Geology and Age 
The White River Group, South Dakota

The oldest rocks in the White River Group are Late Eocene (late Chardonian Age) to mid-Oligocene and range from 30 to 40 million years old. In South Dakota, the Chadron Formation consists of siltstones, sandstones and sandy conglomerates which were deposited in stream channels.

At the top of the Chadron Formation, these sediments shift into the Brule Formation which itself is divided into the lower Scenic Member and the Poleslide Member. The Scenic Member dates to the Middle Oligocene, 35 to 32 million years old and is composed of siltstones and sandy siltstones with some infrequent channel conglomerates. The Poleslide Member is Late Oligocene and dates from 32 to 25 million years old. These beds contain sediments which are sandier than the Scenic Member, and like beds in the early Oligocene, contain occasional ash beds.

Overall the White River Group was deposited in an alluvial flood plain environment characterized by siltstones, sandstones and lenticular conglomerates. These conditions were perfect to facilitate the preservation of many unique and interesting mammal species.

Written by Ryan C

The climate during the Oligocene was one that is quite different than that of today. By looking at the types of rocks found in these bed and the fossils within them, a picture of the Oligocene paleo-environment begins to reveal itself. The prominence of clays and lenticular sandstones indicates a mature river valley with a relatively flat topography, meandering rivers, punctuated by lakes and swamps. The land may have been covered by small shrub-like vegetation, with perhaps forests in parts. The landscape was populated by a great diversity of animal life, particularly. Mr. O'Harra writes a beautiful description of the life during the Oligocene:

"Here the gently flowing streams push their muddy way through reedy marshlands and vigorous forests and furnish a lazy playground for countless turtles and occasional crocodiles. In favored recesses groups of rhinoceroses may be seen, some heavy of bulk and water loving, others graceful and preferring dry land. Little fleet-footed ancestral horses nibble the grass on the hillsides or trot unhindered across the muddy flats. Here and there we see a group of predaceous dogs and not infrequently do we get a glimpse of a ferocious tiger-like cat. On the higher ridges, six horned herbivores reveal their inquisitive pose, and scamper from the nearing presence of some stealthy foe. But the dominating beast is the titanothere. In great numbers we see his majestic form as he moves among his kin and crops at his leisure the coarse grasses of the lowlands. Here and there are gophers and squirrels busy with their toil and their play, and hedgehogs and moles and swine and deer and tapirs and camels, and many other creatures too strange to mention without definition. To all these animals it was home. Here they fought for food and life and supremacy. To them the sun shone, the showers came, the birds sang, the flowers bloomed, and stately trees gave convenient shade to the rollicking young of many a creature.

But 'everlasting hills' have their day and rivers do not flow forever. One by one, group by group, they died, the bodies of most of them quickly feeding the surrounding elements but a chosen few, tucked away by the kindly hand of nature, remaining as unique monuments of the dawning time of the great mammalian races, are now being revealed as gently by nature again in these days of man."

Such a vibrant description!

Written by Ryan C
Quotes from Cleophas C O'Harra