Tendoy Mountains

I-15, MP 15, Lima Rest Area

About four million years ago, this part of the North American Plate slid over a gigantic source of heat in the mantle known as the Yellowstone hot spot. In Yellowstone National Park, this heat is responsible for the geysers, mud pots, and hot springs, but in southwestern Montana, it is partially responsible for the mountains and the valleys. The thermal energy deep underground bulged the crust above it as the tectonic plate moved over the hot spot, causing the Earth’s upper crust to stretch. The stretching was so great that crust broke into blocks separated by steep faults that allowed these blocks to move up or down relative to adjacent blocks. This site in the Red Rock Valley is one of the down-dropped blocks, whereas the Tendoy Mountains to the west were uplifted. Geologists refer to the break separating them as the Red Rock fault. You can see the Red Rock fault as the line along the foot of the mountains closest to the rest area.. The triangular faces or facets that terminate the ridges coming toward you are evidence that this fault is very young. It ruptured as recently as 3000 years ago and earthquakes indicate that it is still actively moving.