The Teton Range began to rise around ten million years ago when movement on the Teton fault generated a series of massive earthquakes, lifting the bedrock west of the fault into mountains while the block to the east dropped to form the valley of Jackson Hole, and this process will continue with future earthquakes. Today the mountains rise more than 7,000 feet above the valley floor, and across Jackson Lake the forty-mile-long Teton fault marks the sharp boundary between the steep mountain face and the flat valley floor. Beginning about two million years ago, glaciers flowing from the high peaks and south from Yellowstone ground and polished bedrock, carved U-shaped canyons, dug basins, and left moraines of boulders, gravel, and rock flour that now form natural dams around the valley’s lakes. Jackson Lake, more than 400 feet deep, shows the power of glaciers to reshape the land. Water, wind, and ice still erode the mountains, though less forcefully than during the ice age, and the small glaciers of today, formed around 1400, are now receding after later glacial periods that included thick ice over parts of Jackson Lake, glacier cover over Jackson, and the carving of lakes such as Jenny and Jackson.