(Above) Reconstruction of the generalized Hell Creek landscape. In the years since this paper’s publication “Cissus” “marginatus” has been renamed Platanites marginata and “Dombeyopsis” trivialis has been renamed Marmarthia trivialis. (taken from: A pedotype approach to latest Cretaceous and earliest Tertiary paleosols in eastern Montana, Retallack 1994). Click image for full-view.
66 million years ago, much of the Hell Creek Formation was located at roughly 55N latitude. Due to continental drift, the rocks preserving Hell Creek are several hundred kilometers south and west of their ancient location. As the earth’s gravitation and axial tilt have not changed significantly in hundreds of millions of years, it is likely that Hell Creek had the same seasonal light cycle as present day Minneapolis, Winnipeg, and Churchill. This means that daylight varied from 8 hours to 16 hours of a ~24 hour cycle, depending on season. At the same time, we know from plant fossils and other sources that the climate was warm-temperate, much like Northern Florida is today. This combination of year round warm temperatures and long periods of seasonal darkness is unparalleled in the modern world.
Hell Creek is believed to have had seasonal variation, in the form of a hot, rainy season during the portion of the year with extensive daylight, and a cooler, dry season that coincided with the darkest months of the year. Warm global temperatures combined with higher concentrations of oxygen in the atmosphere made the Latest Cretaceous a fire prone world. Fires were probably most frequently caused by lightning strikes, also more prevalent in the Cretaceous as the warmer temperatures generate more storms. Warmer temperatures also meant that even plants with high moisture content could burn easily, making fire a very common occurrence in the ecosystem. Hell Creek received between 900-1200 millimeters (over 3ft!) of rain a year. The vast majority of this rain fell during the hot, wet season, making Hell Creek prone to periodic flooding.
Hell Creek was sandwiched between a rising mountain range to the west (the beginning of the Rocky Mountains) that formed a towering plateau similar to that of Tibet, and the remnants of the Western Interior Seaway to the east. While most of the mountain building was subterranean, Volcanoes were scattered across the landscape as well. The rocks of the Hell Creek Formation show that the landscape was a nearly flat plain, crisscrossed by numerous rivers featuring braided channels. Swamps were very rare, with virtually all standing water occurring in oxbow lakes. This landscape was covered in a mosaic of open woodlands and rockier terrain in the uplands, with lava flows near volcanos, and denser woodlands along river valleys. In the last 100’000 years of the Cretaceous, the Western Interior Sea began to rise, resulting in an expansion of swamps in what is now North and South Dakota.
(Above) The common Hell Creek ground floras Humulus sp. (a hops), Palaeoaster inquirenda (a poppy), a rosaceae herb and Blechnum sp. (a fern known from pollen). All of our herbs and tree leaves are directly based on fossils as shown here. (Fossil photographs taken from: Megaflora of the Hell Creek and lower Fort Union Formations in the western Dakotas: Vegetational response to climate change, the Cretaceous-Tertiary boundary event, and rapid marine transgression, Johnson 2002, Geological Society of America Special Papers.) Click image for full-view.