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Rocks At Purgatory

        A diverse range of rocks can be found at Purgatory Chasm. This is partially due to the glacier that moved through it, but also because of the rocks naturally found there. The bedrock beneath Purgatory Chasm is Worcester Formation which consists of carbonaceous slate, phyllite and minor metagraywacke from the lower Devonian and Silurian time periods. It is light gray and strongly foliated. The rocks that make up the bedrock are biotite, muscovite and granite to granodiorite gneiss. Inside the bedrock, there are also small to large inclusions of rocks such as black to gray aluminous mica schist, quartzose schist, and aluminous phyllite. The many rocks that make up the bedrock of the area can be seen everywhere throughout the chasm and are beautiful sights.

This image shows the bedrock of different areas in Massachusetts. Purgatory Chasm is located in the light pink area, which means its bedrock is granite. 

Types of Rocks/Minerals:

The walls of the chasm are made of granite gneiss, which has been metamorphized through heat and pressure.
Granite Gneiss 

Most of the rocks found in the chasm are composed of Granite Gneiss which forms when igneous granite rocks go through tremendous heat and pressure over a long period of time. This type of Gneiss is:

  • Light gray, tinted with red by feldspar

  • Medium Coarse

  • Granular texture

  • Foliated

  • Made of feldspar and quartz that are granulated

  • Made of biotite that “occurs in very thin, indistinct, laded units”

  • Looks as though it was once porphyritic

  • Found with magnetite and hornblende

  • Considered the oldest of the rocks on the Central Massachusetts plateau

  • Found on the southeastern border in towns of Douglas, Uxbridge, Northbridge, Upton, and Grafton

Granite 

Another rock found everywhere in Purgatory Chasm, granite is an igneous rock that is formed by cooling slowly inside a magma chamber beneath the surface. Because it is highly resistant to weathering, there is a huge amount of it still intact in the chasm. 

This cluster of rocks is all granite. Their different colors are caused by differences in the minerals which compose it.
The lighter parts of this granite are clusters of white quartz.
Quartz

Quartz is a mineral that can be found at Purgatory inside other rocks as clumps and clusters or as veins through other rocks. It can come in many different colors such as:

  • white

  • yellow

  • purple

  • brown

  • black

  • pink... etc.

Feldspar

This is a mineral that appear at random along the bottom and the top of the chasm. This indicates that they were plucked and dropped by a glacier. They are mostly found as a yellowish tan color at Purgatory Chasm and are one of the most abundant minerals on Earth. 

The yellowish/tan tint in the rock is feldspar!
Granite Pegmatite

Much of this rock was found along the ground and paths of Purgatory Chasm. It made of granite with many clumps and clusters of others minerals such as:

  • mica

  • feldspar

  • beryl

  • quartz

We were able to find most of these minerals inside the rocks along the chasm.

This clump of different minerals within the granite is an excellent example of granite pegmatite.
Alaskite

We found this rock in the form of an erratic along the chasm (click here to learn more). It is a type of granite which contains no dark colored minerals, resulting in a very light colored rock. This rock's light colored appearance earned it its name of Alaskite.

Here is the erratic made of Alaskite which doesn't match the mineral composition of a lot of the other rocks in the chasm.

So how old are these rocks? 

As we stated before, the bedrock of this area contains minerals from the lower Devonian and Silurian periods which is from 444 million to about 400 million years ago. Using the cross-cutting relationships principle of relative age dating (which states that a fault or intrusion must be younger than the rock it cuts through), we can assume that the fault that helped form the chasm had to have occurred after this time. 

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Pictures:

https://mrdata.usgs.gov/geology/state/state.php?state=MA

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*DISCLAIMER* This is a website that we created as a school project, we are in no way certified geologists.

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