About 2–3 Ma, in the Pleistocene, continental ice sheets expanded from the polar regions of the globe to cover lower latitudes far north of the region, starting a series of cold glacial periods that were interrupted by warmer interglacial periods. Snowmelt from alpine glaciers on the nearby Sierra Nevada during glacial periods fed rivers that flowed into the valleys of the region year round. Since the topography of the Basin and Range region was largely formed by faulting, not by river erosion, many of the basins have no outlets, meaning they will fill up with water like a bathtub until they overflow into the next basin. So during the cooler and wetter pluvial climates of the glacial periods, much of eastern California, all of Nevada, and western Utah were covered by large lakes separated by linear islands (the present day ranges).