Sorry about the long delay, but in my last post in late June, we saw how an iceberg left by melting glaciers managed to be buried in sand, gravel, and clay near Corona Street, resulting in the Frozen Well. This time, I’ll look at how sand and gravel trapped inside the glacier solidified into the Hogback formation visible nearby from Pearl Street. This all started between 13,000 and 13,500 years ago, as ice that was roughly a mile thick over Brandon slowly melted.

But I’m getting ahead of the story. The most recent of many ice ages, named the Wisconsin, started around 80,000 years ago. An alignment of several factors, having to do with orbits and not the atmosphere, combined to result in cooler weather, mostly cooler summers. More specifically, any time summers are so cool that not all of the previous winter’s snow is able to melt, the snow starts to pile up year after year. Snow and ice reached almost two miles deep over Hudson Bay. The pressure of all that ice caused it to spread out over the surface of the earth as far south as Pennsylvania and Cape Cod, reaching its peak around 20,000 years ago. The contour lines below give ice depths in hundreds of meters at glacial maximum. Vermont would have been under roughly a mile of ice and Hudson Bay under roughly 2 miles of ice.

By 20,000 years ago, global temperatures were warm enough that snow and ice started to melt faster than they accumulated each winter. By 13,500 years ago, the ice-free zone had reached southern Vermont, and at this point the story gets interesting. Stagnant ice, combined with gravel below and buried by a layer of clay and more gravel above, is likely what caused Brandon’s Frozen Well. On the other side of Corona Street, the Hogback was the result of a formation of melting ice known as an esker.

As glaciers melt, rivers form both on the surface and deep within the ice. And this isn’t like your refrigerator’s ice maker’s ice, this is like your end-of-winter dirty roadside ice, full of stones, gravel, and the occasional boulder. The images below show how a river deep within a glacier can result in a ridge of gravel once a glacier fully melts.

Diagram of esker formation:

An esker in progress in Alaska:

Looking east towards downtown Brandon, the photo below shows how the layers of the Hogback slope from south to north, reflecting the flow of the channel within the glacial ice.

The slope of the layers in the Brandon Hogback suggests that the sand, gravel, and water flowing through the esker under the glacier were flowing south to north: down Otter Creek through the limestone-rich Marble Valley. The high lime content in the water, combined with the pressure of the ice above, acted to cement the sand and gravel into the feature that is visible today off Pearl Street.

Originally, the Hogback was probably a small section of a much longer ribbon of gravel left by the subglacial esker. However, erosion combined with a long history of gravel extraction has left the concretion portion of the esker standing above the surrounding landscape.

In a 1927 postcard of the Hogback, the formation itself is recognizable, but the surrounding landscape looks very different. There has been over 100 years of gravel extraction in the area, and the sediment of an esker is pretty much all gravel. It seems likely that some smaller concretions have been destroyed during gravel operations, but it is a bit hard to know how quickly the Hogback has been eroding and whether that is natural or due to human intervention. The Hogback is currently on Town of Brandon property, and the immediate surroundings are flat with a steep bank rising up to Pearl Street. The 911 address 250 Pearl Street roughly corresponds with where the Hogback can be best viewed.

This series is a project of Wolf Tree Forest, a wild, working woodlot 2 miles from the Brandon Hogback.