The most striking feature of the mountains--certainly the one which
comes first to a visitor's attention--is the color banding. No matter
where one looks this feature greets his view. If he enters the park at
the St. Mary Entrance, there ahead on the sides of Singleshot and East
Flattop Mountains are white and purple bands. Should he enter first the
Swiftcurrent Valley, he would soon note the banding in the mountains
lying to his right and left, and finally culminating in the precipitous
Garden Wall at the head of the valley. The visitor soon realizes that
every mountain within the park is composed of rock layers of various
colors. With very few exceptions these strata are of sedimentary origin;
that is, they accumulated by depositions of muds and sands in a body of
water and are now mainly limestones, shales, and sandstones. These
sedimentary rocks all belong to a single large unit known as the Belt
series, so named because of exposures in the Little Belt and Big Belt
Mountains farther south in Montana. In Glacier National Park these
rocks, which have a maximum thickness of more than 20,000 feet, are in
the form of a large syncline (downfold), the east and west edges of
which form the crests of the Lewis and Livingstone Ranges (Figure 3D).
Throughout the large area of western Montana, northern Idaho, and
southern British Columbia where Belt rocks occur, they are important
mountain-makers. In addition to the ranges already mentioned they are
the principal rocks in many others, including the Mission, Swan, and
Flathead in the region south of Glacier Park; the Bitterroot and Coeur
d'Alene between Idaho and Montana; and the Purcell in British Columbia.
Further, rocks of similar age form the core of the Uinta Range in Utah
and the lower section of the Grand Canyon in Arizona.
During the Proterozoic Era of Earth history a long, narrow section of
North America extending from the Arctic Ocean southward, probably as far
as Arizona and southern California, slowly sank to form a large,
shallow, sea-filled trough known as a geosyncline (Figure 1). Streams
from adjacent lands carried muds and sands into the sea, at times almost
completely filling it. Inasmuch as thousands of feet of sediments were
deposited, the geosyncline must have continued to sink throughout the
period of sedimentation. Eventually the muds were compacted into shales,
or limestones if they contained a lot of lime, and the sands into
sandstones. These are the rocks we now know as the Belt series. The
surfaces of many of the sandstone layers are covered with ripple marks
which could have been made only by wave and current action in shallow
water. Mudcracks on many of the shale beds prove that at times the
sediments, probably near the mouths of rivers were exposed to the air
long enough to dry out. Great thicknesses of limestone and numerous
fossils of calcareous algae, primitive marine plants, are evidences that
the body of water was a sea.