Surficial Geology

The sediment deposits that characterize Putney’s surficial geology are highly varied. 20,000 years ago the climate around Putney was quite a bit cooler. The same hills and valleys existed, but they were covered with several thousand feet of ice, part of a magnificent continental ice sheet that moved slowly south from arctic regions in Canada. As you might expect, the impact was colossal; the glacier scoured the surface of the Vermont landscape completely clear of all plants and soil, in addition to softening pointed peaks and rounding tight valleys in its wake. Like a giant conveyor belt, the ice picked up tons of sediment of all sizes as it chugged along, carrying boulders, gravels, sand, silt, and clay in its frozen depths. In some places, the scraping of rock on rock left linear striations visible on the surface of rocks, such as those visible on a small outcrop across from the entrance to Landmark College.

Some 6000 years later, as the glacier began retreating from southern Vermont, it melted and discharged staggering volumes of water – both raising global sea levels and releasing the fragments of rock that had been picked up along the way. Many rocks dropped directly out of the ice without the sorting action of water, in a jumble of sediment sizes called glacial till. While the majority of Putney’s uplands are blanketed in till, there are also other prominent surficial features as well, particularly at lower elevations. Where glacial meltwater played a sorting role, sediments were deposited according to their size; the largest, heaviest cobbles and gravels were deposited first, followed by finer grains of sand, silt, and clay. In fact, so much sediment was released by the glacier’s melting that the deposits accumulated and plugged up many of the drainage outlets. A great dam of debris blocked the flow of the Connecticut River, leading to the formation of a giant glacial lake that flooded the entire river valley and its many tributaries, including Sacketts Brook. In Putney, Lake Hitchcock, as it is now called, is estimated to have been at about 500 ft, which is 50 feet above the elevation of the Central School today! Where upland streams emptied into the margins of this still water they lost their momentum and dropped broad deltas of gravel and sand. Farther out in the calm depths settled the tiniest particles of clay.

Eventually the dam failed and the floodwaters flowed out, leaving behind numerous sediments that had accumulated over several thousand years. With the lake gone, the Connecticut River was now free to course through broad valleys, slicing into the soft sands and muds, and creating new landforms like the terraces and floodplains that we see today. In other places the waters carved through all of the compliant materials on top, revealing the hard rock layer below; the unyielding bedrock better resisted the water’s persistent scouring, generating powerful waterfalls like the ones along East Putney Brook.

The glacial till, sand, and clay deposits that have accumulated on Putney’s landscape are directly linked to the town’s cultural history. With regard to the till, early settlers used this bounty of stones for the foundations of their houses and barns, and also removed the rocks to cultivate the soil, which resulted in miles of stone walls as boundaries for crop and hay fields. This glacial legacy remains still today; a walk through now-forested lands like the Putney Central School Forest will uncover remnants of these walls that once criss-crossed the Vermont landscape. In contrast, sandy terraces were favorite spots for early cemeteries, because they are easy to dig and drain well. Both the Old North Burying Ground and the Mount Pleasant Cemetery in Putney are sited in sandy glacial outwash deposits that flank Sacketts Brook. In the past several decades, these sediment resources have been tapped for use as sand and gravel pits. Indeed, when we drive along Interstate 91 in Putney we are riding on a road bed that is made of these stable post-glacial sands. Clay, too, has historically been put to good use in several Putney brickyards. One notable operation was located across the road from the Putney Inn, near Brickyard Lane, and another in East Putney on River Road. Most of the brick buildings in town were probably built from these local sediments, including the historical Pierce’s Hall.

Finally, it is important to remember that just as in the past, surficial landforms are influenced by dynamic processes which persist today in the present. Without a doubt, these transformative forces continually shape the physical topography and our associated human endeavors. Perhaps most notably, much of Putney’s farming heritage is dependent on the fresh loads of sediment deposited annually by streams and rivers. These fertile materials, called alluvium, provide the restorative foundation that feeds local farms, and ultimately families.