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. Around 10 million years ago, Jackson Hole's first large freshwater lake was impounded by east-west fault movement in what is today the southern end of the park. Geologists call this fault-scarp dammed body of shallow water Lake Teewinot and it persisted for around 5 million years. The resulting Teewinot Formation of lakebed sediments sits directly on the Colter and consists of limestones and claystones mixed with volcanic material and fossilized clams and snails. All told, sediments in the Tertiary period attained an aggregate thickness of around 6 miles (10 km), forming the most complete non-marine Tertiary geologic column in the United States. Most of these units within the park are, however, buried under younger deposits. Eventually all the Mesozoic rock from the Teton Range was stripped away and the same formations in Jackson Hole were deeply buried. A prominent outcrop of the pink-colored Flathead Sandstone exists 6,000 feet (1,830 m) above the valley floor on the summit of Mount Moran. Drilling in Jackson Hole found the same formation 24,000 feet (7,300 m) below the valley's surface, indicating that the two blocks have been displaced 30,000 feet (9,100 m) from each other. Thus an average of one foot of movement occurred every 300 years (1 cm per year on average). Quaternary volcanic deposits and ice ages Massive volcanic eruptions from the Yellowstone Volcano northwest of the area occurred 2.2 million, 1.3 million, and 630,000 years ago. Each catastrophic caldera-forming eruption was preceded by a long period of more conventional eruptions along even earlier volcanic episodes. One such event sent large amounts of Rhyolitic lava into the northern extent of Teewinot Lake. The resulting obsidian (volcanic glass) has been potassium-argon dated to 9 million years and was