A formation is a very useful concept in geology. They were originally defined as rock strata (layers) created by the "same formative processes." Formations are recognizable rock layers that might have a uniform history, texture, composition, or thickness. Many formations vary in one or more of these attributes. They are usually named after a "type locality", either the first place they are recognized or somewhere that the formation is outcropping extensively. Any type of rock can be part of a formation: sedimentary, igneous, or metamorphic.

Different environments produce different types of sediment, which are grouped by geologists into different formations. Because many different types of environments and climates coexist on the surface of the Earth at a given time, formations are geographically restricted. As a result, different formations can represent sediments of equivalent ages, but no single formation can extend over the entire surface of the Earth. The boundary between one formation and another might be sharp and well defined, or it might be more gradual. As sediment layers get deposited and stacked atop the previous one, incorporating the remains of animals and plants soon to become fossils, time is recorded from bottom to top, older to younger. Some formations are only several meters thick, while some are several hundreds, and the time they represent can be highly variable. One particularly interesting formation is the Hell Creek Formation, which crops out in North Dakota and Montana and represent stacks of sediments such as sandstone, mudstone, shale, and coal, deposited in terrestrial coastal environments. It formed during the last million year of the Cretaceous period and represents a window in the past that helps geologists and paleontologists to reconstruct the ecosystems in which the very last non-avian dinosaurs lived, up to their demise at the Cretaceous-Tertiary boundary and mass extinction event, 66 million years ago. There are several other terrestrial fossil-bearing formations of the same age outcropping nearby, such as the Lance Formation of Wyoming.

The Lance Formation formed during the same time period as the Hell Creek Formation, but on the opposite side of a shallow inland sea that covered North America, the Western Interior Seaway. Because these formations are close in time and space, they also have comparable sediments and their fossil content reveal comparable ecosystems. Fish, frogs, turtles, salamanders, crocodiles, champsosaurs, and pterosaur fossils have all been found in the Lance Formation. Early mammals have been identified too, and of course, dinosaurs. Dinosaurs of the Lance Formation include: Tyrannosaurus rex, Triceratops, Edmontosaurus, Ankylosaurus, Pachycephalosaurus, and Troodon. The climate was subtropical, warm and wet.

During the last five million years of the Cretaceous, the Western Interior Seaway slowly retreated, which led to a transition of different environment. Geologists have thus recognized different formations representing this transition from the bottom up, starting by the Pierre Shale Formation, representing homogeneous grey shales deposited in shallow marine settings, the Fox Hills Formation, representing shales and sands from near-shore to coastal lagoon and beaches, and the Hell Creek Formation, which is fully terrestrial, represented by floodplains, forested soils, lakes and rivers. Both the Pierre Shale and Fox Hills formations produce fossils such as sharks, rays, marine reptiles, ammonites, brachiopods, and other shellfish, witness of a diverse and thriving marine ecosystem.

Above the Hell Creek Formation, the Fort Union Formation is characterized by colorful banded layers of sandstone, mudstone and coal, which were deposited within a wet and swampy forested environment with lakes and rivers. The Fort Union Formation is very thick and contains important economic deposits of coal, coalbed methane and uranium. The transition between the Hell Creek and the Fort Union formations closely coincide in time with the Cretaceous-Tertiary boundary mass extinction. As such, ecosystems of the Fort Union Formation are drastically different from those of the Hell Creek: fossils of non-avian dinosaurs are gone, and many other species of animal and plants disappeared altogether, yielding fossil assemblages of very low diversity compared to what was known from the Hell Creek. The Fort Union Formation is particularly interesting as it records the history of the post-catastrophe recovery of fauna and flora after the Cretaceous-Tertiary mass extinction event.

<P>The Hell Creek Formation is one of the most fascinating dinosaur graveyard sites ever discovered. Barnum Brown, a renowned American paleontologist, found the first Hell Creek site in Montana. We now know that this formation extends through Montana, North Dakota, South Dakota, and Wyoming. The Hell Creek Formation was laid down between 66.8-66 million years ago. This covers a very interesting span of time, including the last of the dinosaurs up to their extinction at the Cretaceous-Tertiary boundary. </P>
<P>In the Cretaceous period, the land of the Hell Creek Formation looked very different from the dry badlands in the area today. Forests thrived in a warm and moist subtropical climate. A seaway cut through the middle of the United States, stretching from the Gulf of Mexico to the Arctic Ocean. Because of the interaction between land and water in this area, both terrestrial (land) and aquatic animal fossils can be found in the Hell Creek Formation. This, combined with plant fossils, gives us a good idea what the environment was like. </P>
<P>The Hell Creek area was very diverse in the Cretaceous Period. On land, dinosaurs ruled. The first fossil of Tyrannosaurus rex was found here. Large herds of Triceratops roamed the land, along with dromeosaurs, hadrosaurs, pachycephaosaurs, ankylosaurs, ornithopods, and other theropods. Small mammals lived in the shadow of dinosaurs, including some of the first placental mammals. In the water there were turtles, crocodile-like champsosaurs, fish, and amphibians. Birds shared the sky with pterosaurs. </P>
<P>Hell Creek plant life was just as interesting and varied. There were many tall trees, but also low and medium-level shrubs. Flowering plants were well established by then, and there were many plants similar to the ones alive today. Gingko trees, conifers, cycads, moss, and ferns all lived in the area. There were no grasses, but there were willows, maples, magnolias, and sycamore trees. </P>
<P>When the asteroid struck off the Gulf of Mexico devastating extinctions followed. The Hell Creek area lost all dinosaurs (except for birds!) and many other species. Animals that lived in the water did better than animals that lived on land and more cold-blooded animals survived than warm-blooded. Large animals were especially vulnerable. Amphibians, champsosaurs, crocodiles, and turtles survived with little change. Primitive groups of mammals were dominant before the extinction, but placental mammals were more common after. There were still many fish in the habitat, though different species than there were before. </P>
<P>Plants were also affected by the extinction. Many species of flowering plants went extinct. After the impact, paleontologists found metasequoia trees, conifers, sycamore trees, katsura trees, and duckweed. Even though the extinction caused great ecological impact, the general range of species before and after the asteroid were very similar. The big difference is that the non-bird dinosaurs went extinct, leaving many ecological niches empty. It would take millions of years to recover from this loss.</P>

Conifers can be traced back 350 million years (to the Carboniferous). While swamps were dominated by horsetails and seed ferns, drier uplands were host to conifer trees. Early conifers were probably only 2-3 m (about 10 feet) in height. Leaves of these early conifers were needle-like but their cones were not as compact as modern cones. By the Permian, conifers had diversified into many forms, including large, woody trees much like modern conifers. At the end of the Permian, an enormous extinction event caused a major change in land ecosystems. Many of the primitive conifers went extinct, but survivors gradually spread during the Triassic until they became dominant forest trees. The early and middle Mesozoic (about 250-130 million years ago) became the age of conifers. Like modern forms, Mesozoic conifers were large, woody plants that made seeds. With the exception of one group that went extinct (the Cheirolepidiaceae), families of Mesozoic conifers are still on Earth today.