Glaciers & Ice Ages.

Glacial erosion is already covered as a short addendum near the Rock Cycle — but that only explains what a glacier does to rock, not what a glacier actually is or why Earth has gone through entire ages defined by them. Worth telling the fuller story.

A glacier forms wherever more snow falls each year than melts. Over years and decades, undmelted snow piles up, and the sheer weight of new snow on top compresses the layers beneath into denser and denser ice — not by freezing liquid water, but by squeezing the air out of packed snow until it becomes solid glacial ice. Once that ice mass gets thick and heavy enough, gravity takes over and it begins to flow — slowly, sometimes just inches a day, but flow all the same, which is exactly what separates a glacier from a stationary block of ice.

There are two broad kinds. Alpine (valley) glaciers form in mountainous terrain and flow downhill through existing valleys, carving them into the broad U-shape described in the Glacial Erosion section. Continental ice sheets are vastly larger — thick enough to bury entire landscapes, mountains and all, flowing outward from their own weight rather than following existing terrain. Antarctica and Greenland carry the only two continental ice sheets left on Earth today, but during past ice ages, ice sheets like these covered enormous portions of North America and Europe.

An ice age isn't one single event — it's a long period during which Earth's climate runs cold enough for large ice sheets to persist, punctuated by colder glacial periods and warmer interglacial periods within it. We're actually still technically living inside an ice age right now (Antarctica and Greenland's ice sheets are the evidence), just in one of its warmer interglacial stretches. The most recent glacial period peaked around 20,000 years ago, when ice sheets extended across much of Canada and the northern United States — and its effects are still visible today, well beyond glacially-carved valleys: the Great Lakes themselves were carved out by that ice sheet's movement, and much of the flat, fertile farmland across the American Midwest sits on sediment those glaciers ground up and deposited as they eventually retreated.