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To get some idea of the origins of these fossils one must understand something about geology, and specifically about the Cretaceous Period. This period lasted some 70 million years in the time between 135 and 65 million years ago. During this time vast and ubiquitous limestone deposits known to geologists as "The Chalk" were deposited in many parts of the world. This "Chalk" is most dramatically exposed in what are well known as "the white cliffs of Dover" in England. As for the term "Cretaceous" it is derived from the Latin 'creta' meaning 'chalk'.
The Cretaceous Period of North America is characterized by the movements of vast bodies of water. Eventually the bodies united into one prodigious whole, the Mid-Continental Seaway. This sea was at one time about a thousand miles wide and stretched from Canada's north to the Gulf of Mexico. It divided the continent into two vast islands and lasted for millions of years.
The seaway must have provided a fairly stable environment for many aeons. The climate of the time was much different from what it is today. There was little daily or seasonal temperature change. It remained remarkably stable as a tropical or sub-tropical area throughout the Upper Cretaceous Period.
The animal life during the Upper Cretaceous period was teeming and varied. Reptiles, including four types of mosasaurs, plesiosaurs and great sea turtles lived in abundance in this warm salty water. In addition there were at least two species of birds, Hesperornis and Ichthyornis, as well as squids, sharks, and numerous species of fish. All of these forms have been found as fossils in southern Manitoba, as well as in Saskatchewan and Alberta.
When the Cretaceous Period finally came to an end the nature of animal life changed dramatically. The dinosaurs which were the dominant animals living on the land, and the great marine reptiles which dominated life in the sea, both became extinct. The cooling climate and significant disturbances in the earth's crust may have been factors contributing to their dramatic decline. When this period ended so did all the mosasaurs and plesiosaurs. Most of the marine fossils in southern Manitoba have been found in the Morden - Miami area. They are usually found associated with bentonite exposures, along the Manitoba Escarpment, the ancient shoreline of glacial Lake Agassiz, which came into existence millions of years after the fossils were buried. The bentonite layers alternate with shale layers, which are the sea's normal deposits. The fossils are usually found in the top two layers of shale, although they may be found in lower layers as well.
The beds of bentonite and shale form the Pembina Member of the Vermillion River Formation. Bentonite contains minute amounts of radioactive potassium enabling us to determine that these layers were deposited eighty million years ago. From this we get the approximate age of the fossils.
Bentonite is a rock composed of clay minerals formed by the chemical alteration of minute glass particles that once composed volcanic ash. The layers of this volcanic ash were deposited when extinct volcanoes in what is now Montana erupted, spewing tons of this material into the air. This volcanic ash was carried by wind and water currents to form numerous beds all over most of the interior of North America. However, Manitoba is one of the few places where these layers are close enough to the surface and of sufficient thickness to be economically mined. As the bentonite is mined, fossils are discovered in the pits.