The subfield of geography and geology concerned with the study of the origins, evolutions, form, and spatial distribution of the earth's landforms is

The answer is - Geomorphology.
Geomorpholgy is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological procesess operating at the surface or near the surface of the Earth. Geomorphologists seek to understand why are the landscapes the why they are, to understand the dynamics and history of the landforms, as well aspredict the changes through a combination of field observations,numerical modeling and physical experiments.

I'm pretty sure it's c. extremely gradual but not 100% sure though.