Geological Survey Bulletin 614 Guidebook of the Western United States: Part D. The Shasta Route and Coast Line

GUIDEBOOK OF THE WESTERN UNITED STATES.

PART D. THE SHASTA ROUTE AND COAST LINE.

By J. S. DILLER and others.

From Alaska to Lower California the western coast of North America is bordered by a belt of mountains which is from 100 to 200 miles wide. This belt as a whole has been named the Pacific System. It separates the Pacific Ocean on the west from an interior plateau region, which also stretches from Alaska to Mexico. This plateau is of varied character but, as is well illustrated by the State of Nevada, much of it is arid and is seamed with mountain ranges.

Attention need be directed here only to that part of the Pacific System which lies chiefly in the States of Washington, Oregon, and California, a section 1,300 miles in length. (See Pl. I.) Within these three States the Pacific System embraces a number of distinct mountain ranges from which project some of the loftiest peaks on the continent. It includes also broad valleys that are in part close to sea level. Consequently it is a region of strong contrasts in altitude and of wide diversity of scenery and climate. Before the snow disappears each summer from the crest of the Sierra Nevada the burning sun in the interior valley of California has yellowed the grain and has tinted the fruit in orchard and vineyard. Throughout the year the snow cap of Mount Rainier is in full sight from the fertile valleys of Washington, where winter severity is unknown.

PLATE I.—RELIEF MAP OF THE COUNTRY TRAVERSED BY THE SHASTA ROUTE AND COAST LINE. (click on image for an enlargement in a new window)

For its entire extent, from Alaska to Lower California, the Pacific System may in general be divided lengthwise into three parts—two long lines of mountain ranges and an intervening belt of valleys that may be called the Pacific valley belt. The northernmost member of this belt that need be considered here is the valley of Puget Sound. A low divide separates the sound from Cowlitz Valley, the next member of the belt to the south. Cowlitz Valley drains southward to the Columbia and is succeeded on the south by Willamette (wil-lam'et) Valley, whose waters flow northward into the Columbia. Willamette Valley extends southward for about 120 miles, or about halfway across the State of Oregon. Between its head and the north end of the Great Valley of California, the next member of the Pacific valley belt, there is a tract of generally mountainous country, where, for 200 miles, the threefold longitudinal division of the Pacific System into two ranges and an intervening belt of valleys is less evident than it is elsewhere. South of this interrupting mass of mountains, which is dominated by Mount Shasta, the Great Valley of California stretches for more than 400 miles, having a width in general of 40 to 50 miles. Its northern part, known as the Sacramento Valley, drains southward, and its southern part, known as the San Joaquin Valley, drains northward, Sacramento and San Joaquin rivers both flowing into San Francisco Bay. The south end of the San Joaquin Valley is separated by the transverse Tehachapi Mountains from the Mohave Desert. This in turn is separated by the San Bernardino range from the Salton Basin, which is nothing more than the former upper part of the Gulf of California, the sea having been excluded by the great mass of sediment dumped into the gulf by Colorado River. By some the Mohave Desert and the Gulf of California are regarded as parts of the Pacific valley belt. By others, however, notwithstanding the general alignment of these features with the Great Valley of California, they are regarded as parts of the plateau region that lies east of the Pacific System of mountains.

As the Pacific valley belt separates in general two parallel ranges of mountains, so these ranges in turn may be subdivided into mountain groups that are sufficiently distinct in form or structure to have received individual names.

At the north end of the western hue, between Puget Sound and the ocean, stand the Olympic Mountains, which attain a height of 8,000 feet. Very little is known of their structure or of the rocks that compose them.

South of the Olympic Mountains, stretching along the coast of Washington and Oregon for 350 miles, is a comparatively low range which, to distinguish it from the similarly situated range in California, may be called the Oregon Coast Range. This consists chiefly of rather soft sandstones carrying fossil shells from which it has been determined that the sands were deposited in the sea during early and middle Tertiary time. Lavas associated with these rocks show that volcanoes were active in this region in early Tertiary time. These rocks are no longer in the original horizontal position in which they were deposited but have been crumpled by pressure. Moreover, in some places they have been traversed by long cracks or fissures, and the rocks on one side of a crack have been shoved past the rocks on the other side. The geologist would describe these effects briefly by saying that the beds have been folded, fractured, and faulted.

South of the Oregon Coast Range, partly in Oregon but chiefly in California, are the Klamath Mountains. They are composed of rocks that are older and more complicated in structure than those of the Oregon Coast Range and are closely related in materials and in structure to the Sierra Nevada. The Klamath Mountains extend south-southeastward into California for 150 miles and overlap to the east for 80 miles the north end of the California Coast Ranges.

