The Swenson Granite quarry in Concord is expanding to the north, partly to meet demand from customers seeking to reduce their climate impact by using an alternative to curbing made of concrete.

“Concrete has such a gigantic carbon footprint ... A lot of that concrete curb is being switched to granite, or back to granite,” said Scott Herrick, senior advisor at Swenson. “We hear from specifiers they are glad to know they have a greener alternative.”

The manufacture of cement, the main component of concrete, involves a surprisingly large release of CO2, the main greenhouse gas.

By some estimates, the manufacture of cement accounts for 8% of all CO2 emissions worldwide. Swenson mostly produces granite known as Concord Gray, and its main product is curbing stone.

Herrick said Swenson Granite, which has been operating on Rattlesnake Hill off North Main Street since 1883, is in the process of removing “overburden” – dirt and rock lying atop “the good stone” underneath – on the north side of the quarry.

“That has increased the amount of blasting we do,” he said. “That’s going to taper off over time, but we have to blast more when opening up a new area.”

The new area will be part of a drive-in quarry, where stone can be hauled out directly instead of having to be lifted more than 300 feet out of the main quarry by a huge boom crane. That will greatly lower the cost of quarrying the stone.

Neighbors of the property raised questions recently before the Concord Planning Board about the company’s shift of a lot line between 9-11 Lake St. and 369 North State St., adding roughly 10 acres to 187 acres that the company owned, and whether this indicated development of some kind.

Herrick said the change was being made as a buffer between the business and homes.

“It’s basically a buffer. It appears no residential (development) can go there … and we have no intentions of working there. Any time you can get a little buffer from an operation like this, that’s good,” he said. “There is no stone down there.”

Swenson and Vermont quarry Rock of Ages were purchased in 2016 by Canadian investment firm TorQuest Partners, which also bought Quebec’s Polycor and joined the three to create an industry giant.

Granite is an igneous rock, created when molten magma cools underground in specific conditions. Its color, strength and other characteristics depend on nearby minerals and the details of this cooling. These vary from location to location and explain why the granite in Bethel, Vermont, is world famous for its white hue, why North Carolina is known for its pink granite, and why Rattlesnake Hill is full of Concord Gray.

It’s no coincidence that the Swenson quarry is mostly on the southeast side of Rattlesnake Hill, Herrick said. When the massive glaciers retreated northward some 15,000 years ago at the end of the last Ice Age, the release of pressure created different patterns of fracturing on north-facing slopes compared to south-facing; stone on the south side tends to be better for commercial use.

People have been digging up Concord Gray stone there since before the Civil War. By the start of the 20th century there were 44 different operators quarrying stone in west Concord, according to the Concord Historical Society’s history of the city. One of them was John Swenson, who had moved to the U.S. from his native Sweden in the 19th century. He bought out many of the smaller operators until Swenson Granite Works stood alone.

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