Montana Earth Science Picture of the Week

May of 1805 . . .
Monte Dolack's painting depicts the Lewis and Clark expedition near the place where Eagle Creek empties into the Missouri (about 30 miles east of Ft. Benton). The "White Cliffs" shown in the painting owe their existence to sand deposited near the shore of a shallow sea that extended from the Artic Ocean to the Gulf of Mexico. Called the "Western Interior Seaway", the sea existed from about 90 million years ago until about 70 million years ago.

When Dinosaurs Wandered the Area . . .
Apparently the shallow Western Interior Seaway experienced changes, which caused it to shrink and expand. These resulted in fluctuating depths and migrating coastlines. As rivers laden with sand, silt and clay emptied into this sea, the larger pieces (sands) would have settled to the bottom first. So, when the Eagle Creek area was at or near the coast, sands were deposited here as beaches and/or offshore sand bars. The smaller sediments (clays, silts) would not have settled right away, instead settling to the bottom of the sea in deeper waters farther away from the coast.

Same Layer, Different Location . . .
The same formation (layer) can be seen in Billings where it is known as the "Rimrocks". Geologists have named the layer (or series of layers) the Eagle Formation after the Eagle Creek area depicted in the painting.

When the coast was located farther to the west, the sea would have been somewhat deeper here. As a result of this deeper environment, this would have been a time when smaller particles such as silts, and clays were deposited here. Those materials make up the layer known as the Marias River Shale shown in the photo of the Eagle Creek area below as well as on the painting.

Mountain Building (and the subsequent erosion) to the west of here provided an abundant source for the sands, silts, and clays deposited in this part of central Montana. The rock layers seen along the river tilt gently to the east, so as travelers move downriver (eastward), they see increasingly younger rock layers along the river.

Source: The book entitled "Montana’s Wild and Scenic Upper Missouri River" by Glenn Monahan and Chanler Biggs.

Term: fluctuating