Bulletin of the Geological Society of America (1923) (14595362747)

Identifier: bulletinofgeolo341923geol (find matches)
Title: Bulletin of the Geological Society of America
Year: 1890 (1890s)
Authors: Geological Society of America
Subjects: Geology
Publisher: (New York : The Society)
Contributing Library: Smithsonian Institution Libraries
Digitizing Sponsor: Biodiversity Heritage Library



Text Appearing Before Image:
m-nants of these rocks may be seen as rock-defended terraces along bothsides of the valley, in many places overlying the sediments of the CbuChua formation. The separate flows are nearly horizontal, but locallyshow dips up to 20 degrees. POST-TERTIARY ROCKS These are of three types: (1) the glacial drift and erratics wliichmantle the gently undulating uplands beyond the rim of the trench; (2)the giacio-fluvial and Eecent gravels, sands, and silts are now carved GEOLOGY OF THE TRENCH 567 into a series of flats and terraces clinging to the sides of the main valleyand its tributaries to a height of 1,000 feet above the bottom, and (3)Recent lava flows of small ascertained extent, covering some of the Recentvalley fill in U-shaped glacial tributary trenches. Cretaceous Peneplaix AYhen one views the topography in the vicinity of the trench from analtitude of 4,000 or 4,500 feet, he is struck by the fact that he is stand-ing on a gently undulating elevated plain, into which the North Thomp-
Text Appearing After Image:
FiGURK 4.—2\ort)i Thompson River at Ch:i Chua Lhe view was taken looking east across the river. The foothills on the east side of theriver are underlain by early Eocene sediments. son and its tributary streams have incised deep youthful channels. Fromsuch an elevated point in the vicinity of Chu Chua, for example, a viewdown the valley to the south reveals four important physiographic fea-tures: (1) a dissected upland plain, (2) several steep rocky monad-nocks, (3) the deep youthful valleys, and (4) a very broad, shallow,elongated depression of the upland surface, paralleling the course of theNorth Thompson Valley, intermediate in position between the top of thepresent river valley and the general level of the undulating plain. Thisdepression varies from 15 to 25 miles in width, and its axis lies alongthe course of the present North Thompson River. It represents the 568 AV. L. UGLOW PEXEPLAIX IX BRITISH COLUMBIA valley of the ancestor of the present river as it flowed with a very lo