About 1.5 billion years ago during the Precambrian Era, the earth's environment in what would one day become northwest Montana was desolate, with no trees, fish, animals, or birds. Shallow seas with extensive near-shore flats were fed by streams that deposited great amounts of sand and mud, and rain frequently fell and pooled in vast shallow lakes and ponds. In this hostile environment, blue-green algae mats often trapped fine particles of calcium carbonate to form stromatolites that grew in shallow near-shore settings. Rock surfaces often display mud cracks, ripple marks, and sometimes the spatter marks of primeval raindrops. The earth's crust slowly sank for about 100 million years, forming a large geologic basin in which Belt Supergroup sediments accumulated to as much as 10 miles thick. These rocks are common in northern and central Idaho and western Montana, extending east to the Little Belt Mountains in central Montana, and the sedimentary rocks along Interstate 90 between Lookout Pass and Alberton are almost entirely part of the Belt Supergroup. They are distinguished by brown, grey, 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 lies along the Lewis and Clark Fault Zone, a series of faults stretching between northwest Washington State and the Helena area. These faults had significant movement about 70 million years ago when the Rocky Mountains were uplifting and remained active until at least 25 million years ago, and Interstate 90 and U.S. Highway 10 in western Montana follow their trend along a straight canyon eroded along the fault zone.