This guide introduces the local geology of the Dartmouth-Upper Valley area using some of the hikes described in the Dartmouth Outing Club Guide. Our aim is to tell the story of the relatively recent Earth history. By recent we mean the last 500 millions years or so. Alas, the first four billion years of Earth history is poorly represented in this area, but some pre-Cambrian rocks can be seen in the core of the Green Mountains and the Adirondacks.
The rocks that are here show evidence of the Earth’s upheaval. A geologist can, by interpreting the composition, geometry, sequence and regional arrangement of the rocks beneath our feet, read the story of the movement of the continents across the surface of the Earth, volcanic eruptions, seas rising and falling, and mountains erected to Himalayan heights and then worn down to hills. Right here in the Upper Valley we can see the formation of the super-continent, Pangaea, during the Paleozoic Era from 500 to 200 million years ago. Later in the Mesozoic Era, Pangaea split apart. This separation ultimately led to the present continental configuration of the Earth. Further south in the Valley, in Massachusetts and Connecticut, the break up of Pangaea, is evident.
During the last 2 million years, long after all significant mountain building had run its course, and the Appalachian highlands were subsequently laid low by erosion, the Dartmouth Region again underwent a remarkable change. Here, as well as over most of North America to the latitude of (about) Long Island, the land was covered several times by thick sheets of ice moving south from, and then retreating to, the Arctic. Only in the last 10,000 years or so has the climate become the one we enjoy today…or, rather, enjoyed yesterday. It is becoming clear that the next 10,000 years will be unlike the last 10,000 years. But then, as the rocks suggest, the Earth has always been in flux.
We describe the geologic, tectonic events and processes that created the land that we now live on and why it looks like it does. Like the landscape around us, this document is a dynamic one subject to correction and change. We will amend and improve it as we add more examples and as the science advances. We have to some extent simplified the geologic explanations. In situations where controversies exist, and when alternative explanations are possible, we tend to choose the easiest to explain and understand. Thus, for knowledgeable readers, we apologize in advance for mistakes we may have made, but carry on nevertheless, reveling in the knowledge that someday it will probably all be worked out…
We thank the following Geologists for their contributions to our knowledge, both general and specific: Bradford B. VanDiver, J. B. Lyons, C. G. Doll, Gary Johnson, Dick Birnie, Alex Gates. We apologize to anyone we forgotten to mention.
At the end of each section, we will offer references and suggestions for further reading.
I'd love to learn more about local geography. I think I know more about foreign lands because of the European river cruises I've been on.ReplyDelete
It is true that the 1855 fire burned off the trees and soil of Cardigan's neighbor hill 1/2 mile north, and the locals named it Firescrew.ReplyDelete
It is less certain that the 1855 fire burned off the trees on the summit of Mt Cardigan. I recall examining the names and years chiseled into the ledge at the summit by 19th century tourists. IIRC, several were older than 1855. From that I surmise the rock was treeless before the 1855 fire, on the grounds it is quite unlikely a tourist would spend the energy removing soil to chisel their name into the ledge. If other observers confirm my observation, then we have to admit that at least the summit was treeless before 1855, and the article should be revised accordingly.
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