Geological Survey 12th Annual Report (Part I) The Eruptive Rocks of Electric Peak and Sepulchre Mountain, Yellowstone National Park

In order to obtain a clear idea of the geological relations between the various groups of eruptive rocks coming within the scope of this paper, it will be necessary to sketch briefly the leading features of the geology of the region; the more so since the connection between the intrusive and extrusive bodies must be traced across a profound fault, which has affected a large area, and has permitted subsequent erosion to expose deeply seated intruded bodies by the side of contemporaneous surface extrusions.

Electric Peak, 11,100 feet in altitude, lies on the northern boundary of the Yellowstone National Park, 10 miles from its western line. It is the highest point of that portion of the Gallatin Mountains situated within the Park limits. These mountains have been carved out of a block of sedimentary strata composed of limestones, shales, and sandstones of Paleozoic and Mesozoic age, which range from Cambrian to Cretaceous. This block, about 14 miles wide, at present occupies the trough between two great bodies of Archean rocks, and trends northwest and southeast. It has been subjected to a succession of dynamical forces, which have bent it into a general synclinal fold, the axis of which lies near the northern body of Archean, and trends northwest and southeast. They have also produced a number of smaller transverse folds and faults with a nearly north-northeast and south-southwest trend.

The general synclinal movement was accompanied by a series of intrusions of igneous rocks, which found their way between the sedimentary strata, wherever the fissile character of the beds presented planes of least resistance to the dynamical forces engaged in bending them. These intruded masses formed immense laccolites and thinner sheets, that penetrate the more fissile strata for miles, with only occasional changes of horizon.

One large body of eruptive rock is located about 4 miles southwest of Electric Peak, and appears to have been the source of a great number of the sheets, which are intercalated between the Cretaceous shales and sandstones of this mountain. As a result of the main synclinal movement the strata at Electric Peak have a general dip toward the northeast.

After the intrusion of the eruptive sheets a more local synclinal break occurred in the neighborhood of what is now Electric Peak, its axis trending northeast and southwest. The southeastern side of the fractured mass suffered the greater displacement, the strata being turned up vertically in some places. This break produced one or more large fissures and numerous smaller crevices, along which igneous rock was again forced through the shales and sandstones, in the form of a stock and dikes. The stock is located near the axis of the break, and the dikes, which are mostly vertical, branch out into the sedimentary beds for a short distance, and cut across the intruded sheets or cut between them where they had been previously turned up on end.

The igneous magmas which accompanied the convulsive movements of the ruptured strata and forced their way between them to cool as intrusive bodies, also reached the surface of the earth in places and took the form of extrusive masses. The ejected rocks were probably erupted from a number of different vents whose position was governed by the nature and extent of the fissures in the sedimentary rocks. They poured out as flows or massive eruptions and were subsequently blown to pieces and thrown into breccias and were occasionally cut by dikes. They undoubtedly formed very extensive bodies of volcanic ejectamenta which covered a large area of country.

After the intrusion of the stock and dikes just mentioned, and after the accumulation of the volcanic breccias, the region was broken by great faults. These faults trend nearly north and south and have caused great changes in the relative vertical position of the severed rocks, so that, after extensive erosion, deep-seated strata and intrusive bodies of erupted rocks are now exposed by the side of extravasated surface lavas.

The great erosion which carved the faulted blocks into the steep mountains and valleys of the Gallatin Range was followed by the eruption of the vast flows of rhyolite and less abundant basalt that form the plateau country to the south, since which time glaciation and erosion have still further modified the contour of the country.