in Thoreau’s Journal:

I am surprised at the sudden change in the Walden ice with five days.  In cutting a hole now, instead of hard, dry, transparent chips of ice, you make a fine white snow, very damp and adhering together, with but few chips in it. 

The ice has been affected throughout its twenty-six inches, though most, I should say above.  Hard to say exactly where the ice begins, under the two inches of snow.