Teacher Notes in Yellow

 

    
Rocky Point, at World's End Park in Hingham - teacher guide

While this location is a wonderful place to see different rock types, we have found it difficult to come up with conclusive identifications for some of the rocks found here. Different sources use different terms, such as "volcanic conglomerate" or "agglomerate" or "tuff" to describe the same type of rock. Instead of getting hung up on the exact identification, we have found it useful to try to focus on the general origin of the rock as one could infer from studying them.

This "Field Trip" and t highlights the fact that, while the Boston area is very geologically inactive at present, this was not always the case. For much of its history, the Boston area was next to tremendous and violent geologic activity. These rocks formed in the late Precambrian - probably between 550 and 570 million years ago. This was a period of time when many volcanoes were found in our area.

This site (and others in Nantasket Beach, Marblehead, Dane Park and the Blue Hills) are good places to discuss the different types of volcanic rocks. The rocks below are extrusive rocks. These rocks, formed on the surface of the earth. The rocks here formed from volcanic ash. These rocks lie on top of Dedham Granite, which are formed deep below the surface of the earth - probably thousands of feet down. Yet, how is it possible that all of these rocks appear, currently, exposed at the same altitude? Discuss how uplift, erosion, faulting can alter landscapes to make this possible.

Rocky Point is in the World's End Park in Hingham. This park is really one of the most beautiful hiking places in the Boston area. Rocky Point has a great deal of exposed rock, showing several layers of volcanic rock on top of one another.

View along the North side of Rocky Point, showing the variety of
exposed rocks visible

View along the South side of Rocky Point, looking across
the Wier River towards Hull

 
Below you see a picture of an outcropping found in the middle of a forest. Can you guess which type of rock this is? What evidence would you use? What more information would you need to identify the rock? This is from the same outcropping - a small chip was removed. Can you identify the rock now?
The point here is to highlight the fact that most rocks in our area are highly weathered which makes identification almost impossible without breaking through the surface. Only by looking inside can we do any kind of meaningful identification. In fact, this rock is basalt, which is easily shown in the picture on the right (where a chip of rock had fallen off recently).

In the picture below, you'll see two different types of rock on either side of a basalt dike. What can you say about which rock erodes most quickly?

In this picture, you'll notice three distinct types of volcanic rocks. The picture looks down upon an exposed cliff - the dark basalt is on the bottom, then a type of green volcanic "conglomerate", and then a lighter grey volcanic "conglomerate". Try to draw a picture showing how this outcropping was formed.

Students should be able to see that the basalt weathered more quickly than the other rocks. It's common in our area to see, at the seashore, steep cuts through rock where a basalt dike had formed and eroded away - while the walls (of a different, harder rock) remain. The point here is to get students to think about time sequence in an outcropping like this. Some possibilities include: basalt forming and two different volcanic events happening later, or the volcanic rock forming and the basalt intruding later.
The next two pictures show two different types of volcanic "conglomerate" - rock formed by cemented volcanic ash that includes bits of other rock. The picture on the left is contains amygdules - bubbles of gas that leave behind mineral deposits. The picture on the right contains pieces of basalt. While both of these types of rock are formed from ash, their composition and appearance are very different. Can you describe how these two rocks formed differently?
In the two pictures below you can see giant pieces of rock that got trapped in the ash flow before it all solidified into rock. Can you describe how this process might have happened?
In the pictures below, you see a series of volcanic rocks. Notice that there are at least three different types of rock here - black, green and tan. Notice also that layering that has occured. Try to describe the process that led to these different rocks being formed together.
This may be a hard exercise for students, but again, the point here is to get students thinking about time sequence. All these layers are probably created by different types of volcanic ash flow. Some (such as the bottom and top) are probably a "conglomerate", which includes bits of other rock in the ash flow, and the whitish layers are probably a type of "tuff" consisting of layers of very fine-grained ash.
Here are two rocks that look similar, yet formed very differently. One of them is the Roxbury Conglomerate, which formed by weathering and erosion of sand and pebbles being deposited in a shallow waterway. The other is a form of volcanic conglomerate, in which small pebbles got caught up in an ash flow, which eventually hardened together into one rock. Can you figure out which is which? What clues did you use to decide?
The rock on the left is Roxbury Conglomerate, the right is the volcanic conglomerate.