Sediment collected in various lakes in the area from around 17 to 15 million years ago, becoming the Miocene-aged Colter Formation. Beginning around 13 million years ago (also in the Miocene), a 40-mile (64-km) long steeply east dipping normal fault system called the Teton Fault started to vertically move two adjacent blocks. One block, the Jackson Hole basin, moved down while the other block, containing the westward-tilting eastern part of the Teton Range, moved up; thus creating the youngest mountain range in the Rocky Mountains. Most of the downward movement occurred right next to the fault, resulting in a 15° tilt of the Colter Formation. No sediment was deposited on top of the tilted Colter Formation for up to three million years, resulting in an angular unconformity as the tilted Colter partially eroded away.