108 Part II. Chapter 1. 
c. Columbia Plateau. 
West of the Rocky Mountains is a great plateau region which is drained 
by tributaries of the Columbia River. Hence it is called the Columbia 
Plateau. It is a complex of tablelands of diastrophic and volcanic origin, 
relieved by a few great mountains and with many beautiful valleys. The 
region in Cretaceous time 'had extensive plains, broad valleys high mountains 
and some portions were under the sea. Granite, quarzite, and mica-schist 
were the rocks with those of later age on their flanks. Extensive diastropism 
occurred in the Eocene and with it volcanic activity by which mountain cones 
were erected. Later than this in geologic time (Neocene), the more fluid 
lavas broke out in many new places forming thin coulees and frequently filling, 
or obstructing valleys. These eruptions continued for a long period, until the 
old topography was completely changed. Lakes were formed by the damming 
of valleys and some of the best agricultural land occurs where these lakes 
were subsequently drained. The streams of the region have cut narrow canons. 
Snake River runs for several hundreds of miles through a canon carved in 
the lavas and is the most important stream of this district. ‘The walls of 
the gorge are often precipitous and late lava coulees, stretching across the 
river bed, are the cause of waterfalls of which Shoshone Falls is the most 
notable example. The region of the Columbia Plateau comprises large portions 
of the present states of Washington, Oregon and Idaho with a small extension 
into the northern edge of Nevada and Utah. 
d. Colorado Plateau. 
The Colorado Plateau, or system of plateaus was covered by a shallow sea 
during the Cretaceous period. This great extension of the ocean stretched 
far to the east, northeast and southeast nearly to a line drawn east of the 
present Mississippi River. The Park Mountains formed an archipelago of islands 
during this time. At the close of the Cretaceous and in the early Eocene 
periods, the sea water gradually retreated by an irregular upheaval. The 
drainage waters were impounded by this upheaval and great basins were 
formed which became great inland lakes. The slow upheaval continued through 
Eocene time with frequent vulcanism. Great plateaus were also formed to be 
subsequently modified and dissected by the rains and the streams. The rocks 
forming beneath the sea were in a horizontal position and when the upheaval 
occurred, they became dry land and were inclined at an angle to their former 
position. As the elevation of the region continued, all the lakes were drained 
by the cutting of outlet channels and the whole region became arid. In late 
Neocene time, when the lakes were drained, their bottoms became valleys 
which were occupied by streams running into the Colorado and the Rio 
Grande del Norte. Thus, this plateau region is dissected by a vast network 
of streams. The streams usually have deep channels. Rills born of showers 
have often cut deep but larger gorges and the rivers have cut mighty canyoN®. 
