Where did all of these rocks come from?

 Background: 

Between 25,000 and 20,000 years ago a large ice sheet moved down from around Hudson's Bay in Canada and covered all of New England, moving as far south in this part of North America as Long Island. This ice was as much as a mile thick, so was very heavy. As it moved, it scraped up and froze into the ice any loose rock or smaller material it encountered. These rocks at the very bottom of the ice smoothed and sometimes scratched the underlying bedrock. The scratches, called glacial striations, help glaciologists to determine the exact directions of ice movement at a particular location. 

When the climate warmed, the ice began to melt. Just like with plowed snow piles along the streets in winter, whatever was mixed in with the ice was left in place as the ice melted. You have already heard about these deposits -- till and stratified drift -- in the Sleeping Giant exercise. In the till, all sizes of materials are found. Over time, especially on slopes, rainwater has carried away the finer particles, usually to be deposited in stream channels as alluvium. This leaves behind the larger boulders, called erratics. 

Figure 1. Erratics along one of the trails in the woods at Bluff Point State Park.

 Erratics are usually more rounded than the plucked rocks you saw at Chatfield Hollow, because as the ice carries them along, they rub against other rocks and lose their sharp edges. 

Objectives: 

In this woods and beach exercise you will learn: 

1.      What erratics are and what they look like. 

2.      How to tell local from more distant erratics.

 Procedure: 

Begin this exercise by examining the bedrock along the shore to the east of the beach (to your left when you are facing the water at the beach).

Figure 2. A large erratic, Split Rock, on the rocky beach of Bluff Point.

1.      Describe the bedrock found in the outcrop along the beach, including grain size, mineral content, color, texture, etc. 

2.      Walk along the trail which go into the woods from the bedrock point. Watch for loose rocks in the woods. Examine as many of them as possible. Keep a record of what you find in the table below. 

3.      Discuss the percentage of local erratics versus the number of non-local ones. Is there any difference in the shape and roundness between the two types? 

4.      What does this tell you about the distance ice generally transports rocks, at least the larger ones? 

5.   The ice moved from north to south. Do you think any of the rocks in the woods came from the rock making up the beach outcrop? Explain.

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