Purgatory Chasm

Trails:

Located in Sutton MA (42.1288, -71.7132), Purgatory Chasm features many loops and leisurely trails through the woods. A trail map and link are below. The Chasm Loop Trail is full of massive boulders and has rock scrambles. Little Purgatory trail follows along a brook at the end where you can see interesting water and rock formations.

MA DCR: Purgatory Chasm Trail Map

chasm-map.pdf

Geological History:

Multiple theories attempt to explain the creation of Purgatory Chasm. One theory suggests that glacial meltwater ripped out boulders from the area around the end of the Last Glacial Period (roughly 12,000 years ago) (1). Additional related theories suggest that the area was at first a fault, or a fracture in a large block of crust, that then lost many of the rocks within due to glaciers (2).

Our Findings: May 9, 2021

In this rock, Christian discovered a beautiful vein of biotite mica. Mica's crystals form very thin sheets that stack on top of each other. These sheets are pretty bendable and can even be peeled off with your fingers. Biotite mica is one of the main minerals in granite.

In this outcropping of granite, we saw clumps of quartz, one of the hardest minerals (a 7-7.5 on the Mohs scale) (4). Due to weathering and walking, the other minerals in the rock have eroded away, leaving these quartz clumps extruding out of the rock.

As we walked into the center of chasm, we noticed it got cooler and more humid. This temperature change suggests that the gulley of the chasm is wetter than the surrounding areas. We observed on the underside of cliffs in the chasm that the granite became softer. Christian even peeled off a sheet of the rock from the overhang. The water likely eroded some of the minerals in the granite, which led to the rock crumbling and losing structure. When seeing this picture, our project advisor, Ms. Latham, also had an interesting idea about this rock: she suspected it could be "a shear zone, along which movement within the rock has occurred after the rock itself formed. The layered sheets are perhaps where the rock was stretched and squeezed, ultimately forming pathways for water to travel through" (3).

This final image exhibits another sample of the beautiful mica we found along our hike. Unlike the previous finding of black biotite mica, this mica is white and clear. This means it is muscovite mica.

References