Geological Survey Bulletin 611 Guidebook of the Western United States: Part A

SHEET No. 23. (click on image for an enlargement in a new window)

Beyond Ritzville are the stations Essig (see sheet 23, p. 168), Paha, and Ruby, but there is little of interest until, as the train descends a valley at milepost 164, the main line of the Chicago, Milwaukee & St. Paul Railway is seen on the left. This road diverges from the main line of the Northern Pacific at De Smet, a short distance west of Missoula and crosses the Coeur d'Alene Mountains by a more direct route than that of the Northern Pacific by Pend Oreille Lake. At Lind the two roads are on opposite sides of the valley, but a mile below the town the St. Paul road crosses the Northern Pacific on a substantial steel and concrete viaduct. Below the crossing the St. Paul line keeps down the valley, but the Northern Pacific climbs the slope beyond the creek so as to pass into another valley draining to the South. In ascending this ridge it is interesting to note that in a number of cuts where the rocks are well exposed the basalt is overlain by white silt or clay and fine sand that covers the entire hillside and conceals every irregularity of the hard rock beneath. This material has been taken as indicating the presence of a lake in glacial time, similar to glacial Lake Missoula, but the absence of lines of stratification (bedding) shows that the material has had a different origin. The question of the origin is bound up in the general Tertiary history of this region.¹

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¹The Tertiary history of Oregon and Washington begins before there was a Cascade Range, at a time when the territory now occupied by that range, as well as much of the region lying to the east, was a lowland in which shallow estuaries extended for long distances from the coast, while to the east were fresh-water lakes. In these shallow waters and on the low land near them was laid down a thick series of beds of shale, sandstone, and coal, now known as the Swauk and Roslyn formations, of Eocene (Tertiary) age. The deposition of these beds was interrupted by a flow of basaltic lava which was the beginning of a great series of flows that reached their maximum development in the succeeding Miocene epoch.

Sheet after sheet of basaltic lava was poured out over a territory 250,000 square miles in extent that included the greater part of Washington, all of eastern Oregon, part of California, and a large area in the Snake River valley of Idaho. (See map of Columbia River plateau on sheet 23.) It is probable, however, that the eruptions in the Washington area began at a somewhat earlier date than those of the Snake River plains of Idaho, which seem to have been in part contemporaneous with the last flows that occurred along Columbia River.

It is a generally accepted view that the molten rock was forced up through great cracks or fissures in the crust of the earth. This conclusion is based mainly on the following evidence: (1) The volume and extent of the lava floods are so enormous that it is difficult to conceive of their eruption from ordinary craters; (2) the rarity of the fragmental materials known as tuffs, breccias, etc., indicates a welling up through fissures without the explosive action characteristic of crater eruptions; (3) old fissures through which the floods of lava came forth have been discovered—fissures now filled with the cooled and hardened basalt in observable connection with overlying sheets of lava.

The basalt was not poured forth in one great outburst, but is made up of a great number of layers or individual flows which in places are thousands of feet in aggregate thickness. About 20 such flows are exposed in some of the bluffs of Columbia River. Each one represents a distinct outpouring of lava, the eruptions being separated by intervals of time, some of which must have endured for centuries, for they are marked by beds of soil in which trees grew to considerable size before being charred and buried by later flows. The eruptions of lava, especially in the later stages of activity, were separated also by periods in which other materials accumulated, consisting of volcanic ash and beds of sand, clay, and gravel laid down in lakes or rivers.

In addition to the massive flows that constitute the greater part of the formation there are beds of fragmental volcanic material, such as bombs and smaller angular fragments of lava. Such materials have either been thrown up to great heights and rained down upon the surrounding country, or, mixed with hot water, have flowed over the surface as mud. The fragmental materials are not so widely distributed as the massive flows.

The quantity of material poured out during this period was enormous. The greatest thickness of the lava and associated deposits is not less than 4,000 feet, but if it averages only 500 feet thick over the entire area, seemingly a very moderate estimate, it would make a mass of 24,000 cubic miles, or a cube nearly 30 miles in height. Even this great volume may be far less than that actually poured out.

