Ridges of rock debris formed by a moving glacier are called moraines. Moraines are visible when a glacier retreats but are often destroyed when the glacier continues its advance. The rocks in a moraine are a jumble of clay, silt, sand, gravel and boulders.
Terminal moraines occur at the terminal end of an advancing glacier and help anchor the glacier's ice. Glaciers may also gather rocks for lateral moraines along the sides from valley walls. When a tributary glacier joins the main flow, a medial moraine may form in the center of the glacier. Medial moraines form swirls and loops of dark material.