Looking deep into the structure of the place where you live

By BILL CHAISSON
By Bill Chaisson

I’m not a “hard rock geologist.” In fact, the old fashioned pejorative for my type of geologist is “gardener,” because the materials that I looked at weren’t even rock yet most of the time; they were still unconsolidated sediments. But a hard rock geologist is one who studies igneous and metamorphic rocks and the terrains they create. After living in western New York, which is almost entirely made up of relatively flat-lying sedimentary rocks, it was a delight to get back to the raw-boned landscape of New England, which is primarily metamorphic rocks, shot through regularly with igneous rocks (giving the Granite State its name).

As I drive in from Unity each morning, I spend a fair amount of time staring at Green Mountain, which looms over Claremont in the northeast. Since I moved here, I have been wondering what gives it its distinctive shape. I have also been wondering what causes the mountains around here to be distributed in the way they are.

I attended graduate school at the University of Massachusetts, Amherst and had several hard rock geologists as professors. Very quickly I joined a research community of deep-sea marine geologists and paleoceanographers, most of whom do not care about rocks much at all. But because I took classes in structural geology with a hard rock guy, I learned the habit of looking at the landscape and wondering how it’s made. It is a bit like being really into cars; when you hear an engine throb, you probably want to look under the hood to see how it runs.

I don’t claim to be good at this; I got a B in structural geology. But nevertheless I still look at highway outcrops when I drive by them, and if I’m alone and not in a particular hurry, I’ll stop and have a closer look. (All hobbyists know the “if I’m alone” clause; stopping at every yard sale, comic book shop, metamorphic rock outcrop is no fun for the non-fascinated.) I also purchase geological maps that are available for a new area that I move into and read the available literature, popular and academic, that attempts to explain the local geology.

“Attempts to explain” is germane when you are talking about New England bedrock geology, which is famously complicated. At various points in the past, there were economic geology reasons for studying New England, mostly related to mining, but generally speaking trying to figure out why New England looks like New England has been a purely academic pursuit, which is to say, there isn’t much grant money in it.

The western quarter to a third of New Hampshire is part of something called the Bronson Hill anticlinorium. Anyone who has taken geology at any point has probably heard of an anticline; when bedrock is folded by tectonic forces it can bend into a series of recumbent S curves. The portion of the curve that is convex upward is the anticline (the other part is the syncline).

You can think of an anticlinorium as being something like an auditorium in that it is metamorphical room full of anticlines all bunched into a giant convex upward structure. If you see a picture of this, it might remind you of old-fashioned ribbon candy. The Bronson Hill anticlinorium looks like ribbon candy that has been warmed slightly and bent into S curves along its length. Now imagine that someone has taken a band sander to the ribbon candy and given it an uneven surface with highs and lows, exposing the bands of folded candy at odd angles on the surface. That is about the simplest explanation I can give of New England bedrock geology.

The initial folds in the ribbons were caused by continental collisions. What is now New England is a collection of “terranes” (essentially small continents and large archipelagos) that were folded when Laurentia (now called North America) collided with the African portion of Gondwana. The axes of these terranes flow roughly south to north from Connecticut to New Hampshire, but then bend eastward into Maine in northern New Hampshire and Vermont.

What imparted the bending along these axes I frankly don’t know and I’ll bet it’s something that at least 50 to 100 people in the world care about a lot and fight about constantly at meetings. But the erosion (the proverbial band sander) is caused by tens of millions of years of rain and a couple million years of continental ice sheets grinding southward across the region.

Generally speaking, the hard, erosion-resistant stuff stays high and the softer, erosion-susceptible stuff is brought low by erosion. Erosion is the combination of weathering, which is the actual breaking of bedrock into small pieces, plus transportation of those pieces away to another place, usually by water, but also by ice and wind.

So you would expect the mountains around Claremont to be made of harder rocks than those in the river valleys, and again, generally speaking, this is true enough. The local resistant formation is called the Clough Quartzite. Formations are simply bodies of rock that were originally formed in an environment with similar conditions and materials. A quartzite is a metamorphic rock that most often began as a sandstone, a sedimentary rock, which are quite often formed from beach environments. Sandstones become quartzites when they are buried a few miles beneath the Earth’s surface under a region where continental collisions are creating a lot of heat and pressure.

Beaches, you’ve probably noticed, are relatively narrow environments and quartzites are often fairly narrow, ribbon-like formations. The Clough Quartzite is no exception; it is continuous in some places and discontinuous in others where it either dives below the surface or the “band sander” has removed it.

I have climbed to the top of Unity Mountain a few times and discovered that, yes, the Clough Quartzite is exposed at the summit. I have inspected geological maps of the Claremont area and, yes, Green Mountain is “held up” by the Clough. The curious curved shape of the ridge of the summit is made from an anticline of quartzite “ribbon candy” that is diving under the surface up there and was exposed by the “band sander.” In fact the geological map shows an oblong ring of quartzite with Green Mountain at the top, Arrowhead/Flat Rock Hill on the west side, and some of the high hills in Unity and Acworth at the bottom.

I inch my way through this topic bit by bit, looking for ordering principles like the Clough, or patterns of faults to explain the shape of the land. Around here, it is quite a rewarding hobby in that you’re never really done figuring it all out.

Bill Chaisson is editor of the Eagle Times and carries a hand lens everywhere with him to look at rocks.