Surficial Geology

If you had been standing in Jericho—or anywhere in Vermont—14,000 years ago, you would have been standing beneath a mile-thick layer of ice. Slowly, this ice began to melt, and it is the deposits left by this glacier’s retreat that form the surface shape of the landscape that we see today. While the ridges, hills, and valleys were formed by ancient folds of underlying bedrock, the materials that cover the bedrock have been deposited only within the last 13,000 years or so.

Lying just at the boundary between the Green Mountains and the Champlain Valley, patterns in glacial melt are especially interesting in Jericho. Across Vermont, the ice did not melt all at once. While the glacier retreated to the north and left the southern part of the state ice-free, the highest hills where the ice was thinnest uncovered early in the process. In Jericho, the first place to be free of ice was the southeast corner, surrounding the area that is now Mill Brook. As ice melted from the surrounding hillsides, it collected in the Mill Brook basin where Nashville Road now lies. With high mountains to the east and remaining ice to the west, the water had nowhere to go, and glacial Lake Nashville formed, covering all land in the valley below about 750 feet elevation.

As ice continued to melt, Lake Nashville was eventually able to drain a short distance to the west until it hit an ice wall once again. At this point, much of the land in the northern portion of the town had also been uncovered, allowing the water to fill a much wider area. The water level of this new lake, Lake Mansfield, reached elevations of approximately 700 feet, entrenched in the areas that now form the Browns River, the Lee River, Mill Brook, and a portion of the Winooski River.

This lake lasted until the outlets to these rivers were free of ice to the west, and they could flow into Lake Vermont, the ancestral lake to Lake Champlain. Due to the enormous quantities of water freshly melted from the glaciers, this lake was much larger than Lake Champlain is today. Covering the entire Champlain Valley, the lake reached a height of about 600 feet elevation. In Jericho, this means that the lake covered much of the western edge of the town. Meanwhile, the Browns and Lee Rivers and Mill Brook levels dropped and these water ways began to resemble the passages we see today.

How does this glacial history contribute to the features we see on the land? In large part, they are related to the soils we find at the surface. As the glaciers retreated, they left boulders, rocks, and debris of all sizes and shapes behind that piled high across the landscape. We call this debris glacial till, characterized by its mix of varying particle sizes. In many places, this till was then covered by better-sorted sediments, deposited by the water that followed the glacier. The clay soils that we find in many of the Jericho lowlands correspond to the three glacial lakes—Lake Nashville, Lake Mansfield, and Lake Vermont—as the small particles once suspended in the calm waters eventually settled to the bottom. At the shorelines of these lakes, sand deposits are more common, dropping out of the tumbling water where smaller streams suddenly hit the calm of the lake. Along the shorelines of these lakes, more sand and silt deposits are also common, found at elevations corresponding to lake levels. The places where glacial till remains at the surface are hilltops, such as South Hill. Look for clay deposits along Skunk Hollow and Barber Farm Roads—or anywhere near one of the town’s stream valleys. Going just uphill from any of these streams will likely bring you to a sand deposit at the edges of a past lake.