Montana Governor and First Lady's Math and Science Initiative
The Shore of an Ancient Sea
At Black Otter Trail on Montana Highway 3 near Billings
About 80 million years ago this area was near the shore
of the Western Interior Seaway that stretched from
the present-day Gulf of Mexico to the Arctic Ocean.
Rivers draining highlands to the west carried sediment into the
seaway and near-shore currents concentrated the sand creating
barrier islands. As the sea level alternately rose and fell, the
barrier islands migrated, forming an extensive layer of finegrained
sand across much of central Montana. The sand was
eventually buried, compacted, and cemented into the rocks that
now compose the rims and that geologists have named Eagle
Sandstone. When observed from the south, the rimrocks reveal
cross beds called accretion surfaces. These surfaces record the
deposition of sand washed over the barrier and deposited on the
other side by waves, causing the sand bar to grow shoreward.
The seaway was shallow, warm, and probably no more than
a few hundred feet deep. Oysters lived in dense banks along the
shore, while ammonites fed on clams that lived in the shallow
water offshore; sharks cruised the shallows preying on whatever
animals appeared tasty to them. For several million years, two
predators were at the top of the food chain in the sea: the longnecked
Plesiosaurs and the snakelike Mosasaurs. Neither were
dinosaurs, but were air-breathing reptiles adapted to living in
the oceans.
A Brief History
The rims near the confluence of the Yellowstone River and Alkali Creek is one of Montana’s historic hot spots. Native Americans utilized the area for at least 8,000 years before William Clark passed through here in July 1806. Near here in May 1823, Blackfeet warriors ambushed and killed seven Missouri Fur Company trappers led by Michael Immel and Robert Jones. The rims were later a spectacular backdrop for the rip-roaring riverside settlement of Coulson in the 1870s and for Billings beginning in the early 1880s.
