Quincy Quarries
Site Description and History
Only a few miles from the Blue Hills Reservation, the Quincy Quarries offer interesting perspective into the geological history of our area. See our "Blue Hills" sub-page for more geological information. Additionally, from 1826-1963, it was an active quarry that provided granite for nearby construction. In fact, the Bunker Hill Monument utilized building resources from this very site.
Once closed and abandoned, rainwater began to collect in the basin surrounded by the large, left-over granite boulders. The Quarries soon became a popular cliff-jumping spot. With rocks and poles often hidden underneath, fatalities skyrocketed.
The city of Boston addressed this issue by filling the quarries with dirt from the Big Dig. Once drained, the city designated the Quarries as a reservation and encouraged graffiti artists and rock climbers to observe the wonders of this location (1).
Our Findings - May 4, 2021
Large boulders of Granite populate this state park. Often, these are covered with graffiti, but we nonetheless found excellent samples with which to begin our scientific analysis.
In this picture, one can see a typical example of Quincy Granite. Notice how you can observe each mineral in the rock. Given more time to cool, the minerals in the Quincy Granite have had a chance to grow into larger crystals, unlike the fast-cooling rhyolite that we observed in the Blue Hills.
We can also observe the types of minerals that make up granite. The white, glass-like grains are quartz, the white or pinkish grains are feldspars, the black, sparking grains are biotite mica, and some of the black grains are hornblende (a group of minerals). Quartz, feldspar, and hornblende all fall between 5 and 7 on the Mohs Hardness Scale making Quincy Granite a very hard rock.
Sample of Quincy Granite. Fingers for scale.
In the sample on the left, noticed that this piece of Qunicy Granite lacks the red-ish hue of the piece above. We suspect the color differences are due to different mineral make-ups, and in particular due to the feldspar in the rock. Feldspar commonly comes in two flavors: white plagioclase feldspar (containing calcium or sodium) or reddish orthoclase feldspar (containing potassium). The same magma pool can form both these types, but the temperature of cooling impacts which feldspars form. The color differences between these samples suggest they cooled at different temperatures leading more potassium-rich (reddish) feldspar to form in the other rock (2).
For anyone eager to explore the Quincy Quarries, we recommend visiting the Blue Hills as well. By seeing both sites, you can compare and contrast the rock types to think about how the ancient volcano and its cooling magma created the rock we see in our area.
