That igneous rock is an old friend of mine

By BILL CHAISSON
I continue to be a bit surprised when I meet people who don’t notice the geology around them. This would be more understandable to me if we were in Iowa where hardly any rocks stick out of the ground, but here in New Hampshire in most the state you are almost never out of spitting distance of a piece of bedrock or at least a really good-sized boulder. 

I noticed the geology around me often enough to become a geologist. I went to school for it at the University of Massachusetts, Amherst and, as it happens, the bedrock formations that make up the upland just east of Amherst run northward up through New Hampshire, many of them continuing over to Maine. So, when I moved back here after my 25 year in upstate New York, it was like seeing old friends again. “Hey, aren’t you the Rangeley Formation? It’s been years. How’ve you been?”

The main organizational unit for the geology of western New Hampshire is the Bronson Hill Anticlinorium. An anticline is the upward arching part of a fold in bedrock (the downward part is a syncline), and an anticlinorium is a lot of anticlines mashed together. 

A lot of people take earth science in school or watch geology shows on television and get introduced to the idea of plate tectonics. It is the central theory of geology; plate tectonics explains in a unified manner nearly everything that has been observed on the Earth. The mashing together of anticlines is caused by large volcanic islands and continents colliding with North America one after another over hundreds of millions of years. The geology of New England is a salt-water-taffy train wreck of collided “terranes.”

These collisions, which take place over thousands of vertical feet from the surface downward, generate heat and pressure that transforms the existing rocks into new types of rocks. The igneous and sedimentary rocks sitting in this zone of collision became several different kinds of metamorphic rock as a result of various combinations of heat and pressure.

All of this gives what is called “fabric” to the landscape. The mountains, ridges, hills, and valleys of New Hampshire and Vermont aren’t randomly distributed on the land; they are distributed in rough arcs that run northward out of southern New England and then bend northeastward through New Hampshire and Vermont into Maine and Quebec. 

This is why it is such a bear to drive east or west in these states: there is a near constant up and down to traverse, some of it impressively steep. Immediately before living here, I lived for several months in the High Peaks region of the Adirondacks. While there I was continually puzzled by how long it seemed to take to get anywhere. It wasn’t until I moved here and was driving home to Unity one night. I realized that in the Adirondacks they never built road over the mountains; they were content to stay in the valleys and go around them. In the Adirondacks you look at the bedrock from a great distance quite often, but you don’t drive straight over it very often.

New Hampshire is the Granite State, but there are only isolated patches of granite exposed at the surface across the region. In most of the state what you encounter sticking out of the ground are metamorphic rocks like gneisses and schists. Gneisses look a lot like granites and may include the exact same minerals, but they appear foliated — they look like layers of piled leaves, foliage — giving them an ordered banded appearance that granite lacks.

The granite in the Granite state is much younger than the metamorphic rock around it. Granite is a plutonic igneous rock, which means it cools far beneath the surface from a molten state. The granite emplacements of this region are in rough rows extending to the northwest across the fabric of the metamorphic rocks. This is because the North American continent is drifting north and west over a “hot spot,” a plume of heat extending upward from deep in the Earth. At intervals, as the continent drifts by overhead, the plume melts through from below and “batholith” of molten rock rises upward until it is far enough from the heat source to cool into solid rock. A mixture of the minerals quart, feldspar (usually two kinds) plus either mica or hornblende (the dark minerals) makes it granite. If it doesn’t have much quartz in it, then it’s a syenite, which is what makes up the bulk of Mount Ascutney.

Rocks aren’t just good company, they’re a source of revenue. Granite has gone in and out of fashion in architecture, but when people suddenly wanted kitchen counters made out of it, I’m sure the quarries were happy. The boulders along the Black River in Cavendish are dotted with garnets. They aren’t gem quality, but they were good for making garnet paper. Muscovite mica used to be called “isinglass” and transparent sheets of it were used on the doors of boilers and furnaces so that you could see into them.

I am interested in commercial uses of local bedrock and minerals. Anyone with sources of information about this subject can write to me at [email protected].

 

Bill Chaisson is editor of the Eagle Times and does not have a rock collection.