Reading the rock record
Along the northern shore of Lake Huron, a thick sequence of ancient sedimentary rocks called the Huronian Supergroup holds a dramatic climate story. Within this group, the Gowganda Formation, dated to roughly 2.3 billion years ago, contains some of the most compelling evidence for the planet's oldest and longest ice age. This period of glaciation occurred during the Paleoproterozoic Era, lasting from about 2.4 to 2.1 billion years ago.
The tell-tale signs are preserved in the stone itself. The Gowganda Formation is characterized by diamictites—a type of rock with a chaotic mixture of large granite boulders and small pebbles suspended in a fine-grained matrix of mud. This jumble of unsorted debris is a classic signature of glacial till, the material left behind when a glacier melts and drops everything it was carrying. Geologists call this lithified glacial till a "tillite". Further evidence comes from dropstones, which are isolated boulders found in finely layered lake or sea sediments. These rocks were carried out on icebergs and dropped into the mud below as the ice melted, often deforming the soft layers beneath them. Some outcrops even preserve varves, the distinct annual layers of sediment that form in glacial lakes.