Thompson Formation slates were cut by dikes 1.2 billion years ago.  Just downriver from the site here is an exposure of a small anticline formed in the slates.  The amount of lava flows associated with the exposed plumbing system that relates to a hot spot once under present-day Lake Superior (estimated at 480,000 cubic miles) was greater than the amount created by the Deccan Trap.  Some of the dikes show columnar jointing.