An Alluvial Fan is sort of a triangle shaped load of sand and minerals that moves throughout other areas nearby the fan.  An Alluvial Fan is usually at the bottom of a canyon or mountain, it is where a deposit of different soils is deposited near a stream or river.  Sand from the Whitewater Preserve acts as an Alluvial Fan for the Coachella Valley and brings the loose and fine sand all over the Coachella Valley.  There is also an Alluvial Fan near the Tramway in the Coachella Valley. What happens to make this loose sand is a lot of erosion and breaking apart because of wind and water.  This makes mountains become boulders and boulders become gravel and gravel become fine sand.