The Arnold Arboretum shows lots of evidence of glaciation - the process by which glaciers alter the landscape. What's interesting about this location is that you can see evicence of relatively recent glaciers (~22,000 years ago) and very ancient glaciers (~600 million years ago).
The Arboretum has several hills, of which Peters Hill (on the south side of the park) is the largest. These hills are drumlins. A drumlin is a hill composed of a pile of sand, rocks, gravel, boulders, etc. that have were left behind as glaciers retreated over 16,000 years ago.
Not too far away, at the Chestnut Hill Mall (behind Bloomingdale's), you can see more evidence of the glaciers advanced. They left behind clear scratches in the rock as they slowly advanced. We think of glaciers as having moved south as they covered Massachusetts, and in general, they do. You'll notice that the surface has two sets of scratches almost perpendicular to each other. Both of these sets of scratches were caused by glaciers as they advanced South. (We call this "glacial scarring".) It is uncommon to find two sets of scratches, since the later glacial event generally wipes out the earlier one.
But wait! How is it possible for two sets of glacial scratches (one earlier than the other) to both be oriented south, if they are perpendicular?? What could have happened here? Hint: think about the difference between the entire ice sheet moving and what individual, much smaller, parts of it might have done.
Can you tell which set of scratches is older? What is your evidence? |