Rising more than 4,000 feet above the plains, Chief Mountain demands attention and has long played a prominent role in the cultures of indigenous people. The peak is called Ninaistaki by the Blackfeet and is the sacred home to the creator of thunder. It typically appeared on early maps of the area, and in 1901 Bailey Willis of the US Geological Survey recognized that Chief Mountain and the other Belt rocks had moved east many tens of miles along the “Lewis Overthrust.” This Sevier-style, low-angle compressional fault ramped up from the basement, carrying 1.47 billion years old Belt Supergroup rocks eastward over Cretaceous shale during the collision of the North American and Farallon plates. Long after the collision, rising land and erosion left this huge block of Belt rock isolated from the other rocks above the thrust, what geologists call a klippe.
The Lewis thrust broke close to the base of the Belt Supergroup, ramping upward until it could move horizontally on weak Cretaceous shales. It slowly transported a slab of rock that was at least 300 miles long and 2 miles thick about 80 miles from west to east, stopping close to where the mountains now rise from the plains on the east side of Glacier National Park. Thrust faults place older rocks over younger rocks, and this one beats them all, with sandy dolostone of the 1.47-billion-year-old Altyn Formation thrust over foreland basin shale and sandstone deposited in Cretaceous time. Based on cross-cutting relationships of dated volcanic rocks, the fault moved between 75 and 59 million years ago. The fault is complex, ranging from a single fault plane to multiple planes, some of which are rising seahorse-shaped faults called duplex structures. All rocks above the fault are gently folded into a huge downfold called the Akamina syncline.The Lewis thrust plane is nearly horizontal, so friction on its surface must have been very low. Perhaps expandable clays in the underlying Cretaceous shales dropped friction dramatically. Since the fault moved shortly after deposition of the shales, they might have been wet, and when loaded by the heavy Belt Supergroup rocks, pore water might have squeezed out of them and lubricated the fault plane. The Cretaceous rocks below the fault are typically covered with plants, but where we see them, they range from seemingly undeformed rocks that parallel the fault to intensely deformed rocks. Over time, the fold and thrust belt continued to migrate east of the Lewis thrust, but mostly in Cretaceous rocks as compressional energy waned, deforming the weak rocks near the top of the stratigraphic pile. It all ended around 56 million years ago.
Chief Mountain consists of hard rocks of the Altyn and Appenkunny formations over weak shale and sandstone of the Cretaceous Two Medicine Formation. Erosion through the fault plane isolated this stump of Belt rocks from the rest of the fault forming the klippe, a German word meaning cliff. The great thrust slab might have extended a bit farther east, but the evidence has been eroded away, and Chief Mountain is most likely very close to the eastern front of the Lewis thrust. Exposure of this once deeply buried fault shows the ground was raised after it stopped moving and miles of overlying rock were eroded away. For the last 50 million years, western Montana and the plains have both been thermally rising from crustal extension and thinning. As a result, there has been ample time for uplift and erosion to expose this great structure. The large scar on the northeast face of Chief Mountain formed when weakened rock collapsed during an earthquake on July 2, 1992, showing that the landscape is still a work in progress. Source
This really does a good job of explaining its background. A friend and I drove down the 17 and 89 many years ago as young men. It was a sight to behold. Little wonder the natives considered it sacred.
I worked on an oil rig just over the border the next spring. They had drilled 15,000 feet through the overthurst to get to the sedimentary layers underneath. The Livingston range was out the door pushing 10,000 feet plus feet up into the continental divide. It was quite the place.
The mountain is huge but tiny in scale to the rest of the geology. I didn’t know the overthrust was as far as it was. The forces to do all that are impresssive.