Engineers initially believed that the bluff where the tunnel was to be constructed was made of solid rock because a thick layer of limestone was found on top of the hill. However, after work began, the "solid rock" was found to be mostly hard blue shale. While solid when first exposed, the rock rapidly decomposed when exposed to air and water. The shale can not be worked with picks, and on which blasting has little effect. The tunnel, will be the longest tunnel in the state, and as the blue shale when exposed to the air becomes a rotten shale rock, which crumbles to pieces, the tunnel will have to be strongly arched from one end to the other. The Dubuque Daily Times of August 31, 1887 reported "The cost of the tunnel will probably exceed the cost of grading fifty miles of any part of the road."