in Thoreau’s Journal:

I see along the sides of the river 2 to 4 inches above the surface—but all at one level clear drop shaped crystals of ice—either held up by some twig—or hanging by a dead vine of climbing mikania— They are the remains of a thin sheet of ice, which melted as the river went down & in drops formed around & ran down these cores—& again froze & being thicker than the surrounding ice have outlasted it.