During the Cretaceous period, a shallow tropical inland sea, the Western Interior Seaway, split North America in two. Real estate in Montana during this time would have been very expensive -- much of the state sat on the coastline of this tropical sea. Over millions of years, the seaway underwent several transgressions and regressions, or relative rises and falls of sea level, respectively. A portion of the rise and fall of the Interior Seaway is recorded in the sedimentary succession near Choteau, MT.
Egg Mountain is a locality in the Two Medicine Formation, a 600 meter thick package of rocks that were deposited in western Montana from around 83 to 75 million years ago in the Campanian epoch of the Late Cretaceous period.
This is about 8 million years before T-Rex and Triceratops roamed Montana, and about 10 million years before the extinction event that wiped out all non-avian dinosaurs. Within the Two Medicine Formation, Egg Mountain and the other dinosaur-bearing rock layers are found in the Willow Creek Anticline.
An anticline is a structural geology term used to describe a hill where the layers of rock form an upside down U shape. This happens in commonly in mountain building, like the Rocky Mountains, where compressional forces fold and bend rock layers, similar to pushing on the edge of a rug. After millions of years, some of the top layers in the Willow Creek anticline have eroded away, leaving exposed the layers that contain Egg Mountain, dinosaur nests, and an extensive dinosaur bone bed.
The sedimentary rocks at Egg Mountain were deposited at height of a regression, when the coastline of the sea was as far east as Billings, MT. Carbonate-rich sediments shed off the newly uplifting Rocky Mountains were deposited in systems of braided streams and small lakes. This environment was much closer to the mountains than to the seaway 200 miles away. The uplifting Rockies also produced volcanism, recorded in igneous rocks in the Elkhorn and Adel Mountains southwest of Choteau. Layers of volcanic ash, called bentonite, are evidence of volcanic activity and are common features used to obtain radiometric dates.
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1. In front of you what features show remnants of the anticline?
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Further Information.
In late July of 1978 Jack Horner visited the Rock Shop and Museum in Bynum, MT. Local resident Marion Brandvold had discovered baby dinosaur bones that she showed Jack. Laurie Trexler had also found an adult duckbilled dinosaur skull nearby. The baby bones were the first to be found in North America, and the first in a nest anywhere in the world. Both the babies and the skull were discovered while Dave and Laurie Trexler and Marion Brandvold were working on an adult dinosaur they planned to display in their museum. At the time, this display would have been only the second dinosaur on display in Montana.
Bob Makela and Jack Horner collected the skull and site materials from the baby locality. They also borrowed the baby bones collected by Marion. These were studied that winter and a preliminary report naming “Maiasaura peeblesorum” was published in Nature in 1979, before any eggs were discovered. A volunteer of Jack’s, Fran Tannenbaum, found the first eggs at Egg Mountain, approximately 3/4 of a mile from the original Maiasaura locality, in July of 1979.
Both Egg Mountain and the Maiasaura locality are rich in egg, baby, and adult fossils. The animals themselves roamed a broad flat coastal plain along the edge of the Cretaceous Interior Seaway approximately 77 million years ago. The Egg Mountain locality has produced remains of adult and embryonic Troodon (a small meat-eater) and adult Orodromeus (a small plant-eater), as well as remains of cretaceous mammals, lizards, and pterosaurs. The Maiasaura locality has yielded literally thousands of individual fossils, all believed to be from the single species of Maiasaura. The original Maiasaura nest contained remains of baby dinosaurs that had been hatched for some time before their demise. Because these individuals had remained in the nest after hatching, some individual (most likely the mother!) had to have been caring for the babies. This was the first indisputable evidence that dinosaurs were capable of any sort of complex behavior.
The Maiasaura’s round nests were six or seven feet wide and could hold up to 25 eggs. The hatched babies were about one foot long. Adult Maiasaurs weighed almost 6,000 pounds and were almost 30 feet long. The nests of Troodon at Egg Mountain itself are about half the diameter of the Maiasaura nests, but they contained roughly the same numbers of eggs.