Middlesex Fells

Teacher Notes in Yellow
Middlesex Fells - teacher guide

The Middlesex Fells is part of a much larger formation called the "Lynn Volcanics". These are ancient volcanic rocks that formed in late Precambrian times. The predominant rock in the Middlesex Fells is a mostly light-colored rhyolite known as "Lynn Felsite" . These rocks formed as the microcontinent "Avalonia" was being formed as an arc of volcanic islands. The Lynn Felsite is an extrusive igneous rock, which formed as the volcanoes erupted. In addition, there are intrusions of black basalt running through the Felsite. Basaltic intrusions typically form during periods of rifting; these probably formed as the "Boston Basin" formed. The Boston Basin is a rift basin (a region where plates are moving apart) within the Avalon microcontinent. This was around the same time as the formation of the Lynn Felsite. A question for the students to consider is which came first - the Felsite or Basalt? While the answer may not be immediately clear, by studying the road cut pictures below, you can determine that, at least here, the Felsite formed first. In addition, there is a large dike of gabbro (known as Medford Gabbro) running through the park. In fact, the Fells are the site of an old gabbro quarry. Gabbro was a common building stone in the Boston area for many years. A question we ask in this "field trip" is why gabbro was used, and not the felsite? In fact, the gabbro weathers much more rapidly, which might indicate that the felsite would be a better choice, at least in terms of durability. The answer lies in the fact that the felsite is an extremly hard rock that fractures along planes of weakness within the existing rock - not where you want it to fracture if you were cutting it. Therefore, it would be much more difficult to use it for building stone. The gabbro is much easier to cut and carve. The Medford Gabbro formed about 190 million years ago, so it is much newer than the felsite or basalt. It formed as the continent was stretched and rifts began to open up as the Atlantic Ocean began to form (as Pangaea began to break apart).
The Middlesex Fells is a beautiful park north of Boston. Now set aside as a nature reservation, it onced was the site of a gabbro quarry.
Picture of a pond on the site of the old gabbro quarry.

Weathered Medford Gabbro			Weathered Lynn Felsite
The most common types of rock found in the Fells are "Medford Gabbro" and "Lynn Felsite" (a type of rhyolite). Compare the pictures above of how the two rocks hold up under weathering. What about the way these two rock types form might explain these differences? Given the way the two rocks weather, what might that tell you about why the gabbro was quarried, and not the felsite?
If you were to wander through the park, you would notice a third type of rock, a dark rock called basalt. There are many places in the park where you see contact between the Lynn Felsite and the basalt. Study the pictures below. What patterns do you observe? What clues are present that might help you determine which rock came first (the felsite or the basalt)?








The pictures below show a large roadcut along Route 93, which cuts right through the Fells park. Notice dark bands of basalt and the much ligher Lynn Felsite. With this additional information, can you now tell which rock came first? It's interesting to note that you can't find the basalt cutting through the Medford gabbro. What does that suggest about when the gabbro formed, compare to the other two types of rock.





We highlight the pictures below because they show a very interesting metamorphic process that produces slickenside. This is a process of localized metamorphism in which minerals form on the surface between two rocks that slide past one another. The frictional heating is the source of heat for the mineral recrystallization.
The pictures below show a mineral formation that occurs when rocks slide past each other. This type of formation is called slickenside. The greenish cast is from a mineral called olivine (often found in slickensides). Why does it seem logical that new minerals might form between two rocks that slide past each other (as in a fault)?