The investigation and explanation of landforms and geomorphological processes in drylands. Tooth (2009) PPG 33, 2 identifies three emerging research themes that highlight the growing links between arid geomorphology and other disciplines: the role of fire in arid geomorphological systems; arid fluvial sedimentary systems, characterized by investigations that commonly focus on processes, landforms, and sedimentary products at larger spatial scales and longer timescales; and arid geomorphology on Mars.