Grand Teton National Park: landscape and origins
The Teton Range is the youngest mountain range in North America, and the only big one without foothills. Both have the same cause.
The Rocky Mountains are 50 to 80 million years old, the Appalachians over 300. The Teton Range has been rising for less than ten million years. On a geological scale that is youth, and the reason the peaks are so sharp: erosion simply has not had the time.
The range does not grow steadily but in sudden jerks, along the 40-mile (64 km) Teton Fault. The region is stretching from east to west; when the strain gives way, the mountain block rises by 60 to 90 centimetres (2 to 3 ft) while the valley floor drops by 1.2 to 1.8 metres (4 to 6 ft). That explains the second peculiarity: the Tetons have no foothills, because the valley falls faster than the mountain rises. The mountains stand in the valley without any transition.
How much this adds up to can be read from a single rock layer. The same Flathead Sandstone lies about 1,800 metres (5,900 ft) above the valley floor on Mount Moran and about 6,000 metres (19,700 ft) below it on the east side of the fault. The total offset since the movement began is probably more than 7,600 metres (24,900 ft).
The material itself is far older than the range. Much of it is gneiss 2.7 billion years old, one of the oldest rocks in North America, formed when seafloor was buried up to 29 kilometres (18 mi) deep in a plate collision. 2.5 billion years ago granite forced its way into the cracks; it is harder and forms the highest peaks today. About 775 million years ago came the black diabase dikes, one of which cuts through the face of Mount Moran over a width of 45 metres (148 ft).
The fault is still active and, according to the park, can produce an earthquake of magnitude 7.0 to 7.5. Since the ice left the valley about 20,000 years ago there have been up to a dozen; the three most recent were about 5,900, 8,000 and 10,000 years ago. At the Cathedral Group Turnout on the Jenny Lake Scenic Loop you can see the fault scarp in the landscape.
NPS: geology and the Teton Fault ↗
The park in numbers
- Highest peak
- Grand Teton, 4,199 m (13,776 ft)
- Age of the uplift
- less than 10 million years
- Length of the fault
- 40 miles, about 64 km
- Offset per earthquake
- 0.6–0.9 m up, 1.2–1.8 m down
- Total offset
- over 7,600 m (24,900 ft)
- Oldest rock
- gneiss, 2.7 billion years
- Possible earthquake
- magnitude 7.0 to 7.5
- Park area
- about 310,000 acres, 1,255 km²
- Elevation of Jackson Hole
- over 1,900 m (6,200 ft)
