Geological Survey Bulletin 1292 The Geologic Story of Mount Rainier

The rocks of the cascade Range provide a record of earth history that started nearly 60 million years ago. Even then, as today, waves pounded on beaches and rivers ran to the sea, molding and distributing material that formed some of the rocks we now see in the park.

You may find it difficult to imagine the different landscape of that far distant time. There was no Mount Rainier nor cascade mountain range. In fact, there was very little dry land in the area we call western Washington. Instead, this was a broad lowland of swamps, deltas, and inlets that bordered the Pacific Ocean. Rivers draining into this lowland from the east spread sand and clay on the lush swamp growth. Other plants grew on the deposits, and they were covered, in turn, by more sand and clay. In this way, thousands of feet of sand and clay and peat accumulated and were compacted into sandstone, shale, and coal. We can see some of the rocks formed at that time in cuts along the Mowich Lake Road west of the park (fig. 1). Seams of coal were mined at carbonado and Wilkeson, 10 miles northwest of the park, during the late 19th and the early 20th centuries.

OUTCROP of gray to brown sandstone and dark-gray to black coaly shale in the Puget Group along the Mowich Lake Road (Fig. 1)

These beds of sandstone, shale, and coal make up a sequence of rocks called the Puget Group, which is 10,000 feet thick. Wave-ripple marks and remains of plants show that the rocks were formed in shallow water fairly close to sea level. How could the rocks have piled up to this great thickness? The coastal plain and adjacent basin must have been slowly sinking, and the influx of sand and clay must have just barely kept pace with the downward movements.

A little less than 40 million years ago, the western Washington landscape changed dramatically. Geologists R. S. Fiske, C. A. Hopson, and A. C. Waters have discovered that volcanoes then rose on the former coastal plain at the site of Mount Rainier National Park and became islands as the area sank beneath the sea. When molten rock was erupted underwater from the submerged flanks of these volcanoes, steam explosions shattered the lava into countless fragments. The resulting debris, mixed with water, flowed as mud across great areas of the submerged basin floor.

You can see rocks formed from these layers of volcanic mud and sand in cuts along the highway on the east side of Backbone Ridge and between Cayuse Pass and Tipsoo Lake. Look there for alternating beds of grayish-green sand stone and breccia, a concretelike rock in which the pebbles have sharp corners. These rocks are known as the Ohanapecosh Formation. Like the Puget Group, the Ohanapecosh Formation is at least 10,000 feet thick. Yet, nearly all of it accumulated in shallow water as western Washington continued to sink slowly during the volcanic eruptions.

The long-continued sinking finally ended after the Ohanapecosh volcanic activity ceased. Western Washington was then lifted several thousand feet above sea level, and the Puget and Ohanapecosh rocks were slowly compressed into a series of broad shallow folds. Before eruptions began again, rivers cut valleys hundreds of feet deep, and weathering of the rocks produced thick red clayey soils similar to those that are forming in some areas of high rainfall and high temperature today. Look for the red rocks formed from these old soils in roadcuts as you drive along the Stevens canyon road about 2 miles southeast of Box Canyon.

The next volcanic eruptions, which may have begun between 25 and 30 million years ago, differed from those of Ohanapecosh time. These volcanoes, somewhere beyond the boundaries of the park, erupted great flows of hot pumice that, being highly mobile, rushed down the flanks of the volcanoes and spread over many square miles of the adjacent regions. The pumice flows were "lubricated" by hot volcanic gas emitted from inside each pumice particle, which buffered it from other particles. Some hot pumice flows were 350 feet deep. The heat still remaining in the pumice after it stopped flowing partly melted the particles to form a hard rock known as welded tuff. Repeated pumice flows buried the hilly landscape and eventually formed a vast volcanic plain. The rocks, which are mostly welded tuffs, are now the Stevens Ridge Formation, which you can see along the highway in Stevens canyon 1-2 miles west of Box canyon. You can recognize the welded tuff by its light-gray to white color and its many flattened and sharp-edged inclusions of darker gray pumice (fig. 2).

OUTCROP of light-gray welded tuff in the Stevens Ridge Formation along the road in Stevens Canyon. The angular dark-gray fragments in the welded tuff are chunks of pumice. (Fig. 2)

Another period of volcanism followed, of still a different kind, when lava flowed outward from broad low volcanoes. The flows were of two kinds: basalt, the kind now erupted by Hawaiian volcanoes, and andesite, the type erupted by Mount Rainier. Individual flows 50-500 feet thick were stacked on top of one another to a total depth of fully 2,500 feet. We know these rocks as the Fifes Peak Formation. They form many of the cliffs and peaks in the northwestern part of the park. You can examine them in cuts along the Mowich Lake Road between Mountain Meadows and Mowich Lake. The time of the eruption of the Fifes Peak lavas may have been between 20 and 30 million years ago.

When the Fifes Peak volcanoes finally became extinct, this part of western Washington changed again. The rocks once more were uplifted and compressed into broad folds parallel to those formed at the end of Ohanapecosh time. The rocks buckled and, in places, broke and shifted thousands of feet along great fractures, or faults.

About 12 million years ago one or more masses of molten rock, many miles across, pushed upward through the Puget Group and younger rocks. When this molten rock cooled and hardened, it formed granodiorite, a close relative of granite. Although most of the molten rock solidified underground, some of it reached the land surface and formed volcanoes at a few places within the area of Mount Rainier National Park.

Granodiorite is probably the most attractive rock in the park. It is mostly white, but it contains large dark mineral grains that give it a "salt-and-pepper" appearance (fig. 3). The large size of the grains is a result of the molten rock cooling slowly at a considerable depth below the land surface — the individual minerals had a long time to grow before the "melt" solidified into rock. In contrast, the rocks formed from lavas that flowed onto the ground surface are generally fine grained because the lavas cooled too quickly for the mineral grains to grow appreciably.

GRANODIORITE looks like granite and has a light-gray speckled appearance. The knife is about 3 inches long. (Fig. 3)

Granodiorite underlies the White River valley, the Carbon River valley, and parts of the upper Nisqually River valley and the Tatoosh Range. You can see it in roadcuts between Longmire and Christine Falls and at several places along the road between White River Ranger Station and White River campground.

After the granodiorite solidified, the foundation of Mount Rainier was complete except for one other landscape change that preceded the birth of the volcano. Not long after the granodiorite was formed, the Cascade mountain range began to rise — not rapidly, but little by little over many thousands of years. As the land rose, rivers cut valleys into the growing mountains so that by the time the new volcano began to erupt, the cascades had already been carved into a rugged range of high ridges and peaks separated by deep valleys. Deep erosion thus laid bare the rock layers in which we today read the geologic history of the park (fig. 4).

GEOLOGICAL CROSS SECTION of Mount Rainier and its foundation rocks from Mother Mountain southward to Tatoosh Range. True-scale cross section is nearly 17 miles long. Slightly modified from U.S. Geological Survey Professional Paper 444, Plate 1. (Fig. 4) (click on image for an enlargement in a new window)