Glacial till is formed when a glacier carries materials such as boulders, gravel, sand and clay from one area and deposits them in another area. Material carried in the glacier's base and deposited under it is called basal till, while material carried on or near the glacier's surface and deposited when the glacier melts is called ablation till.
Because a glacier carries sediment in its ice that does not undergo the smoothing process of water transport, the stones in glacial till are typically sharp and jagged. However, as the glacier moves, large objects embedded in the base grind and scrape on the rocky surface of the land, creating a fine sand called rock flour. The glacial sediment also produces scratches in the underlying rock called striations and shiny surfaces called glacial polish.
The sediment the glacier carries is called glacial drift, and the mixture it deposits is called till. Unlike river sediment, which forms patterns depending on the size and weight of materials, glacial till is unsorted, with all the sizes and types of sediment mixed together. When till creates a mound or ridge, it is known as a moraine. Moraines formed along the sides of alpine glaciers are called lateral moraines, and when lateral moraines merge at the convergence of two glaciers, the result is a medial moraine. The moraine at the leading edge of a glacier is called a terminal moraine. When a glacier is steadily retreating, the till it leaves behind in a layer on the ground is known as a ground moraine.