During the glacier’s southward advance, already existing loose rocks became buried in the ice mass or hitched a ride on the surface and were transported far from their original location (typically 50 to 200 miles). The melting glacier then left behind these hitch-hikers called erratics. The one you are looking at probably came from New Hampshire. Geologists decide a rock is an erratic when its composition is dissimilar to nearby bedrock but is similar to that found north of the area. The biggest erratic, 23’X37’X85’ in size, is located in Madison, NH. Read more.

Now look around you and you will actually see hundreds of rocks and most are likely erratics. They are everywhere! But particularly interesting erratics are ones that were laid down on top of a local rock, and at once we have a balance rock. While there are no balance rocks in the basin, you might visit one in Holliston, MA or numerous other places to see more outstanding examples.

Glacial erratics were the bane of the earliest settlers as they struggled to remove them from fields urgently needed for crops, using them to build stone walls at the edges of pastures. So it is the erratic that defines the quintessential New England landscape that attracts visitors from around the world.

This is an excellent opportunity for you to get a taste of the investigative methods used by scientists to identify land features. Suppose you are the glaciologist and have just arrived at this rock. Where did the rock come from? Did it detach from a nearby hill and roll here? Probably not, because there are no elevated features nearby. Did the rock chip off from the local bedrock? There is no evidence of that.

So it is likely that the rock was transported here. But how? Was it moved here by a stream? No, it is much too heavy. Of course you know that a glacier covered the area and could have supplied a force powerful enough to move this rock. So now you suspect it is an erratic.

If you had the geological and chemical analytical tools of the scientist, you could have confirmed your suspicion. In your own explorations, put on the scientist’s hat and see what glacial features you can identify that you will see on your walk.

Stop #3 is about 75 yds. down the trail ahead of you.