Much of what became the Mendenhall Valley was under glacial ice or ocean water. Since then, glaciers, weather, silt, earthquakes and human hands have changed the land. Glaciers carve valleys and push up stony ridges called morraines. They also move large boulders called erratics. Ice gradually fragments erratics into smaller rocks.
Rain carves out creeks and riverbeds, washing silt to larger bodies of water.
And then the process may begin all over again a few thousand years later. Downtown Juneau is built on two land forms - an alluvial fan created by Gold Creek, and an ancient delta, said Kathy Hocker, a naturalist with Juneau's Discovery Foundation. The delta was created 20,000 years ago during the last ice age. The alluvial fan has built up atop the delta since then, in a mere blink of geologic time. Downtown Juneau and other parts of town have also lifted as a result of post-glacial rebound.