English: Glacial grooves on gabbro in the Precambrian of Minnesota, USA.

Glaciers are rivers of ice. Ice is a mineral (H2O). Glacial ice is a rock (technically, a metamorphic rock). Despite being solid, ice does flow under certain conditions at the Earth’s surface. Occasionally, Earth experiences Ice Ages, during which extensive ice sheets cover and move over significant portions of the Earth’s surface. As ice moves over landmasses, it erodes underlying rocks and picks up small to large pieces of debris. This debris accumulates at the base of the ice sheet and scrapes bedrock as the glacier moves, resulting in glacial scratches (glacial striations) (= thin scratch lines on rock) and glacial grooves (= large channels incised in rock).

This outcrop is glacially grooved gabbro, which is a mafic, phaneritic, intrusive igneous rock composed principally of plagioclase feldspar and pyroxene. This rock is part of the widespread Duluth Complex, a ~1.1 billion year old suite of igneous rocks in northeastern Minnesota that is associated with volcanism in the Lake Superior segment of the ancient Mid-Continent Rift System.

The now-weathered glacial grooves here were formed during the Pleistocene Ice Age.

Geologic unit & age: troctolitic series, Duluth Complex, late Mesoproterozoic, 1099 Ma

Locality: roadcut next to parking lot for the Thomson Hill Information Center (= rest stop) along Skyline Parkway, northern side of Interstate 35, just west of the Rt. 2-Interstate 35 intersection, west of Duluth, Minnesota, USA. (46° 43’ 46.14” North, 92° 12’ 25.27” West)