Geology
of the Willoughby Cliffs Natural Area

Excerpted from:"Willoughby Lake Legends and Legacies," Harriet F. Fisher. Academy Books, Rutland, Vermont. Copyright © 1988.
Reprinted with permission of the author.

It is a well established idea that Vermont is the most beautiful state in the Union.   It is also generally conceded, at least by the local residents, that Willoughby Lake is the most beautiful spot in Vermont. The question is, How did Willoughby Lake get to be so beautiful? How was the deep notch between the mountains created? Why is the lake so deep? And why are the forests and fields surrounding the lake so lush and green? For the answers to these questions, we must take a look at the geologic forces that went to create the lake and the surrounding terrain.

For at least a rough understanding of the geologic events that contributed to making Lake Willoughby what it is today, it is only necessary to consider two episodes in geologic history: first, the time when the rocks in the mountains around the lake formed; and second, the time when the great ice masses periodically ground down across New England.

top

Probably more than half the rocks underlying the area started as sediments – sand, mud, and lime – deposited in a shallow, warm sea about 400 million years ago. More and more material kept being piled on top of these sediments until they were very deeply buried and eventually subjected to downward pressure strong enough to compact them into solid rock. At the same time, upward and sideward pressures tilted and folded these originally flat-lying sedimentary rocks to varying degrees.

The next event was the upward injection of very hot, liquid rock, known in the trade as “magma,” originating from some deep source. This magma filled cracks and was squirted between the layers of the sedimentary rocks, where it then cooled and hardened into what we see now as granite. In this process, the heat from the cooling magma so cooked the sedimentary rock into which it was injected that its original texture is often hardly recognizable to a modern observer. Geologists call rock that has undergone this transformation “metamorphic.” All this happened at great depth and under a great thickness of overlying rock, with enormous temperatures and pressures. Good examples of metamorphic rock can be seen on the face of Pisgah, and in the road-cut a few hundred feet north of the watering trough on Roaring Brook.

top

What happened next was not so dramatic. Little by little the enormously thick overlying rocks were eroded and washed away; this exposed the complex of sedimentary and metamorphic rocks with the intruded granites that we see today. During this process of erosion, the granites and metamorphic rocks, being harder and therefore more resistant than the sedimentary rocks, tended to stand up as hills and mountains. This process was slow, but there was plenty of time – over 300 million years.

top

The final series of events started only two million years ago. In the far north, winter snow started to accumulate faster than it could melt. Tremendous thicknesses piled up until the weight compressed the snow into ice. When it reached a certain critical thickness – about 10,000 feet – the ice began to flow slowly, like a huge mass of molasses or peanut butter. The advancing front of this mass, which formed into continental glaciers, pushed across the land like a giant bulldozer, grinding off the tops of mountains and scooping out hollows where the rocks were softer than average or where an earlier river had carved out a channel. There was a tendency for the ice to slide up the north sides of prominent mountains and then “calve” or divide on the south sides, in such a way that the north slopes were left relatively gentle, while the cliffs occurred on the south, or down-glacier side. This effect is seen clearly on Wheeler and Haystack Mountains.

After each advance of the ice sheet, the climate would eventually warm up again, the ice would melt back, and life would resume where the land had been covered by a ten-thousand-foot layer of solid ice. Recent research has indicated that this periodic advance and melt-back repeated itself more than 15 times during the last two million years. Each time the ice advanced, the land forms left by the last melt-back were erased, much as a blackboard is erased. Because these erasures were so thorough, all we really know now is what was left after the very last melt-back about 11,000 years ago. What was the Willoughby notch like when the ice first advanced? And why did the ice gouge out the deep cleft between the two mountains? We can only guess at the answers. Probably there was an incipient notch between what are now Pisgah and Hor. Possibly the rocks in this notch were sufficiently softer than those underlying the mountains, so that the notch was deepened a little more every time the ice scraped across.

top

One thing that is somewhat more than a guess, however, is what happened during the last advance and melt-back – the last writing on the blackboard. That advance pushed as far south as Long Island. In fact, Long Island itself is made up of soil and rock fragments scraped off New England and pushed ahead of the ice front. But then the climate moderated again, and the ice mass over New England began melting back as usual. During this particular melting period, however, there was a temporary reversal when the ice again advanced for a short distance, with fingers of ice feeling their way through valleys ahead of the main ice front. One of these fingers cut through the Willoughby notch, reaming it out and then dumping the scrapings at the furthest advance of the finger, now seen as the height-of-land south of the lake. This process was almost exactly analogous to that of the valley glaciers that formed the fjords of Norway and Alaska, except that instead of flowing down from central ranges of high mountains, our glaciers flowed down from 10,000-foot mountains of ice to the north. As in the fjords of Norway, the gouging here was deep (over 300 feet opposite Roaring Brook on the lake road), and the cross-section of the lake bottom is U-shaped rather than V-shaped like river valleys. Crystal Lake in Barton, and the finger lakes of central New York State probably had a similar origin.*

Another   effect   of the glaciation on Vermont was to grind up and scatter over the countryside the lime-rich sedimentary material. These rock fragments readily decayed to release their component lime and sand, and thus to form an extremely rich and fertile loamy soil. This fertility resulted in the rich, green color of landscape – in summer, anyway – that gave Vermont its name. Unfortunately, at the same time the ice also left so many granite boulders in the soil that cultivation can be pretty difficult. Some of these granite boulders are of enormous size, and may have been carried by the glacier for very great distances. One of these is Balance Rock, just off Long Pond road, which is estimated to weigh over 200 tons.

There is every reason to think that we are now living in the interval between two ice ages. Chemical studies of deep-sea cores in areas not covered by ice during the past ice ages have indicated that the intervals between the fifteen melt-backs and the following readvances of the ice range from 9,000 to 20,000 years. It is a sobering thought that we are already 11,000 years into our present interglacial period. How soon will the ice come grinding down across Canada again and scrape away everything on the land surface at Willoughby? We can only hope we won’t be here to see it happen.

*Editor’s note: There is an old wives’ tale to the effect that an underground connection exists between Willoughby and Crystal. Objects lost in one lake are said to have been found in the other.

top