A Lost World: Precambrian Belt Rocks

Imagine a world very different than we know today. About 1.5 billion years ago during the Precambrian Eon, the earth’s environment was desolate, with no trees, fish, large animals or birds. Shallow seas with extensive near-shore flats were fed by streams that deposited great amounts of sand and mud. Rain frequently fell and pooled in vast shallow lakes and ponds in what would one day become northwest Montana. Despite the hostile environment, blue-green algae mats grew locally, often trapping fine particles of calcium carbonate to form structures called stromatolites, that grew in shallow near-shore environments. The surface of the rocks often display mud cracks, ripple marks, and, sometimes, the spatter marks of primeval raindrops, indicating that they were deposited under conditions of periodic wetting and drying. The earth’s crust slowly sank for about 100 million years forming a large geologic basin in which Belt Supergroup sediments accumulated as much as 10 miles thick! The rocks are common in northern and central Idaho and western Montana, and extend east to the Little Belt Mountains in central Montana. The sedimentary rocks along Interstate 90 between Lookout Pass and Alberton are almost entirely rocks of the Belt Supergroup. These rocks are distinguished by brown, gray, red, green, purple, and yellow colors and locally form dramatic cliffs where resistant, well-cemented sandstones are exposed in the canyon. Interstate 90 from near Lookout Pass through Missoula is located near and along the Lewis and Clark Fault Zone, a series of faults that stretch from northwest Washington State and the Helena area. The faults had significant movement about 70 million years ago when the Rocky Mountains were uplifting and were active until at least 25 million years ago. Faulting and folding disrupted the original positions of the Belt Supergroup and younger rocks. Some of the faults across the border in Idaho are associated with silver ore. Interstate 90 and US Highway 10 in western Montana follow the trend of the faults along a straight canyons that eroded along the fault zone.