Rocks on the Earth's surface are continually broken down by the forces of flowing water, chemical reactions, blowing wind and crushing ice. This broken rock is eventually washed as sediment into the ocean. The net effect of this erosion is to lower the surface of the continents to sea level. From the point of view of geological time, weathering and erosion are comparatively rapid. In the course of just 18 million years, the continents would be reduced to sea level and the oceans would cover the Earth.