The oldest rocks

If we were to walk backward in time at the rate of one century per step, the first step would return us to 1872, the year that Yellowstone National Park was established. But to return to the oldest recorded event in its geologic history, we would have to walk (at 3 feet per step) some 15,000 miles, or three-fifths of the way around the world! Occurring far back in the antiquity of the Precambrian Era—approximately 2.7 billion years ago according to radiometric dating ([fig. 6])—the oldest event resulted in rocks so crumpled and changed by heat and pressure that their original character is obscure. These rocks, having been transformed from still older ones, are called metamorphic rocks. Considered to form part of the very foundation of the continent itself, they are also commonly referred to as basement rocks.

THE ROCKS of Yellowstone National Park, separated into individual units or formations and arranged according to their geologic ages (see [fig. 6]). A formation is a body of rock that contains certain identifying features (such as composition, color, and fossils) which set it apart from all other rock units. The identifying features of each formation provide valuable clues bearing on its origin. Most formations are given formal names, and usually each formation is thick and widespread enough to be recognized over broad areas. Some, however, change character from place to place, and different names may be used in different areas even though the rocks represent the same geologic time interval. (Fig. 5)

[High-resolution Version]

AGE, IN THOUSANDS OF YEARS ROCK FORMATION OR UNIT
40± to present Stream sand and gravel
Hot-spring deposits
9 to 250± Glacial deposits
60 to 600 Plateau Rhyolite
600 Upper Unit, Yellowstone Tuff
600 to 2,000 Rhyolite and basalt lava flows
2,000 Lower Unit, Yellowstone Tuff
2,000+ Rhyolite and basalt lava flows

KINDS OF ROCKS SHOWN IN COLUMNS
Sandstone or stream sand
Conglomerate, glacial moraines, or stream gravels
Volcanic breccia
Shale
Limestone
Dolomite
Lava flows
Welded tuff
Travertine or geyserite

ROCK FORMATIONS AGE, IN MILLIONS OF YEARS PERIOD ERA
Northern part of park Southern part of park
Thick lava flows, welded tuffs, glacial deposits, and hot-spring deposits QUATERNARY CENOZOIC
2-3
Pliocene, Miocene, and Oligocene rocks not known to be present
37-38
Absaroka volcanic rocks Absaroka volcanic rocks (Eocene) TERTIARY
53-54
Volcanic and sedimentary rocks (largely eroded away before Absaroka volcanic rocks were deposited) Pinyon Conglomerate (Paleocene and Cretaceous)
65
Landslide Creek Fm Harebell Formation CRETACEOUS MESOZOIC
Everts Formation (Eroded away before Harebell was deposited)
Eagle Sandstone Bacon Ridge Sandstone
Telegraph Creek Fm ″
Cody Shale Cody Shale
Frontier Formation Frontier Formation
Mowry Shale Mowry Shale
Thermopolis Shale Thermopolis Shale
Kootenai Formation Cloverly Formation
136
Morrison Formation Morrison(?) Fm JURASSIC
Swift Formation Sundance Formation
Rierdon Formation ″
Sawtooth Formation Gypsum Spring Fm
190-195
Woodside & Thaynes(?) Formations Chugwater Formation TRIASSIC
Dinwoody Formation Dinwoody Formation
225
Shedhorn Sandstone Phosphoria Fm and related rocks PERMIAN PALEOZOIC
280
Quadrant Sandstone Tensleep Formation PENNSYLVANIAN
Amsden Formation Amsden Formation
Mission Canyon Limestone Madison Limestone MISSISSIPIAN
Lodgepole Limestone ″
345
Three Forks Fm Darby Formation DEVONIAN
Jefferson Formation ″
Bighorn Dolomite (Not exposed, except for isolated outcrops of some formations in Falls River area, in southwestern part of park) ORDOVICIAN
500
Snowy Range Fm ″ CAMBRIAN
Pilgrim Limestone
Park Shale
Meagher Limestone
Wolsey Shale
Flathead Sandstone
570
Gneiss and Schist (Not exposed) PRECAMBRIAN
2,700

