This video presents the Great Unconformity exposed in the Camp Creek drainage east of Melrose, Montana. At this location, rocks metamorphosed during the collision of continental plates some 1.8 billion years ago are overlain by ~520 million-year-old beach sandstones of the Flathead Formation. This huge gap in the geologic record is known as the Great Unconformity, and the contact between these different rocks represents over a billion years of erosion and/or non deposition.
The consensus interpretation of the unconformity is that some 1.8 billion years ago the collision of a continental crustal block called the Medicine Hat Block with the Wyoming Province of the North American craton (continent) resulted in metamorphism and uplift of mountains that might have rivaled the Himalayas today. Once the colliding blocks were sutured and the plate collision ended, the mountains began to erode. Over the next 1.0 to 1.3 billion years, they were eroded down to sea level and much of western North America was a flat, low-lying plain. The ocean rose up around 520 to 515 million years ago, drowning the eroded surface with ocean water and depositing ocean sediments.
As the sea transgressed across the eroded surface, it first deposited beach sand of the Flathead Sandstone. This rock contains some fossils and abundant sedimentary structures that indicate a marine beach environment. With continued sea-level rise, the beach moved eastward, and the Camp Creek area was covered by deeper water, where fine-grained mud that couldn't settle on the beach were deposited. These deposits are called the Wolsey Shale, and they are loaded with burrows, trilobite tracks and a few, nearly intact trilobite shell fossils. As the sea continued to rise, the source of sand, silt and clay were moved so far away from the area that organisms that filter the water and secrete calcite flourished, depositing limestone of the Meagher Formation.
Sea-level continued to fluctuate, leaving behind more shale and limestone, marine and non-marine deposits throughout the period from 520 to 245 million years ago, what geologist's call the Paleozoic Era. Many of these rocks are exposed at Camp Creek and throughout the area. There is a map in the video to help you locate the site. You can find specific directions in the 2nd Edition of Roadside Geology of Montana, so get out, explore and enjoy Montana Geology!