Wanderlust World Wanderlust World When a stream exits a narrow mountain valley onto a flat plain, it slows and spreads, dropping its sediment load in a fan-shaped deposit called an alluvial fan. Composed of sand, gravel, and boulders, these fans can extend kilometers from the mountain front. They are common in arid/semi-arid regions (e.g., Death Valley, Himalayan foothills) and record past flood events.