Inland Sea of Montana

The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that existed during the mid- to late Cretaceous period as well as the very early Paleogene, splitting the continent of North America into two landmasses, Laramidia to the west and Appalachia to the east. The ancient sea stretched from the Gulf of Mexico and through the middle of the modern-day countries of the United States and Canada, meeting with the Arctic Ocean to the north. At its largest, it was 2,500 feet (760 m) deep, 600 miles (970 km) wide and over 2,000 miles (3,200 km) long. For over sixty million years during the Cretaceous Period, much of eastern Montana was underwater, covered by a vast inland sea. As the Rocky Mountains formed to the west, it created a broad, flat coastal plain that was home to many different species of dinosaurs, Indeed, the long life of the sea saw the rise and extinction of many dinosaur species until it finally receded from Montana about 65 million years ago at the end of the Cretaceous Period. The sea was shallow and warm, probably no more than a few hundred feet deep. It was a home to a wide variety of aquatic life. Oysters lived in dense banks along the shore, while tentacle ammonites fed on monster clams that lived in the shallow water offshore. Sharks also cruised the shallows preying on whatever animals appeared tasty to them. For several million years, two predators, synonymous with prehistoric sea creatures, were at the top of the food chain in the sea. Both were carnivores that ate just about anything they could seize and swallow. Fossils of these two predators have been found throughout eastern Montana.

For centuries, the  rimrocks along the Yellowstone River guided travelers between the mountains and the buffalo country to the east. In 1912, regional Good Roads enthusiasts and county officials created one of the first interstate highways in the United States, the Yellowstone Trail, an interconnected network of county roads blazed by distinctive chrome yellow signs with black arrows. The 4,000-mile highway connected Plymouth Rock, Massachusetts and Seattle, Washington with a branch to Yellowstone National Park. Later re-designated U.S. Highway 10, Interstate 90 bypassed it in 1971.

To claim this earth cache, you must: Post a picture (or message to me) of yourself, body part or your gps/phone at the cache site ensuring you were actually there (if you fail to do this, your log may be deleted), and email us the answers to the following questions:

1. What were the two predators called and which one is believed to resemble Scotland’s Loch Ness Monster?

2. Were the two predators dinosaurs? If so, what family, if not dinosaurs, what were they?

3. The river to the south has cut cliffs. Identify the river and the type of rock and why this further supports the idea of a sea being here.

4. The landscape to the southwest has indications of seas pushing towards them. Describe the hills and why they demonstrate a sea was here.

Sources: Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. pp. 487–489.

Montana Transportation Department