MASSIVE BEDS OF BRECCIA of the Absaroka volcanic rocks along the road north of Dunraven Pass. This breccia formed part of a steep-sided volcanic cone, of which Mount Washburn is a remnant. (Fig. 17)
Closeup view shows very coarse character of the breccia, with large rock fragments imbedded in fine ash, dust, and sand. Nearly all the rocks are of andesitic composition, consisting chiefly of feldspar and pyroxene. Most common colors are medium to fairly dark shades of brown, red, purple, and gray.
Absaroka volcanism, however, was not a simple, continuous process—the eruptions were intermittent, the many volcanoes were not always active at the same time, and between eruptions there were long periods of quiescence during which the erupted material was deeply eroded. The repetitive nature of the eruptions is best illustrated by the famous fossil forests of Yellowstone. Here is striking evidence that enough time elapsed between eruptions for widespread forests to become established on the lower slopes of the volcanoes and in the broad valleys between them. Judged from the great size of some of the now-petrified logs ([fig. 19]), several hundreds of years must have passed before another volcanic outburst smothered the forest. Many different forest layers have been recognized in the Specimen Ridge area as well as in several other places throughout the Park.
As the Absaroka magma rose from deep underground, some of it squirted, like toothpaste, into the layered Paleozoic and Mesozoic sedimentary rocks through which it passed. These relatively small masses of molten rock material slowly cooled and crystallized to form intrusive igneous rocks such as diorite ([fig. 20]). The resulting intrusive bodies, called sills, dikes, stocks, and laccoliths, depending on their form, are most abundant in the Gallatin Range and in the vicinity of the East Entrance ([pl. 1]). At the conclusion of volcanic activity, the last of the rising magma solidified in the main conduits to form slender, somewhat cylindrical bodies of rock called volcanic necks that probably conform closely to the shape of the original conduits. The circular intrusive rock body at Bunsen Peak ([fig. 21]), now exposed to view because erosion has stripped away the lava and volcanic breccia that once completely buried it, represents either a volcanic neck or a small stock that solidified directly beneath a volcano.
MASSIVELY LAYERED BRECCIAS, conglomerates, and sandstones of the Absaroka volcanic sequence at Barronette Peak, as viewed from the road near the Northeast Entrance; the ridge is 3,000 feet high. These rocks, deposited as part of an alluvial plain between volcanoes, once filled the Yellowstone region to a level higher than the top of Barronette Peak, but erosion since late Tertiary time has stripped the volcanics from much of the Park area. The volcanic rocks (Eocene in age, [fig. 5]) rest directly on Paleozoic sedimentary rocks along the line indicated. During the Laramide orogeny, in Late Cretaceous and early Tertiary times, the region was folded and uplifted into mountains. Thousands of feet of Mesozoic and Paleozoic sedimentary rocks were then eroded off the rising mountains before the Absaroka volcanic rocks were deposited, (Fig. 18)
GIANT PETRIFIED TREE TRUNKS in Yellowstone’s fossil forest. The enclosing rocks, part of the Absaroka volcanic sequence that forms Specimen Ridge, are approximately 50 million years old. Many of the tree trunks are still upright, having been smothered and buried in their original positions by breccia, ash, and dust from nearby volcanoes. It is evident that more than one “forest” is represented in this view. Prof. Erling Dorf, of Princeton University, counted a total of 27 different forest layers in the rocks now exposed at Specimen Ridge. He also determined that the most common kinds of trees were sycamore, walnut, magnolia, chestnut, oak, redwood, maple, and dogwood. The nearest living relatives of many of these trees are now found in the warm temperate to subtropical forests of the southeastern and southern United States. (National Park Service photograph.) (Fig. 19)
Mount Washburn is the north half of one of the ancient Absaroka volcanoes ([fig. 26]), and many of the rocks and other features related to this volcano, which characterized this great period of volcanism, can be seen along the road between Canyon Village and Tower. In roadcuts just south of Dunraven Pass several thin igneous dikes cut through volcanic breccias. These dikes radiate outward from the nearby central core of the volcano, which lies east of the highway in the vicinity of Washburn Hot Springs. From Dunraven Pass northward for 2-3 miles, the road is lined with lava flows and very coarse breccias that accumulated close to the volcanic neck ([fig. 17]). Farther north toward Tower Falls, breccias and conglomerates predominate, but the average size of individual rock fragments decreases gradually northward away from the center of eruption. Beds of sandstone then begin to appear in the sequence, having been deposited mainly by streams that drained the north slope of the volcano.