In addition to its cultural and historic significance, Pompeys Pillar is a living classroom for natural history.
The Pillar and the cliffs across the river are composed of sandstones and shales of the Upper Cretaceous Hell Creek (Lance) Formation. The Hell Creek Formation, ranging from 75-65 million years ago, represents the last strata of the Cretaceous Period. The Hell Creek Formation has been found to contain the fossilized remains of dinosaurs and primitive mammals. Although no animal or plant fossils have been documented at Pompeys Pillar, significant fossils have been found in similar sandstone beds nearby.
In fact, William Clark may been the first to record a paleontological find in the area immediately downstream from Pompeys Pillar. “I employed my Self in getting pieces of the rib of a fish which was Semented within the face of the rock. ...it is 3 feet in length tho a part of the end appears to have been broken off.” It is thought that this reference is to the discovery of a fossilized rib in the uppermost Hell Creek Formation. The rib probably came from a terrestrial dinosaur. The most common terrestrial dinosaurs of that period in this area were the hadrasaurus, triceratops, albertosaurus and tyrannosaurus.
Yellowstone River and Associated Riparian Habitat
The Yellowstone can be characterized as a meandering braided prairie river. The Yellowstone is the longest free-flowing river in the lower 48 states and provides rich, fertile farmland and habitat for many wildlife and bird species. The riparian areas along the river contain several cottonwood community types. Many standing cottonwood trees within the flood plain are estimated to be more than 100 years old.
The area is more than a static piece of history or a crossroads for bygone cultures. It is alive with wildlife. Clark noted seeing wildlife in abundance here and elsewhere along the Yellowstone, “for me to mention or give an estimate of the different Species of wild animals on this river particularly Buffalow, Elk Antelopes & Wolves would be increditable. I shall therefore be silent on the subject further.” Pompeys Pillar is still home to many wildlife species and is designated a Watchable Wildlife Viewing Area. Deer, fox, coyotes, raccoons and numerous small mammals, amphibians and reptiles call the Pillar home. Much of the wildlife population is a result of the site’s thriving riparian zone, a healthy plant community of grasses, willows and cottonwood trees that stabilize the river bank and provide important habitat. At the time William Clark traveled through the area, he made note of seeing hundreds of buffalo, elk, wolves and deer. Today, although the area supports a variety of wildlife, it is not to the extent nor diversity that Clark noted. The more common species include mule and whitetail deer, raccoon, fox, bobcat, coyote. There have been a few mountain lion sightings.
The Yellowstone River near Pompeys Pillar supports a remarkable diversity of bird species. Many neo-tropical migratory bird species use the riparian corridor for nesting and/or as safe resting cover during migration. More than 160 bird species have been observed on or near Pompeys Pillar, including waterfowl, songbirds, shorebirds, woodpeckers, raptors, swallows and game birds. Some birds observed in the area are on the BLM's sensitive species list: the ferruginous hawk, the loggerhead shrike, Franklin’s gull, Forster’s tern, the northern goshawk, the sage grouse, the peregrine falcon. Peregrine falcons have historically occupied rocky cliff habitat near Pompeys Pillar. Twenty-one peregrines were released east of Pompeys Pillar in 1996.
The Yellowstone River near Pompeys Pillar is a transition zone between a cold and warm water fisheries. As such, fish species representative of both temperature zones have been recorded in this reach. Common fish species include goldeye, common carp, flathead chub, emerald shiner, western silvery/plains minnow, river carpsucker, shorthead redhorse, longnose sucker, white sucker, mountain sucker, channel catfish, stonecat, burbot, smallmouth bass and sauger.
Threatened and Endangered Species
Two threatened/endangered species occur in the Pompeys Pillar area or have biogeographical ranges that overlap the area. Bald eagles (threatened) have occupied nests in the vicinity of Pompeys Pillar, but no black-footed ferrets (endangered) have been found in the vicinity of the Pillar.