The surface of Iowa, and of the neighboring states north and east as well, is strewn with boulders of granite, diabase, porphyry, and other igneous rocks in various stages of decomposition, and of all sizes, from a few inches to 50 feet or more in diameter. These are all more or less rounded or sub-spherical, though often flattened. They have all been transported by ice from the north. When traced back to their original ledges the latter are usually found to be very smooth and fresh, with practically no indications of decay.
Proceedings of the Iowa Academy of Sciences
©1894 Iowa Academy of Science, Inc.
Keyes, Charles Rollin
"Secular Decay of Granitic Rocks,"
Proceedings of the Iowa Academy of Science, 2(1), 27-31.
Available at: https://scholarworks.uni.edu/pias/vol2/iss1/9