Abstract The Milwaukee Formation, a sequence of Givetian (Middle Devonian) marine strata deposited in what is now southeastern Wisconsin, USA, was recognized as preserving an exceptionally diverse biota of invertebrates, placoderms, and plants, when local cement companies mined the rock for the manufacture of hydraulic cement from 1876 to 1911. It is now recognized as containing one of North America's most diverse biotas of its age coming from a single formation, having roughly 250 species, 100 families, 16 phyla, and four kingdoms. Intense collecting has revealed the rich biodiversity that prevailed during the Devonian Period in what is now the Milwaukee area. When the cement mines shut down, the quarries were filled and what was already regarded as a localized series of outcrops became even more restricted. Today the formation is known almost exclusively from limited exposures in Estabrook Park and the adjoining Lincoln Park, in Milwaukee County, in the area once occupied by the cement quarries. These outcrops (the type locality) are protected by a Milwaukee County Park Ordinance that prohibits removing materials from the parks, thus enabling the type locality to remain intact for educational and non-destructive research purposes, while largely unstudied museum collections remain available for ongoing research.

Abstract Paleozoic reefs were first identified by James Hall (1862) in the Silurian rocks of the American Midwest. Scientifically, they were crucial in defining ancient reefs, documenting the early history of this marine ecosystem, and interpreting regional stratigraphy, palaeogeography and palaeoecology. Schoonmaker Reef in Wauwatosa, Wisconsin, is historically the most significant of these structures, as the first to be correctly identified as a ‘coral’ reef by exhibiting the transition between a reef mound and the surrounding seafloor. After nearly two hundred years of quarrying, Thornton Reef in Thornton, Illinois, is next in importance as it displays an unequalled three-dimensional exposure, demonstrating the impact of Silurian climate change on reef development. Silurian reefs were discovered and studied only through stone industry quarrying. Extensive nineteenth-century fossil collecting by local naturalists was also a key component in reef studies. These reefs are also an important part of the region's geoheritage, comprising elements of its economy, history, education and scenic landscape. Recognizing their scientific and historical importance, some reefs were part of geoconservation projects and are protected sites. However, other reefs and irreplaceable reef fossil collections need further initiatives to preserve the scientific value and historical legacy of these relatively small geological features.