FIRST GEOLOGICAL MAP OF MILWAUKEE AND ITS ROLE IN THE DISCOVERY OF FOSSIL REEFS

In 1859, Thomas J. Hale created an unpublished map and cross-section depicting the bedrock geology of Milwaukee, Wisconsin, under the direction of James Hall. Preserved in the Hall Papers in the New York State Archives, this map, entitled “Geologic Map & Section of Milwaukee and Vicinity”, covers approximately the same area represented in modern editions of the U.S.G.S. Milwaukee 7.5 min topographic map. During the 1850s, geologic maps for this region, at any scale, were rarely published. Most covered an entire state or territory, featuring a continuous portrayal of general stratigraphic units found at the bedrock surface. Hale’s map differs in that he was able to accurately portray the location and character of individual rock exposures and quarries by focusing on a small, but important, area. Although there were few bedrock exposures in the Milwaukee area then, the map is reasonably accurate when compared with more modern maps that utilize extensive, more recently available subsurface information.

Besides representing what could be considered to be the first example of a “7.5 min” geologic map from the region, Hale’s map provides critical information on the discovery of Silurian reefs in the Milwaukee area — the first ancient reefs correctly identified in North America and possibly the world (two are now National Historic Landmarks). During the 1840s and early 1850s, local naturalist Increase A. Lapham began collecting fossils at Milwaukee area sites and, with the help of James Hall, established an accurate comparison with the New York section. In this work, Lapham discovered highly fossiliferous localized rock hills, which he thought were remnants of an eroded rock unit. Hall visited Lapham in Milwaukee on several occasions in this time period, including an 1859 trip when he was State Geologist of Wisconsin. In 1862, Hall published his research on the area, proposing that the Lapham’s rock hills were Silurian coral reefs as shown by sedimentlolgic and paleontologic evidence. Hall’s notes from this work appear to be lost, but those of his assistant Hale (1859-1862) are preserved in the Hall papers. In addition to his 1859 map, Hale’s detailed sketches and notes of facies changes at the critical Schoonmaker Reef (Wauwatosa) provide important information about the evidence used to make the reef interpretation for these rocks.