Bloody Bluff Fault Zone

Bloody Bluff Fault Zone, Lexington

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.

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. A tiny bit of this fault can be seen highlighted by the black line in the picture above.
Going Deeper... 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. 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.


In the pictures above, you see rocks that were once basalt, but has now been metamorphosed. Note that basalt is usually a uniformly dark color, and yet the basalt in this picture has many light-colored, twisted veins running through it. This is evidence of metamorphism. Can you describe what kind of process might cause solid rock to change like this? The metamorphosed basalt is known as amphibolite. You can also notice a light colored granite. Which rock to you think is older? How do you know? Hint: look carefully for clues showing one type of rock intruding on another.

These pictures show an outcrop that was left after a new hotel and parking lot were constructed. At the top of the outcrop 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. Samples of this rock show evidence of metamorphism, such as the fact that this once hard igneous rock is brittle and easy to break in layers (this is called foliation). Do you think this is the same rock - granite - that we saw in the previous pictures? Why or why not?

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). Evidence of this metamorphism include: foliation (rocks that were previously not layered, being squeezed into layers), deformation (rocks that look like they've been twisted and squeezed), recrystallization (veins of minerals that form as the rock is heated and squeezed), and mineral formation along lines of contact between rocks. 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?