It is a series of fresh and brackish-water clays, mudstones, and sandstones deposited during the Maastrichtian and Danian (respectively, the end of the Cretaceous period and the beginning of the Paleogene) by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits along the low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway.

The most complete hadrosaurid dinosaur ever found, an Edmontosaurus, was retrieved in 2000 from the Hell Creek Formation and widely publicized in a National Geographic documentary aired in December 2007.

The teeth of sharks and rays are sometimes found in the riverine Hell Creek Formation, suggesting that some of these taxa were then, as now, tolerant of fresh water.

[14] It is a series of fresh and brackish-water clays, mudstones, and sandstones deposited during the Maastrichtian and Danian (respectively, the end of the Cretaceous period and the beginning of the Paleogene) by fluvial activity in fluctuating channels and deltas and very occasional peaty swamp deposits along the low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway.

The Hell Creek Formation, as typified by exposures in the Fort Peck area of Montana, has been interpreted as a flat, forested floodplain with a relatively subtropical climate that supported a variety of plants ranging from angiosperm trees to gymnosperms such as the conifers, cycadeoids and ginkgos to ferns and moss.

These formations are composed largely of sandstone and mudstone which have been attributed to floodplain, fluvial, lacustrine, swamp, estuarine and coastal plain environments.

HC-530 and HC-312) in the Apex sandstone and Jen-rex sand where individual bones were counted and (2) in two thirds of the formation reflected in L3 and U3 records of dinosaur skeletons only.

[20][21] Historically, numerous teeth have been attributed to various Dromaeosaurid and Troodontid taxa with known body fossils from only older formations, including Dromaeosaurus, Saurornitholestes, and Troodon.

However, in a 2013 study, Evans et al. concluded that there is little evidence for more than a single dromaeosaurid taxon, Acheroraptor, in the Hell Creek-Lance assemblages, which would render these taxa invalid for this formation.

[22] Fossilized teeth of various troodontids and coelurosaurs are common throughout the Hell Creek Formation; the best known examples include Paronychodon, Pectinodon and Richardoestesia, respectively.

The Hell Creek Formation was a low floodplain at the time before the sea retreated, and in the wet ground of the dense woodland, the diversity of angiosperms and conifers were present.

The Hell Creek Formation of this layer contains over 300 tablets, of which angiosperms are by far the most diverse and dominant flora of the entire population, about 90 percent, followed by about 5% of conifers, 4% of ferns, and others.

In sharp contrast to the Great Plains today, the presence of many thermophilous taxa such as palm trees and gingers meant the climate was warmer and wetter then.

The plants of the Hell Creek Formation generally represent angiosperm-dominated riparian forests of variable diversity, depending on stratigraphic position and sedimentary environment.

For this reason, Kirk Johnson and Leo Hickey divided it into five zones and described them as HCIa, HCIb, HCIIa, HCIIb, and HCIII as a reflection of floral change through time.

[24] For example, the HCIa zone is dominated by "Dryophyllum" subfalcatum, Leepierceia preartocarpoides, "Vitis" stantonii, and "Celastrus" taurenensis, and is located 55 to 105 meters below the K–Pg boundary layer.

Although the HCIb zone is a very thin layer, about 5 meters of rock, it bears unusually high diversity of herbaceous and shrubby plants, including Urticaceae, Ranunculaceae, Rosaceae, and Cannabaceae.

[29] However, there was an extremely high angiosperm diversity—common plane trees, "Dryophyllum" subfalcatum, Leepierceia preartocarpoides, and palm trees—along with extinct cycadeoid Nilssoniocladus, Ginkgo, araucariaceous, podocarpaceous, and cupressaceous conifers.

On the other hand, there is a great increase in the abundance of fossil fern spores in the two centimeters of rock that directly overlies the impact fallout layer.

Among the localities, the Mud Buttes, located in Bowman County, North Dakota, is probably the richest megaflora assemblage known and the most diverse leaf quarry from the Hell Creek Formation.