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With Dr. Misra

The Mystery of the Fells


Welcome to the Middlesex Fells! We are at the bottom of our hike to the top of the mountain. Let's zoom out to see the Fells from the highway.

These cliffs are actually man made! They were cut through to make a highway. We can now see the inside of the tall rocks. The Fells tower over the highway, taller than anything else.

As a geoscientist, it's my job to figure out the history of the earth. I do this by making observations and predictions. We can use what we can see as well as what we know from past discoveries to talk about ancient geological history!

I use a field book to help track my observations and insights. I like to draw, write, and take pictures in my book. Take a look!

You can make predictions too based on what you can see here. I'll help you with some extra clues too. During this hike, we will be paying attention to three rocks: gabbro, basalt, and felsite. These rocks will help us determine why the Fells formed here. They give us clues!

I have a challenge for you! What rock in the Fells came first and why? This is something you can figure out yourself by inspecting the landscape and using previous geological research.

You'll need a field book too! Use your field book to draw, write, ask questions, and answer questions. There are questions for you to answer. You can also add whatever you like!


I'll guide you through the hike, pointing out details and giving you clues. I'll also tell you what geoscientists do when working in the field!

We'll start going up the mountain to see a view of Boston! Let's go!

Start using your field book now! Click on the blue pencil icon to access your field book. Click on it every time you see it.

Igneous rocks are rocks that form when hot magma cools and hardens into a rock. Gabbro is an igneous rock. Gabbro is made of slow cooling magma. This magma comes from below the earth's surface. Gabbro is an intrusive rock, coming from magma pushing its way towards the earth's surface, but does not erupt. Gabbro has large crystals we can see.

What kind of rock is this?

Geologists use a hand lens to look at rock's finer details in the field. This is much faster than a microscope.

This is what gabbro would look like under a hand lens. You can clearly see the size of the crystals and grains. They are black and white. Do they look large or small to you?

What does a hand lens do?

Gabbro is made up of bunch large crystals. Geologists also use microscopes to look at rock's details.

Take a closer look!

2mm scale We can see a bunch of different colors because each mineral interacts with light differently. We can see gabbro's large crystals up close. This is one of gabbro's characteristics.

What does a microscope do for geoscientists?

When geoscientists look at rocks without a microscope or a hand lens, we notice the rocks' color, layers, and texture. Texture is the size of the minerals in the rock. We use these words to describe different rocks.

Color: Color is what the rock looks like on the inside, not the outside. Rocks are covered in dirt and debris. Texture: Texture refers to the sizes of the mineral grains in the rocks. We can describe the grains as large or small or the rock as smooth or coarse. Layers: Some rocks have layers of different colors or textures. What words would you use to describe a rock?

How would you describe this rock?

Weathering is when rocks break apart because of outside elements interacting with them physically or chemically. Gabbro and Basalt weather easily. They break apart easier than other rocks, and do not last as long as stronger rocks.

Why is this rock round and bumpy?

Basalt is very similar to Gabbro. Remember, Gabbro comes from magma and cools slowly to become a rock made up of coarse, large crystals. Basalt also comes from magma deep in the earth's crust, but cools quickly, and is made up of fine, small crystals. Basalt erupts when it reaches the earth surface, while gabbro does not. Basalt and gabbro are made up of the same chemicals such as magnesium and iron.

What kind of rock is this?

This rock has light gray lines, different from its original dark gray. This color change indicates weathering. What could have happened to this rock to have caused this weathering?

How do you know this rock has been weathered? What kind of rock do you think it is?

Erosion works alongside weathering. Erosion is when rocks are displaced by water, wind, animals, or humans. Rocks affected by weathering stay in their original location and break down from heat and pressure over time. What is the difference between erosion and weathering?

We know that Basalt and Gabbro both formed from magma deep within the earth's crust. How did this magma find it's way to the earth's surface?

Sometimes scientists need to rely on each other to get the full story. We can make predictions based on what we see, but sometimes it's helpful to learn more about what we can't see.

Rifts are when cracks form in the earth's surface. Magma pushes through these cracks to form igneous rocks we have seen like Basalt and Gabbro. These rifts can create valleys or even new oceans. Rifts can sometimes fail and not form fully. A failed rift can create a valley. A successful rift creates new oceans. The Red Sea is experiencing a rift. Two parts of Africa are pushing apart to create a larger Red Sea. This rift has been happening for millions of years. Do you think the Fells are a successful rift or a failed rift?

What happened to the Fells millions of years ago?

Why is this rock smooth?

Some rocks were smoothed over by glaciers that came down from Canada 20,000 years ago. These glaciers glided over the Fells to smoothen out the tops of rocks like this one. Here is another example of a rock that was smoothed over.

Felsite is a light colored and fine grained rock that came from magma and erupted when it reached the earth's surface. Felsite is resistant towards weathering and hard to break apart. Felsite is chemically different from basalt and gabbro. Compare felsite, basalt, and gabbro.

Why is this rock lighter than the other rocks?

Geoscientists sometimes work to determine the ages of rocks. Think about basalt, gabbro, and felsite. Which one was here first? Keep thinking about this question as we finish our hike.

We can see all of Boston from up here! Let's take a closer look at the cliffs from the highway.

Notice the different colors on these cliffs. What do these colors remind you of? Can you name the two rocks in the cliff?

Why are the cliffs two different colors?

This is a dike from another part of the Fells. What kind of dike is shown here?

A Dike is when magma intrudes through another rock creating a vertical line. This dike is a dark color. The dark colored stripe is gabbro and the light colored rocks surrounding it is felsite. Dikes form from the earth moving during a rift. This is a gabbro dike. The main dike in the Fells spans several miles, but smaller dikes can be found during the hike and on the cliffs like this one.

Which rock was here first? The gabbro, the basalt, or the felsite? Which rock is younger and why?

The felsite was here first! We can see how the gabbro intrudes through the felsite when we look at the dike from the highway. We also know that felsite does not weather easily unlike gabbro and basalt. The felsite has lasted millions of years even through the rift when the gabbro pushed to the earth's surface.