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.
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.
[Illustration: Mount Rainier]
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.
[Illustration: Outcrop of gray to brown sandstone and dark-gray to black
coaly shale in the Puget Group along the Mowich Lake Road. (Fig. 1)]
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 sandstone 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).
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.
[Illustration: 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)]
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.
[Illustration: Granodiorite looks like granite and has a light-gray
speckled appearance. The knife is about 3 inches long. (Fig. 3)]
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 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.
[Illustration: 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)]
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).