Cap au Gres is a name that was given by the early French voyageurs to a prominent sandstone headland rising from the east side of the Mississippi River a dozen miles above the mouth of the Illinois. The point is of special interest geologically on account of having, side by side, beds of the earliest and latest Paleozoic. The sandstone is Cambrian in age and the contiguous limestone middle Carboniferous. The cliff marks the position of the most profound dislocation, or fault, known in the Mississippi Valley. Near the line of the slip the horizontal strata are abruptly bent upward at high angles, as much as 80 degrees, immediately against the fault plane. (See plates I and II). Hence it is that within the very short horizontal distance of less than a mile the greater part of the entire Paleozoic section of the region is well displayed.
Proceedings of the Iowa Academy of Sciences
©1897 Iowa Academy of Science, Inc.
Keyes, Charles R.
"Some Geological Formations of the Cap-au-Gres Uplift,"
Proceedings of the Iowa Academy of Science, 5(1), 58-63.
Available at: https://scholarworks.uni.edu/pias/vol5/iss1/12