aka Dave Hoover
That Westslope could have come down from a lake, but also could have come up from the SF. The far upper reach of the SF is dominated by Westslopes.
I doubt that it would have come the SF. The Westslopes on SF are only way up high on it with too many Falls in the way. As you and Joe suggest, I would imagine it likely came from one of the Lakes feeding the MF.