Geologic units in Eau Claire county, Wisconsin

Cambrian, undivided (Cambrian) at surface, covers 93 % of this area

Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations

Gneiss, migmatite, and amphibolite (Late Archean) at surface, covers 4 % of this area

Quartzofeldspathic gneiss and less abundant amphibolite and migmatite. Includes granitoid rocks. Granitic gneiss at Port Edwards, WI on Wisconsin River has a U-Pb zircon age of 2870 +/- 13 Ma, and gneiss at Jim Falls in Chippewa River valley has a U-Pb zircon age of 2522 +/- 22 Ma (Sims and others, 1989)

Layered metagabbro (Late Archean) at surface, covers 2 % of this area

Interlayered gabbro, mafic accumulates, gabbroic anorthosite and anorthosite (Cummings, 1984) in Eau Claire River (T. 27 N., R. 8 W.). Age uncertain, but interpreted as Archean (Sims, 1990).

Volcanic rocks, undivided (Early Proterozoic) at surface, covers 0.4 % of this area

Mafic to felsic flows, pyroclastic rocks, impure quartzite, and conglomerate in Eau Claire River, Eau Claire and northern Clark Counties. Rhyolite has zircon ages of 1,858 +/- 5 Ma. Possibly correlative with Milladore Volcanic Complex.