Montana Governor and First Lady's Math and Science Initiative
A Lost World - Precambrian Belt Rocks
Mile Post 4 on I-90, Dena Mora Rest Area
Imagine a world very different than we know today. About
1.5 billion years ago during the Precambrian Era, the
earth’s environment 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. 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
often trapped fine particles of calcium carbonate to form
structures called stromatolites, that grew in shallow nearshore
environments. The surface of the rocks 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 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 along the Lewis and Clark Fault Zone, a series of
faults that stretch between 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. Interstate 90
and US Highway 10 in western Montana follow the trend of the
faults along straight canyons that eroded along the fault zone.
A Lost World
