I think the reason these posts got stalled back in April was that I kept meaning to go back to the geologic maps and do some homework. I still haven't, but it's time to move on.
There is a band of Tertiary bedrock (reddish sandstone) that runs roughly east-west through the southern part of Marrowstone and Indian Islands. It first appears a short distance north of Mystery Bay (on the inner shore of Marrowstone) and is exposed well into the southern part of the harbor.
What I found most impressive was the exposure above the rock seawall on the lead photo. That's till on top, but look at those beautifully cross-bedded, angular sandstone cobbles! I'm surmising that this is the result of the glacier advancing southward across the sandstone surface (but those stones are dipping north) -- this is another of those spots where I wish there had been a better geologist along to help explain this.
AERIAL VIEW
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