Coincident with the later stages of the lava eruptions occurred a subsidence of the area east of the present Cascade Range, and a large lake was formed, in which accumulated material derived from the land area on the west. Before the basaltic eruptions had ceased a great volcanic disturbance occurred in the Cascade region and lava of another kind (andesite) was poured forth. The eruptions were accompanied by explosions, which produced large quantities of tuff, volcanic ash, and pumice. While the volcanic eruptions were in progress, the rocks of the Cascade region were folded and crushed and some what uplifted, and the streams, made more powerful by the increase in grade due to the uplift, carried vast quantities of the andesitic material eastward to the lake basin previously described. In this body of fresh water the waste material was deposited as mud, sand, or gravel, together with some sheets of basaltic lava that apparently marked the last expiring gasp of the giant forces which had previously poured forth such irresistible floods of this material. In the Yakima Valley the beds thus laid down have been named the Ellensburg formation. Here the sediments are coarse, indicating nearness to the margin of the lake and to the source of supply, but farther out in the basin, according to I. C. Russell, the material deposited in this lake is largely white silt, composed chiefly of volcanic dust and ash that were blown out of the new set of volcanoes which then were active to the west. This material is exceedingly fine and forms the White Bluff on Columbia River some distance above Pasco (Pl. XXII, A) and the isolated hills at Fishtrap.

Up to the close of the lake period of central Washington the Cascade Range, as it is known to-day, did not exist. During the deposition of the Ellensburg formation the area now occupied by the range was being worn down by streams, and finally it reached that state of low relief that is called a peneplain. Its surface was then not very far above sea level and it bore little resemblance to the region as it is known at the present time,

The great uplift of the Cascade Range came as the closing event in the Tertiary history or the opening chapter in the Quaternary. These mountains were not formed by a volcanic outburst, although such events were of common occurrence in this country, but they are the result of a gradual uplift of this part of the earth's crust, which produced a great arched plateau about 5,000 feet above sea level.

Volcanic activity was not entirely suspended in this region, and here and there vents were formed from which poured forth lava and volcanic ash, and large cones were built upon the surface of the deeply eroded plateau. These are the great conical peaks which are such majestic features of the Cascade Range.

The next great epoch in the geologic history of this region was that of glaciation, when great ice sheets came down from the north and also from the local mountains. The plain of the Columbia was invaded by two ice masses—one from the northeast by way of Spokane, and the other coming down the valley of Okanogan (o-kan-og'an) River and then across the immediate canyon of the Columbia That great river in this time of flood was temporarily diverted to a course near Coulee City, but on the retreat of the ice it resumed its original channel. The great cold of the glacial epoch left large parts of this region without a cover of vegetation, and much of the soft material laid down in the lake previously described was picked up by the winds and whirled away to the east, where it finally settled as a mantle over the entire region. As the winds which transported the dust were not markedly periodic and as the fine material settled particle by particle like snow, the deposits are homogeneous and not marked by lines of stratification.

Such, therefore, seems to have been the mode of origin and the manner of distribution of the white earth, generally called by geologists loess.

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After reaching the summit at Providence the railway begins the long descent of a little valley which leads eventually into the open plain above Pasco. This is one the least interesting stretches, so far as scenery is concerned, along the Northern Pacific, for the valley is so narrow that the views from the train give no idea of the character of the upland. The valley is carved in basalt, which shows in places white loess mantling everything.

Near milepost 192 a branch line of the road enters the valley from the right and gradually descends along the valley wall. This branch unites with the main line at Connell and serves to connect this place with Coulee City and the Big Bend country to the north.

Below Connell the valley walls grow more rugged, and at Cactus siding (milepost 200) a very prominent wall of lava is seen on both sides of the valley, but more particularly on the right (west). This cliff is about 150 feet high and in the lower part shows beautiful columnar structure (see Pl. XXII, B), but the columns are small, and they may not be visible from the train. The rugged walls continue as far as Mesa, but beyond that station the valley opens and the walls are lower and have lost much of their rugged character.

PLATE XXII.—A (top), WHITE BLUFF OF THE COLUMBIA, 20 MILES ABOVE PASCO, WASH. Composed of white volcanic ash (Ellensburg formation). Man on horseback on top of bluff indicates size of bluff. B (bottom), COLUMNAR LAVA AT CACTUS SIDING, 5 MILES SOUTH OF CONNELL, WASH. On cooling the lava has formed columns or prisms which in places are remarkably regular. The columns stand at right angles to the cooling surface of the flow.