Alltrikes

A pilot friend of mine pointed out an interesting "loophole."

The inner ring of the class C goes from the "surface" of the airport to approximately 4000AGL (rounded up to nearest 100 ft).   The Airport is up on a mesa, next to a river valley.  The river passes through the Class C inner ring, but it over 400 ft below the airport.   Is it legal to fly in this swath of airspace, below the Class C inner ring without contacting the tower?  The river valley is in  a 1/3 mile swath of empty flood plain, with no buildings.

This came up because I saw a plane flying very low over the river last weekend (low for a GA, about 200AGL) just outside the inner ring, but headed in that direction.  We were trying to figure out what he was up to.