GEOLOGY OF THE UPPER MEDICINE LODGE VALLEY  


Contents and Photo courtesy of Peter Fiske PhD

The Hildreth Livestock Ranch is situated not only in some of the most beautiful country in the United States but also some of the most interesting mountains in the world. The Beaverhead and Tendoy Mountain Ranges run roughly north south and form the east and west edges of the valley. They are part of the Rocky Mt. Range - a long belt of rugged mountains pushed up over the last 100 million years. In fact, the crest of the Beaverhead Mts form the axis of the Continental Divide, as well as the boundary between Montana and Idaho, in this part of Montana.

The rocks that are exposed in these uplifted mountains represent over 1 billion years of geologic history. The Hildreth Ranch is situated on some of the oldest rock - granites and granitic gneiss - that were the roots of an old mountain range that long ago was completely weathered away. From roughly 1,000 million to 600 million years a large ocean basin existed in this region. Several thousand feet of sandstone and shale was deposited during this time. From 600 million to 300 million years ago other sedimentary rocks were laid down on top including shales, sandstones and limestones. Some of the grey limestones of the Madison Formation are exposed near the crest of the Tendoy Mountains.

This neat stack of sediments was then folded and squeezed during the mountain building period 200-100 million years ago that formed most of the geology of the Rocky Mts. Evidence of the spectacular faulting and folding can be seen in the cliffs south of Morrison Lake, where limestones and shales are folded in a huge U shape.

The final event forming the landscape was a period of stretching, which broke the landscape into large north-south blocks which tilted to the east forming the Tendoy and Beaverhead Mountain Ranges.

But perhaps the most important event was the formation of the Beaverhead Impact Structure. Scientists are unsure exactly when this huge meteorite impact took place (roughly 1,000 to 600 million years ago) but the evidence that remains suggests that it is the largest meteorite impact crater in the continental U.S. The crater itself has long since disappeared - eroded, faulted, and deformed by later geologic activity. But the evidence for the impact remains in the rocks in the form of "shatter cones" - enigmatic conical fracture patterns that are distinctive evidence of meteorite impact. The Hildreth Livestock Ranch has been the site of several years of geological investigations and has hosted a major international meeting about the impact features in the area.