ice age logic

Bunch of guys in Maine go ice fishing, and one invites a geology professor from the college. They are standing around the hole they’ve cut, and one says the ice must be two feet thick. The geology professor tells them, “You know, back in the ‘glacier days,’ the ice here was more than a mile high.” They look up and down, think it over, and finally one says, “A mile high, huh?.. But just over the lake, right?”