The city is located in the Southeastern Wisconsin glacial till plains that were created by the Wisconsin glaciation during the most recent ice age. The soil is clayey glacial till with a thin layer of loess on the surface. The city has some limestone deposits, including the Devonian Thiensville formation in north-central Mequon and the Silurian Little Menomonee River Reef District, which contains dolomite marine fossils. The Wisconsin Department of Natural Resources considers the eastern part of the city along the lake to be in the Central Lake Michigan Coastal ecological landscape, while the western part of the city is in the Southern Lake Michigan Coastal ecological landscape.