Bedrock Geology

Though it may not be obvious, the rocks that constitute the landforms beneath the Putney landscape have played an eminent role in determining the cultural heritage of the town, and continue to impact the lives of citizens today. Yet the story of Putney’s bedrock extends far back in time before the first human beings inhabited the land.

In fact, Putney’s hills and valleys are underlain by a foundation of stone that goes back several hundred million years to an era called the Devonian, a time before plants even existed on dry land! The location of the earth’s continents was much different back then, and geologist tell us that the land which we now call Putney was at that time below tropical waters on the floor of an ancient ocean called the Iapetus.

Over time, this sea floor accumulated a thick blanket of sediments, with great variability in both sediment size and mineral composition. Some of the sediments eroded from nearby continental highlands such as the Taconic and Green Mountains. Others were biologic in origin, like the calcium carbonate shells created by tiny marine animals. Volcanic eruptions also contributed igneous sediments to the mix. Over tens of millions of years, the sands and mud built up into thick deposits, became compacted, and cemented into stone.

Then something truly awesome occurred: great tectonic forces caused the collision of the Northern European and North American continents, in an event known as the Acadian Orogeny. It is quite difficult to imagine the time-frame over which this impact took place – geologists believe that it took 50 million years altogether – yet the result is readily visible in the bedrock of the surrounding landscape. The upshot of all this tectonic activity was that the rocks of the shallow marine basin were thrust up into massive peaks, and being subject to extreme heat and pressure they were metamorphosed into new rock types called slate, phyllite, schist, marble, and amphibolite. Like a rug that is pushed from opposite ends, many of the rock layers buckled and folded into long north-south ridges with jagged angles, and some even broke along fault zones. Since those chaotic times, erosion by water, wind, and ice has diminished those once-grandiose mountains into the familiar rolling hills of Putney.

The majority of Putney’s bedrock is comprised of schist, or schist combined with other materials. In the Northfield Formation, the thin band of rocks along the western edge of Putney that forms the Putney Mountain ridgeline, the schist is relatively pure. Directly east of the Northfield is the Waits River Formation, which has impure marble (containing both calcium carbonate and quartz sand) mixed in with the schist. The marble tends to be highly weathered, giving it a punky, porous texture.

On the eastern side of town there are noticeable quantities of slate, part of the Littleton Formation. Though much of this slate is of poor quality, some of it was quarried by early colonial settlers. Commercially, the old slate quarry along Signal Pine Road was in active use during the 19th century, and a careful search along the ridgeline of Bear Hill will reveal a few small pockets where nearby farmsteads extracted slate slabs for local roofing projects. East Putney also contains thin bands of amphibolite and other rocks of volcanic origin.

As already alluded to, bedrock geology is a defining characteristic in molding not only the physical terrain, but also the ecologic and human communities on a landscape. Hence, variations in the distributions of various rock types can help explain patterns of flora and fauna in addition to human settlement. The marble in the Waits River Formation supplies calcium carbonate to the local soils, providing a boost of nutrients and promoting the growth of certain “rich” soil plant communities like the basswood, sugar maple, wild leeks, and maidenhair fern.

In contrast, the slate below Bear Hill is relatively nutrient poor and home to forest generalist plant species like beech, white birch, and wood ferns. Bear Hill’s rocky, difficult terrain has also prevented any significant human settlement in that area of town. Putney’s early industrial development can likewise be tied directly to the bedrock; the first mill sites and industrial centers in town were established along East Putney Falls and Sacketts Brooks, where hard, erosion-resistant bedrock created cascading waterfalls, perfect for generating power. Even today, Putney Paper continues to operate in the same place where paper making has been a continuous process since 1818!