If any of you have wondered where the Commonwealth’s beautiful gray granite curbstones come from, today’s photo is the answer: The ~375 Ma (millions-of-years-old) Chelmsford granite. This aerial photo is of the Fletcher granite quarry in Westford, which has been in operation since the mid 20th century. Quarrying of the Chelmsford, and other granites, in Massachusetts has been occurring since the 1600s, according to various town histories.
What is of particular interest in this photo is how well the Chelmsford granite naturally splits at 90 degree angles— something historic granite workers immediately noticed and have taken advantage of over the centuries . The old quarry worker’s term for this is “Rift and Grain”. In New England, the rift and grain of granites, in many places, is oriented in vertically at 90 degrees to one another: in North-South and East-West striking planes: Very convenient for quarrying dimension stone.
The origin of this rift and grain is even more fascinating, and has been the subject of many academic papers over the years including ones by the famous Richard Jahns, Don Wise, and Terry Engelder. The rift and grain are parallel to regional fracture systems in the granite that occurred in response to tectonic stresses in the geologic past— the orientation of those stresses are different than the present day stress in the continental crust of New England (created by pushing from the spreading Mid-Atlantic Ridge). In order to take full advantage of the older rift and grain, quarry operators have to isolate large blocks of the granite and let them “relax” for a few decades so the present-day crustal stress can be removed from the rock. This minimizes wastage and ensures nice, 90 degree angles when the stone is quarried. Photo credit: Bing Maps. (via Massachusetts Geological Survey)