It was placed in the Clam Lake area because the granite bed rock was the best to transmit the ELF signals. The Lines for the station covered many miles.
It makes sense, and there is granite, but but from my 30 years ago recollection the area was covered with glacial deposits, glacial till. 75 or 100 miles north —Northern Michigan, Hurley, Iron Mountain Wisconsin—and the rock is at the surface. Of course, its not like I went back into the testing area where they were doing this. It might have been exposed. I was just driving past on my way somewhere else.