Near the close of the Mesozoic Era the earth was subjected to a series
of intense crustal disturbances that geologists call the Laramide
orogeny (orogeny means mountain-building). The origin and nature of the
forces that bent and cracked the crust are unknown, but current theories
being developed about sea-floor spreading and continental drift may shed
light on this major upheaval that began about 75 million years ago. A
significant effect of the Laramide orogeny was the uplift and contortion
of many of the mountain ranges within what we today call the Rocky
Mountains.
At the onset of the crustal disturbance, the gently rolling landscape of
the Yellowstone region began to warp and flex into large upfolds
(_anticlines_) and downfolds (_synclines_) (fig. 13). Gradually the
mountain-building pressures increased, finally reaching such magnitude
that the limbs of the folds could bend and stretch no further;
thereupon, the rock layers broke and were shoved over one another along
extensive _reverse faults_. The severely crumpled rocks within the Park
area can now be seen only along the north edge and in the south-central
part along the Snake River. In both places, the folds and faults are
especially well displayed by the layered Paleozoic and Mesozoic
sedimentary formations (fig. 9).
One of the most prominent Laramide structural features is a large
anticline in the north-central and northeastern parts of the Park (fig.
14, section B-B′); the road from Mammoth to the Northeast Entrance
crosses much of this feature (pl. 1). Although originally forming a high
mountain mass, the anticline has been eroded so extensively that it no
longer appears mountainous (fig. 18). It displays a broad core of
Precambrian gneisses and schists and is bounded along its southwest
margin by a large reverse fault. Along the fault, the ancient gneisses
and schists have been shoved over rocks as young as Late Cretaceous, a
movement amounting to 10,000 feet or more. The Cretaceous rocks are
those that are now exposed at Mount Everts (fig. 10).
[Illustration: COMMON KINDS OF GEOLOGIC STRUCTURES produced by
deformation of the earth’s crust. An original horizontal rock layer
may be upfolded into anticlines, downfolded into synclines, and
broken by either reverse or normal faults. A fault is a fracture or
a zone of fractures within the earth’s crust along which movement
has taken place. A reverse fault is one generally produced by
compression (squeezing together), and the hanging-wall block has
moved up with respect to the footwall block. A normal fault is one
generally produced by tension (pulling apart), and the hanging-wall
block has moved down with respect to the footwall block. All these
kinds of structures are present in Yellowstone National Park. (Fig.
13)]
HORIZONTAL (undeformed)
REVERSE FAULT (compressional)
Hanging wall
Footwall
Forces
ANTICLINE (upfold)
Crest
Limb
NORMAL FAULT (tensional)
Footwall
Hanging wall
Forces
SYNCLINE (downfold)
Limb
Trough
During the Laramide orogeny, many folds and faults formed in the
northwestern part of the Park, in the area now occupied by the Gallatin
Range (fig. 14, section A-A′). In south-central Yellowstone, the
Paleozoic and Mesozoic sedimentary rocks were tightly folded into three
anticlines separated from one another by synclines and faults (fig. 14,
section C-C′). Movement along one reverse fault in this area was locally
more than 10,000 feet.
As the lands were uplifted and contorted, they came under vigorous
attack by the ever-present agents of erosion. Tremendous quantities of
rock were stripped from the highlands, and the debris was carried by
streams into the adjacent lowland basins and deposited mostly as sand
and gravel. As the highlands continued to rise, the basins continued to
sink, and in a short period of time great thicknesses of basin-fill
sediments accumulated locally. One such deposit, the Harebell Formation
of latest Cretaceous age in south-central Yellowstone (fig. 5), is more
than 8,000 feet thick.
Other similar anticlines, synclines, and reverse faults no doubt extend
far into the interior of Yellowstone National Park, and perhaps entirely
across it in places, but they lie buried beneath a thick capping of
volcanic rocks. Nevertheless, it seems safe to conclude that none of the
Park area escaped the effects of the great forces of the Laramide
orogeny. These forces, regardless of how they originated deep within the
earth, seem to have been compressional (fig. 13), pushing the upper
layers of the earth’s crust from the east and northeast toward the west
and southwest. This interpretation is based on the style of the
structural features just described, which shows that the steep limbs of
folds, as well as the direction of movements along reverse faults, point
toward the west or southwest (fig. 14).
By early Eocene time, about 20 million years after they had begun, the
deformational forces relaxed. But the effects of the giant earth
movements were to last for a very long time. Crustal disturbances of
such magnitude commonly produce conditions deep within the earth which,
in places, gives rise to intense volcanic activity; one such place was
Yellowstone.
[Illustration: CROSS SECTIONS SHOWING GEOLOGIC STRUCTURES in
Yellowstone National Park. These illustrate the possible rock
relationships that might be seen along the faces of vertical slices
of the earth’s crust, if it could be cut and pulled apart (much like
slicing a cake and looking at the different layers). The locations
of the sections are shown on the geologic map, plate 1. Reverse
faults and most folds originated during the Larimide orogeny, and
normal faults originated chiefly during Pliocene and later times.
The arrows indicate the relative movements of fault blocks. Geologic
symbols: Qs, Quaternary superficial deposits; Qb, Quaternary basalt
flows; Qy, Quaternary Yellowstone tuff; Tav, Tertiary Absaroka
volcanic rocks; Mzr, Mesozoic sedimentary rocks; Pzr, Paleozoic
sedimentary rocks; pCr, Precambrian metamorphic (“basement”) rocks.
(Based partly on information supplied by E. T. Ruppel and J. D.
Love.) (Fig. 14)]