A CENOZOIC 50 M.Y. MESOZOIC 200 M.Y. PALEOZOIC 500 M.Y. PRECAMBRIAN 4.5 B.Y. B Quaternary—Early man 2 Tertiary 65 Cretaceous 140 Jurassic 190 Triassic 220 Permian 280 Pennsylvanian 310 Mississippian 340 Devonian 390 Silurian 440 Ordovician 500 Cambrian 575 First abundant fossils Precambrian 2700 Oldest rocks in Yellowstone Beginning of the earth 4,500 M.Y. C PRINCIPAL EVENTS Holocene Glaciation, canyon cutting, thermal activity, eruption of Plateau Rhyolite Pleistocene Eruption of Yellowstone Tuff and associated lava flow; collapse of Yellowstone caldera; normal faulting 2 Pliocene Regional uplift; large-scale normal faulting and uplift of mountain ranges; deep erosion 12 Miocene Moderate erosion; possibly some volcanic activity 26 Oligocene ″ 37 Eocene Eruption and deposition of Absaroka volcanic rocks 54 Paleocene Laramide Orogeny—folding, faulting, uplift and erosion of mountain ranges; deposition of sand and gravel in subsiding basins 65 Cretaceous Deposition of sediments in oceans and along beaches and river flood plains ... Cambrian 570 M.Y.

THE GEOLOGIC TIME SCALE—the “calendar” used by geologists in interpreting earth history. Column A, graduated in billions of years (B.Y.) and subdivided into the four major geologic eras (Precambrian, for example), represents the time elapsed since the beginning of the earth, which is believed to have been about 4.5 billion years ago. Column B is an expansion of part of the time scale in millions of years (M.Y.), to show the subdivisions (periods—Cambrian, for example) of the Paleozoic, Mesozoic, and Cenozoic Eras; column C is a further expansion to show particularly the subdivisions (epochs—Paleocene, for example) of the Tertiary and Quaternary Periods. The principal events in the geologic history of Yellowstone National Park are listed to the right of column C, opposite the time intervals in which they occurred. The ages, in years, are based on radiometric dating. Many rocks contain radioactive elements which begin to decay at a very slow but measurable rate as soon as the parent rock is formed. The most common radioactive elements are uranium, rubidium, and potassium, and their decay (“daughter”) products are lead, strontium, and argon, respectively. By measuring both the amount of a given daughter product and the amount of the original radioactive element still remaining in the parent rock, and then relating these measurements to their known rate of radioactive decay, the age of the rock in actual numbers of years can be calculated. The decay of radioactive carbon (carbon-14) to nitrogen is especially useful for dating rocks less than 40,000 years old. (Fig. 6)

Gneiss, a coarsely banded rock ([fig. 7]), and schist, a finely banded rock, are the most common kinds of metamorphic rocks in Yellowstone. Originally, the gneiss probably was granite, and the schist was a shale or sandstone. Outcrops of the gneisses and schists occur only in the northern part of the Park ([pl. 1]), where they form the central cores of some mountain ranges such as the Gallatin Range ([fig. 3]). They also lie buried beneath younger rocks in many other areas of the Park.

From the time of the metamorphic event, when the gneisses and schists were formed, until the deposition of sediments of the Cambrian Period (figs. [5] and [6]), there is virtually no record. It is reasonably certain, however, that several times during this 2.1-billion-year interval the region was intensely squeezed and uplifted into high mountains and then deeply eroded. By the end of Precambrian time, approximately 570 million years ago, the ancient Yellowstone landscape had been reduced by erosion to a flat, stark, almost featureless plain, which was soon to be flooded by a shallow sea encroaching from the west. This very old surface is now partly exposed in some places across the Buffalo Plateau, at the north edge of the Park ([fig. 1]).

LAMAR RIVER. View downstream (west) along the Lamar River in Lamar Canyon. The rocks along the river banks are coarsely banded Precambrian gneisses more than 2.5 billion years old, some of the oldest rocks in Yellowstone National Park. (Fig. 7)

Closeup views show coarse banding and texture of the gneiss; minerals include quartz, feldspar, and biotite (black mica).