THE HYPOTHESIS: Geologic materials exposed at or near the earth's surface undergo continual alteration and wearing away (erosion) due to interaction with the surface environment (air, water, snow, ice, living organisms, etc.). Materials vary in their resistance to this attack; some areas are worn down more rapidly than others. More specifically, valleys or low areas develop in regions underlain by non-resistant materials, where the wearing away is relatively rapid; adjacent regions, underlain by more resistant materials that wear away less rapidly, are left behind as hills or higher areas. The variation in elevation that results is said to be due to 'differential erosion'. One or more major landforms in the NYC region are due to differential erosion.