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 PERIOD ERA
MILLIONS
OF YEARS
Northern part of Southern part of
park park
Thick lava flows, welded tuffs, glacial QUATERNARY _CENOZOIC_
deposits, and hot-spring deposits
2-3
Pliocene, Miocene, and Oligocene rocks
not known to be present
37-38
Absaroka volcanic Absaroka volcanic TERTIARY
rocks rocks (Eocene)
53-54
Volcanic and Pinyon Conglomerate
sedimentary (Paleocene and
rocks (largely Cretaceous)
eroded away
before Absaroka
volcanic rocks
were deposited)
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 & Chugwater Formation TRIASSIC
Thaynes(?)
Formations
Dinwoody Formation Dinwoody Formation
225
Shedhorn Sandstone Phosphoria Fm and PERMIAN _PALEOZOIC_
related rocks
280
Quadrant Sandstone Tensleep Formation PENNSYLVANIAN
Amsden Formation Amsden Formation
Mission Canyon Madison Limestone MISSISSIPIAN
Limestone
Lodgepole Limestone ″
345
Three Forks Fm Darby Formation DEVONIAN
Jefferson Formation ″
Bighorn Dolomite (Not exposed, ORDOVICIAN
except for
isolated outcrops
of some
formations in
Falls River area,
in southwestern
part of park)
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.
[Illustration: 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).
[Illustration: 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)]
[Illustration: Closeup views show coarse banding and texture of the
gneiss; minerals include quartz, feldspar, and biotite (black
mica).]