Alluvial fans are common geologic features in basin and range provinces like the one in the American southwest. They’re composed of sand and gravel that has been eroded from mountains, transported by water, and then deposited in enormous fan-like shapes on more level ground. Some of the most well-known are in Death Valley National Park, and I got to see some of them up close on an undergraduate geology trip last winter.
To understand their formation, it’s essential to understand the tectonic forces involved in basin and range creation. These regions are the result of extensional forces that stretch the earth’s crust over a wide area. As the crust is pulled apart, it fractures and forms normal faults:
Water from snow melt and rain storms carries sand and gravel down the sides of the horsts, slowly carving canyons into the mountains.
When this sediment-laden water exits the canyons, it reaches much more level and wide terrain.
The water velocity drops, and it releases its sediment load. Sand and gravel are deposited in layers with each rainstorm.
Over time this repeated deposition leads to the formation of giant alluvial fans.