︎erratic
n.— «When I rounded the a sharp corner, rising from the dry gully I’d dipped into, the stones were less than 20 paces away, resting before me with the candor and natural equipoise of wild animals. In years past, I’d come upon such specimens in ones and twos—erratics, as they are called, boulders once sledded and deposited by glaciers.» —“In the presence of stones” by Charles Finn High Country News Dec. 24, 2007. (source: Double-Tongued Dictionary)
Further reading
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You’re right, it is in a few dictionaries. The adjective-turned-noun was new to me, though, so I recorded it.
Hmmm . . . this seems to be a pretty commonplace word, especially if you’ve taken Grade 8 geography. I’m pretty certain any reputable dictionary lists “erratic” with this meaning.