Property in Montana|
Map Courtesy of
When dinosaurs roamed . . .
During much of the Cretaceous Period (144 to
65 million years ago) a large portion of
Montana was covered by the waters of a
shallow, inland sea called the Western Interior
Seaway. The sea was formed as west-central
North America was subsiding to an elevation
below sea level. As this sinking occurred the
area filled with water from the Gulf of Mexico
and the Arctic Ocean.
Source of the sediment . .
Between 80 and 40 million years ago, tectonic forces were building
the Rocky Mountains in western Montana and
Idaho. Rivers flowing eastward from these
mountains transported massive amounts
of sediment (sand, silt, clay) to this sea. As
the land cycled between periods of uplift and
subsidence, the seaway expanded or shrank,
resulting in dramatic east-west shifts in the
location of the coastline.
Where do I drop you off? . .
Changes in the
location of the coastline also caused changes
in the type of sediment deposited in different
areas. . . Sand was deposited closer to the
coast, whereas silts and clays settled to the
bottom in deeper waters. Watch this animation. As layers of sand,
silt and clay became buried, they were
compressed and turned into the sedimentary
rocks found today in central and eastern
Montana, and throughout the rest of the
west-central United States. For example, the
"Rimrocks" of Billings are made of sand
deposited at or near the shoreline of the
ancient Western Interior Seaway.
Source: Monahan, Glenn and
Chanler Biggs. Montana's Wild and
Scenic Upper Missouri River. Missoula,
Montana: Northern Rocky Mountain Books,
Terms: subsidence, deposition