Much of what we call “sense of place” is grounded in the physical nature of the land itself, and in its history. With that in mind, consider this: Chewonki Neck (along with adjacent Oak Island and part of the lower east side of Westport Island) is essentially unique, geologically speaking, in Mid-Coast Maine and perhaps the entire State. In this Natural History Mystery, we will take an expedition to The Point, the southernmost peninsula on Chewonki Neck, to discover why two very different rock formations lie right next to each other, splitting Chewonki Neck in two!

Imagine you are standing on The Point. Montsweag Creek is to our east (right), Montsweag Bay opens to the west (left) with Westport Island beyond, and Oak Island lies straight ahead (due south). Notice the bedrock on which we’re standing: solid, blocky and mainly pinkish gray in color with darker streaks. Although it has basically the same mineral composition as granite, this rock technically is not a type of granite. It is called Oak Island Gneiss. 

If you look closely you can see how the pinkish Oak Island Gneiss comes into swirling contact with a very different dark rock that seems to form layers set on edge vertically. This rock represents the Cape Elizabeth Formation, which has a mineral composition quite different from Oak Island Gneiss. Amazingly, Chewonki Neck is divided lengthwise almost in half by these two rock units; Oak Island Gneiss making up the east side, and Cape Elizabeth rock making up the west. Moreover, the two rock types are not only different in composition but also in age, and in their very geologic origin. The Cape Elizabeth rock is roughly 450 million years old, and is about 50 million years older than the Oak Island Gneiss. 

What a geological mystery! How did such different kinds of rock come to lie right next to one another, right down the middle of Chewonki Neck like this? What has happened geologically to create this special place in Maine?

Here’s a Hint:

To solve this geological mystery, think about this question:

  • Why is it significant to know that Oak Island Gneiss has a mineral composition close to granite, and Cape Elizabeth rock doesn’t?

For Further Thought:

 For the most part, the bedrock of Maine contains few fossil remains, whereas in adjacent states and Canadian provinces fossils are fairly common. Why might this be so?

Submit Your Natural History Mystery Answer!

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