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).
Glacial grooves on Columbus Limestone (lower Middle Devonian) from Kelleys’ Island, western Lake Erie, USA (public display, Field Museum of Natural History, Chicago, Illinois, USA).
Glacial striations (glacial scratches) & glacial polish (shiny areas) on limestone from Lemont, Illinois, USA (FMNH G 675, Field Museum of Natural History, Chicago, Illinois, USA).
Glacial striations (glacial scratches) on Columbus Limestone (Middle Devonian). The glacial scratching occurred during the Wisconsinan Glaciation (late Late Pleistocene). ~4.7 cm across.
Locality: quarry ~2.4 miles east of Watkins, immediately northeast of intersection of Watkins Road & State Road, eastern Mill Creek Township, southeastern Union County, central Ohio, USA.
Collected 9 April 1967 by Lee St. John.
Glacial grooves in Columbus Limestone (lower Middle Devonian) at Glacial Grooves State Park, northwestern Kelleys’ Island, western Lake Erie, USA. These spectacular, world-class glacial grooves were carved out during the Pleistocene (the last Ice Age).