The California Coast Ranges extend southward for more than 600 miles to the vicinity of Point Conception, where the coast turns sharply eastward along the Santa Barbara Channel. They consist of rocks of great variety of character and of widely different geologic ages. Their structure also is exceedingly complex. The most abundant rocks were formed in the Tertiary, Cretaceous, and perhaps Jurassic periods. Most of these rocks were deposited as sediments on a sea bottom, but associated with the rocks so formed are in places layers of lava or masses of igneous rock, which were forced in molten condition into the sedimentary rocks. All have been folded and faulted at several periods. Finally erosion, or the action of moving water, in the form of rain, streams, and waves, aided by the crumbling effect of weather, carved out the hills as they now appear. The resulting forms reveal to one skilled in their interpretation some indication of the complexity of the underlying rock structure.

At their south end the Coast Ranges meet another set of ranges, which have a more easterly trend than the Coast Ranges proper and do not fall readily into the general threefold division of the Pacific System of mountains that is so clearly recognizable farther north. The suggestion has been made that these mountains should be distinguished from the Coast Ranges, with which they have commonly been included, by designating them the Sierras de los Angeles, from the city of Los Angeles. Well-known individual ranges classed in this group are the Santa Ynez, San Gabriel, and San Bernardino.

Along the east side of the Pacific valley belt stretch two great ranges—the Cascade Range on the north and the Sierra Nevada on the south. The Cascade Range begins a few miles north of the boundary between Washington and British Columbia and continues southward for 650 miles to the vicinity of Mount Shasta in California. It has a foundation of granite and of sedimentary rocks, such as sandstone and shale, ranging in age from Carboniferous to Miocene. Some of the sedimentary rocks and some associated igneous rocks have been changed by heat and pressure into crystalline laminated rocks known as schists. From Mount Rainier southward the rocks just mentioned are buried to an increasing extent under lavas which probably flowed from many vents at different times. The volcanic cones, such as Shasta, Hood, and Rainier, which form striking features of the Cascades, represent a comparatively late stage of volcanic activity and have been piled up on a preexisting mountain range.

The Sierra Nevada which continues southward the general line of the Cascade Range, is of somewhat different structure and history. It is a great inclined block 350 miles long and 80 miles wide, with a gentle slope to the west and an abrupt descent on the east to the deserts of Nevada. This abrupt descent marks a line of faults, east of which the desert country has gone down and west of which, to some extent, the Sierra Nevada has gone up. The Sierra Nevada consists of a great irregular mass of granite which in early Cretaceous time was intruded in a molten condition into sedimentary and older igneous rocks, both of which sets of rocks, in consequence of the heat and squeezing which accompanied this intrusion, were changed in part to schists and slates. After the range had been deeply worn down by erosion, floods of lava were poured over the surface in Tertiary time and the range was tilted up to its present general form. That form, however, has been much modified in detail by comparatively recent erosion and by the canyon cutting of the present streams.

South of the Dominion of Canada three rivers, the Columbia, the Klamath, and the Pit, flow across the Pacific System from the interior plateaus to the ocean. Other large streams, such as the Willamette, in Oregon, and the Sacramento and San Joaquin, in California, flow northward or southward for many miles in one or another of the longitudinal depressions of the Pacific valley belt before turning westward across the western line of mountains to reach the Pacific.

The valley belt is the great agricultural region of the Pacific coast, and in Washington and Oregon, owing partly to its accessibility to ocean-going ships by way of Puget Sound and the Columbia, it contains most of the population. In California, however, the broad fields, orchards, and vineyards traversed by the Sacramento and San Joaquin are rivaled by the many smaller but fertile valleys of the Coast Ranges. The superb harbor of San Francisco sufficiently accounts for the situation of the metropolis; and the rapid growth of Los Angeles, the second city in the State, is due to a combination of shipping advantages with highly profitable kinds of agriculture and extensive resources in petroleum

NOTE.—For the convenience of the traveler the sheets of the route map in this book are so arranged that he can unfold them one by one and keep in view the one to which the text he is reading relates. A reference is made in the text to each sheet at the place where it should be so unfolded, and the areas covered by the sheets are shown on Plate I. A list of these sheets and of the other illustrations, showing where each one is placed in the book, is given on pages 137-138. A glossary of geologic terms is given on pages 133-136 and an index of stations on pages 139-142.