Teacher Notes in Yellow

 

     
Bloody Bluff Fault Zone, Lexington - teacher guide

There are 4 good locations to scope out here, if you decide to go yourself. Click here for a map so you can find these locations.

A key part of understanding Massachusetts geologic history is understanding the process accretion of several terranes onto the existing continent of Laurentia (what we call now North America). The easternmost of these terranes is Avalonia (in which the "Boston Basin" sits). Further west is the terrane of Nashoba.

The Bloody Fault marks the boundary between these two terranes. It can be seen in the outcropping above.

<< Add map on Page 39 from Skehan >>

Another fascinating feature, which can be seen at many places along route 128 from Waltham to Peabody, are exposed "mylonites". These are highly deformed metamorphic rock caused by tectonic plate movements. The movement occurs deep in the crust where the rock is more ductile and less brittle, so the rock gets stretched, pulled and twisted, etc. The extent of shearing is less than you would see from plate movement in cooler, more brittle rocks closer to the earth's surface.

The mylonites in this area (known as the Burlington Mylonite Zone) mostly occured in two distinct time periods. The first occured during Precambrian times as Avalonia was splitting away from Gondwana (around 600 million years ago). The second occured in Silurian times (443-417 million years ago), as the continental margin of Avalonia was being subducted under Nashoba.

 

This spot is famous for a bloody battle during the Revolutionary War. Less famous, but certainly no less important, is the "Bloody Bluff Fault". This fault was one of many faults along the boundary between the "microcontinents" Avalon and Nashoba. This particular fault was active around 300 million years ago. You can see the highly deformed granite and diorite here.

In the pictures above, you see that were once basalt, but has now been metamorphosed. Can you see the evidence of metamorphism?

The metamorphosed basalt is known as amphibolite. Overlaying the dark amphibolite is granite. Which rock to you think is older? How do you know?


 

These pictures show an outcropping that was left after a new hotel and parking lot were constructed. On the top level we see intensely metamorphosed amphibolite. This is likely an extension of the amphibolite in the previous pictures. Note the light-colored rock on the bottom - it shows evidence of metamorphism. Do you think this is the same rock - granite - that we saw in the previous pictures? Why or why not?

 

The light-colored rock is definitely not granite - at least not anymore! The rocks in this area are so highly metamorphosed that it's very difficult to identify the parent rock. It's most likely a light colored mylonite.

 


There are many forces that can cause rocks to become metamorphic rocks. These particular rocks were changed through pulling and stretching deep within the Earth's crust (the process that we see here is called mylonitization).

What evidence do you see in these pictures that the rocks went through this stretching and pulling?

This process happened under high temperature - not high enough to melt the rocks completely, but enough to produce major changes in the rocks. What evidence do you see in these pictures that the metamorphism happened under high temperature?