William H. Keating (1799–1840) served as mineralogist on Major Stephen H. Long’s 1823 expedition to the source of the St. Peter’s (Minnesota) River, concluding, on the basis of grain shape, that the St. Peter Sandstone, at what was later to be its type section, Fort Snelling, in the state of Minnesota, was a chemical precipitate from seawater. This appears to be an echo of the Neptunist teachings of Abraham Gottlob Werner (1749–1817), as interpreted in the American Midwest. While endorsed by prominent geologist James Hall and others, and given some plausibility by the analogy of siliceous sinter depositing from hot springs in Iceland, the ‘purity’ criterion used by supporters of the theory was found fallacious with further advances in sedimentology such as those by Charles L. Dake and George A. Thiel. The word ‘purity,’ which was industrial-commercial parlance, merged into the concept of ‘maturity’ in the ternary sandstone classifications of the 1940s. Because the St. Peter Sandstone appears so featureless, actors projected their latent biases onto the blank walls of the sandstone outcrops.