As used in hydrology , this term refers to rock and soil material that occurs at the earth's surface or within the subsurface which does not permit water to enter or move through them in any perceptible amounts. These materials normally have small-sized pores or have pores that have become clogged (sealed) which severely restrict water entry and movement. At the ground surface, rock outcrops, road surfaces, or soil surfaces that have been severely compacted would be considered impervious. These areas shed rainfall easily, causing overland flow or surface runoff which pick up and transport soil particles and cause excessive soil erosion . Soils or geologic strata beneath the earth's surface are considered impervious, or impermeable, if the size of the pores is small and/or if the pores are not connected.