers. Many of us are used to seeing stonewalls all across the landscape. Thanks to Vermont’s glacial history, early settlers faced the difficult task of clearing rocky soils. Yet they could also use the rocks they pulled while clearing the land to create walls to contain their sheep herds.

But glacial till wasn’t the only material left behind in the wake of the glaciers. During the glacial retreat, ice blocked pathways of flowing water, causing lakes to form in places and redirecting rivers. At one point thousands of years ago, as the Laurentide ice sheet was retreating, Lake Vermont, a glacial lake that when at its deepest was 600 feet above present Lake Champlain, formed in the Champlain Valley. Where the Lake was deepest in the western part of town, clays and silts accumulated above the glacial till. At that point in time, the path of the Winooski River was blocked to the north, and so the River flowed south through the hollow where we today find Hinesburg Hollow Road. The powerful Winooski dumped sand and gravel at the delta where the river flowed into Lake Vermont.

Surficial materials contribute to the “parent material” that forms the soil, and therefore affects plant and animal distribution, as well as human land use. In the case of Hinesburg, the sand and gravel belt associated with the river proved useful to enterprise in Hinesburg; Hinesburg Sand and Gravel is located where this delta used to be!