Geomorphology is the study of how the Earth’s surface takes shape. Two sets of forces are at work: endogenic forces from inside the Earth build relief (mountains, plateaus, rift valleys), and exogenic forces at the surface wear it down (weathering, and erosion by rivers, glaciers, wind and waves).

What UPSC asks

Prelims: seismic waves and discontinuities, plate boundaries and examples, rock types, and the matching of landforms to the agent that made them (a delta with rivers, a drumlin with glaciers, a barchan with wind).
Mains GS I: explaining a distribution or a process — why fold mountains lie along plate margins, how plate tectonics explains earthquakes and volcanoes, how the Himalaya formed.

Key sub-topics

Theme	Topics
Interior of the Earth	Crust, mantle, core; seismic waves; shadow zones; discontinuities
Mobile Earth	Continental drift, sea-floor spreading, plate tectonics, plate boundaries
Rocks and minerals	Igneous, sedimentary, metamorphic; the rock cycle
Surface processes	Weathering, mass movement, erosion and deposition
Landforms	Fluvial, glacial, aeolian, karst and coastal landforms

Start with Interior of the Earth and Plate Tectonics, then Landforms and Agents of Erosion.