Jackie Bennett Is Curious About This Rock Formation On The Back Shore

I wonder if anyone can help me with this rock formation on the Back Shore.  What is the black in the granite.  It looks like it was poured in.

4 thoughts on “Jackie Bennett Is Curious About This Rock Formation On The Back Shore

  1. That is a basalt dike. The basic idea is that this magma like fine-grained rock was injected into and expanded a crack in pre existing granite.

    But I geologist could tell you more!

    Like

    1. Thank you so much. I have noticed it along the shore before but I was fascinated by the way it looked like it was flowing down the crack in the granite. Jackie

      Like

      1. Yes, it’s a dike of, probably, diabase (a basalt relative). If you can find a copy of Martin Ross’s excellent “Cape Ann: Its Physical and Environmental Geology” he explains the many dikes that cut through Cape Ann granite.

        If this is the one I think it is, you can also see it over on Andrew’s Point. Yes, it’s the same structure across the water.

        Like

Leaving a comment rewards the author of this post- add to the discussion here-

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s