×

N.H. is the Granite State, but that doesn’t mean you’re standing over granite

  • Lee Wilder, public outreach coordinator for the New Hampshire Geological Survey, explains the different types of bedrock in New Hampshire. David Brooks / Monitor staff

  • This is a simplified map of New Hampshire's different types of bedrock. Yes, simplified. Courtesy of the state Department of Environmental Services

  • Cannon Mountain's east side, scoured by Ice Age glaciers, was the site of the Old Man of the Mountain. Greg Keeler - Cannon Mountain

  • The Appalachian Trail along Franconia Ridge.



Monitor staff
Tuesday, June 12, 2018

Like most New Hampshirites, I love the nickname Granite State. So it was a little startling when I learned that, geologically speaking, it’s sort of wrong.

“We’re less than half granite,” is how Lee Wilder, the public outreach coordinator for New Hampshire Geological Survey, put it to me recently.

To be specific, he said, less than half of New Hampshire’s bedrock, meaning the solid rock undernearth our soil, is actually granite. Much of it, including Mount Washington and the rest of the Presidential Range, is something called schist.

You know another place where the bedrock is schist? Mount Kearsarge – despite the fact that in an April column about 19th-century mapping of the state I casually mentioned “the granite ledges” on that mountain’s summit.

That mention led Wilder to send me a gentle correction: “The exposed ledges on the top of Mount Kearsarge in Warner, N.H., are a mica schist of the Devonian Aged Littleton Formation. This is a metamorphic rock. Granite is an igneous rock,” he wrote.

Metamorphic, igneous, the Devonian age – I sort of remembered that stuff from school. I reached out to Wilder to brush off some mental cobwebs and also see whether my love of the state nickname was misplaced.

We met outside the state offices of the Geological Survey in Concord. As might be expected from a man who spent 35 years as a public school teacher and also taught at Colby-Sawyer College, Wilder brought along lots of educational aids. He had rock samples, handouts, a flow chart showing how different types of rock are created, even a color map of New Hampshire bedrock geology that looks like it was painted by one of those Spin-Art sets from my childhood.

That map includes 156 different categories with names like “Hornblende granodiorite of Highlandcroft pluton” and “moat volcanics.” There were even a couple of bedrock categories that I’ve heard of, including Concord granite, which is being quarried on Rattlesnake Hill even as we speak.

The map’s complexity reflects our convoluted geological history, Wilder explained.

The bedrock underlying New Hampshire’s soils indicates that over the past 500 million years we have been part of four separate collisions between tectonic plates, the massive pieces of the planet’s outermost shell that carry the continents around and form the ocean floor. When these collisions happened, the resulting uplifting, sinking and folding created a mish-mosh of bedrock.

Granite, as I had sort of remembered, is an igneous rock created by the cooling of molten magma as it oozes up from lower levels. That is compared with sedimentary rock like limestone, created when layers of material gets deposited on ocean floors and squeezed over the course of many millennia.

New Hampshire’s bedrock is entirely igneous rock or a similar form called metamorphic rock, created when heat and pressure alters existing sedimentary or igneous rock. That, incidentally, is why we don’t have any fossils to speak of: Our bedrock has been heated or crushed or melted so much that no fossilized bones are discernible.

Those scores of subcategories on the bedrock map? They have different mineral compositions created by different timing and temperatures during past plate collisions.

Granite is mostly quartz and feldspar with some white Muscovite mica. The smooth gray of Concord granite exists because it contains small grains of the light-colored minerals, Wilder said. Equally famous Conway granite has black biotite mica.

The most obvious granite in New Hampshire is the face of Cannon Mountain as seen in Franconia Notch, former home to the Old Man of the Mountain.

“The reason the Old Man fell is that it had a type of pink feldspar which weathers out first,” Wilder said. “Over time, it was eaten away, until it just couldn’t hold up.”

Schist is a metamorphic rock formed of multiple layers of different minerals squeezed together over time, producing a variety of colors and patterns. That’s one reason hiking along Franconia Ridge is such a joy; the schist rocks are much more interesting to look at than most granite, which tends to be uniform in appearance.

All this bedrock complexity was further complicated by the last Ice Age, which lasted here from about 110,000 to 11,000 years ago.

Glaciers moved over what is now New Hampshire from north to south, then retreated back up north, then moved south again, then retreated again, picking up and moving around and depositing gravel and stones and boulders here and there as they shifted and grew and melted.

“If you pick up a rock in New Hampshire, that doesn’t mean it’s going to be a New Hampshire rock,” Wilder said. “It could have been brought here.”

In other words, the bedrock under your feet is likely to not be granite, and the rocks you can see and reach might be almost anything, but likely not granite.

To cut to the chase: Does this mean we should actually be called The Schist State? Wilder thinks not.

For one thing, he fears the endless jokes this nickname would produce among fifth-graders. Also, he knows that New Hampshire isn’t overwhelmingly schist. We also have quite a lot of gneiss, which doesn’t help because it is pronounced “nice” and would also produce plenty of nickname jokes.

But most importantly, even as a geologist Wilder knows that geology isn’t everything.

“Economically, it’s a very important stone. It’s the stone we’re known for: We have supplied granite throughout the course of our history,” Wilder said.

My concern was misplaced, he said; the state nickname is just fine.

(David Brooks can be reached at 369-3313 or dbrooks@cmonitor.com or on Twitter @GraniteGeek.)