Teacher Notes in Yellow

This short "field trip" highlights a unique conglomerate formation. Very large boulders of granodiorite are found embedded within the conglomerate. The lesson here would be to challenge students to imagine what the conditions were like as the deposits that make up this rock were eroded. These rocks were formed in late Precambrian times (probably around 570 million years ago). The landscape was probably quite mountainous as granodiorite is an igneous rock formed underneath volcanoes. Since these are large pieces, there had to have been very strong rivers or streams carrying these boulders down from the mountains. Conglomerates like these form in shallow waterways - only finer-grained sediments could have been carried out to deeper waters.

We suggest having students study this site in conjunction with the Chestnut Hill and Blue Hills sites to compare different conglomerate formations.

Another interesting conversation to have with your students is about the classification of rocks. For example, why are the embedded boulders called "granodiorite" and not "granite" or "diorite". This is a good time to discuss the difficulty of classifying rocks and the fact that rock types are really a sort of spectrum - one rock type grades into another. The "granodiorite" here is truly "on the fence" - darker than granite but not quite diorite either!

Below are pictures of some of the exposed conglomerate. Notice how large some of the boulders are that are embedded in the conglomerate - some are over a foot long. What types of conditions would create rocks like these?

Can you draw a picture that shows the type of environment under which the sediments that formed these rocks were deposited?