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ORIGIN OF CROSS STRATIFICATION. 25 
of this plain is about one hundred feet above the level of the river, and the bluffs formed by 
the cut edges, limiting the bottom lands, are composed of gravel, sand, and clays; sometimes 
partially cemented by lime. In this, as in most of the subordinate basins traversed by the 
Colorado, its course is exceedingly tortuous; and as the water level is subject to great oscilla- 
tions the channel is constantly changing, and the sediment, with which its current is always 
loaded, undergoing deposition or removal. The sections of these alluvial deposits formed 
along the channel afforded me, at the time we passed—the water then being at its lowest stage— 
many instructive examples of the mode of formation of false bedding and cross stratification 
so frequently observed in the older rocks. Several sketches illustrative of this subject, and 
taken from my note-book, are given in the accompanying wood cuts. 
Fig. 3.— SECTION OF ALLUVIAL BANK OF THE COLORADO. 
@ Surface of bottom lands covered with grass and canes. 
& Beds of clay and sand horizontally stratified. 
e Water line of Colorado. 
dd Channels eroded in }, subsequently filled with curved layers of clay and sand. 
Fig. 4.—ORCsS STRATIFICATION FORMED BY CHANGING CHANNEL.. 
a Old bank covered with small willows, receiving additions from receding channel. 
6 New bank, or sand bar, cross stratified, being added to a. 
¢ Present channel moving from a to d; the dotted line representing the surface of the stream. 
d Oldest bank, being now cut away by current, covered with large cottonwood and willows. 
Sections like this are not unfrequently exposed, where the present stream has cut across its x 
former bed—flowing from a to d, and crossing c. ’ 
The nature of the sediment deposited, whether clay or sand, depends upon the velocity of 
the water in which it was suspended. The bayous and deserted channels, in which the water 
has very little motion, are gradually filled up with argillaceous sediment, while the first deposit 
made by the current water, when its motion is but partially arrested, is sandy—as in the 
formation of sand bars. ee 
The alternations of strata of clay and sand which compose the alluvial banks are simply a 
record of the varying velocity of the current which deposited them. