File:Glacial striations (Straight Lake West roadcut, north of Temagami, Ontario, Canada) (40859886953).jpg

Glacially-smoothed & striated tillite in the Precambrian of Ontario, Canada.

Tillites are ancient, lithified tills (= unsorted, nonbedded, coarse-grained glacial deposits). This is part of the famous Gowganda Formation, a ~2.3 billion year old deposit in Ontario, Canada representing a very ancient Ice Age. Other glacially-deposited sedimentary rocks of Paleoproterozoic age have been reported from Wyoming, Finland, India, southern Africa, and Australia. The glaciation was likely a widespread event, and may have persisted for a long time. This ice age is hypothesized to have been triggered by decreasing levels of methane gas (CH4) in the atmosphere as oxygen gas (O2) levels increased (the methane oxidized to carbon dioxide). This lessened the effects of greenhouse warming, and resulted in climate cooling.

This tillite talus sample has been eroded, smoothed and striated by a much more recent Ice Age, during the Pleistocene. As glacial 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).

Stratigraphy: Coleman Member, Gowganda Formation, lower Cobalt Group, Huronian Supergroup, Paleoproterozoic, ~2.3 Ga