How do mountain wind patterns help create desert sand seas?

Large dune seas, or ergs, often depend on both wind and a steady supply of loose sand. Mountains can help by producing sediment through erosion, and by shaping regional airflow so winds become strong and persistent in certain corridors and basins. Once fine sand reaches an arid basin, repeated wind action can sort it into dunes that cover vast areas. Some desert dune fields are partly fed by sediments carried from mountain ranges far away. In this way, mountain geology can help create the raw material for huge desert sand seas.