Teacher Notes in Yellow

 

    

Ringer Park in Allston - teacher guide

This region features rock types of the "Roxbury Formation": Sandstone and Mudstone. The lesson here is to get students to think about the different conditions that would lead to the formation of these three different sedimentary rocks. Understand that there is still consirable uncertainty among geologists about the exact conditions of the formation of these rocks. However, the general environments under which they formed is clear. Students can use this "field trip" to develop a general sense of how sedimentary rocks form.

Because only finer-grained sediments could have been carried out to deep waters, the fine-grained sandstones and mudstones must have formed in deeper water, or under gentle erosional conditions. Students could contrast the formation of these fine-grained sedimentary rocks to the formation of the "Roxbury Conglomerate" found elsewhere in the Boston area. The Conglomerate must have formed under conditions of rapid weathering and erosion - most likely fast moving, powerful streams or rivers carrying rocks and boulders quickly to the bottom of a mountain. Your students can study examples of conglomerate at the "Chestnut Hill", "Arnold Arboretum" and "Blue Hills" field trips.

Students should consider: Why do we see very fine grained mudstone layers alternating with coarser grained sandstone layers? Was this the result of different erosional conditions or changing sea level (so that, for example when the mudstone sediments were deposited the sea was deeper)? Periods of uplift? Periods of intense mountain building (such as volcanism or converging plate boundaries) followed by periods of relative calm? Note that there is no right answer here, but challenging students to consider their evidence when forming their hypothesis is an important exercise.

Near Ringer Park in Allston you can see examples of sedimentary rocks (with volcanic rocks sitting on top). What about this formation tells you that these rocks are probably sedimentary? Look carefully at the next three pictures to answer the question.
 

Look at the layers of sandtone and mudstone below. Notice the white band of quartz(?) running vertically through the rock. Which do you think formed first - the quartz or the sandstone and mudstone layers? What's your evidence?

In the picture below, you'll notice two rather large trees that seem to be growing right out of the rock. How is this possible? Look again at the pictures above. Which layer - mudstone or sandstone - do you think the seeds started to grow in?
Note that the mudstone is a softer rock which weathers more quickly than the sandstone. It would be easier for plant roots to work their way into this rock.

In the picture below, notice the contact between the sedimentary rocks (on the bottom left) and the volcanics (on the upper right).

Notice that the layers of sedimentary rock are almost horizontal. Yet, the volcanics seem to have formed in a way that cuts through the layers of rock. How do you think this happened?

Once again, there is no "right answer" here. The important thing is to get your students to think about possible answers and to support those answers with their observations. Some possibilities include: the sedimentary rocks were eroded along the right side, and then after this erosion the volcanic rock was deposited. Another possibility is that there was some sort of faulting here - one rock was moved in relation to the other.

 

Matching Game!

The three pictures below show three different rocks formed under very different conditions. Yet their appearance is similar.

Try to match the rock with the description of how it was formed.

A. Pebbles and small boulders are deposited, along with sand, at the bottom of a shallow waterway. Over time the rocks and sand get cemented together to form a hard, dense rock.

B. Small chunks of rock are ejected during a volcanic eruption. These chunks are deposited in fine-grained volcanic ash. The whole mixture gets cemented together.

C. Flowing Lava has small gas bubbles in it. As the lava cools rapidly, these bubbles are trapped. These bubbles fill with minerals as the lava cools.

Rock 1

Rock 2

Rock 3

 

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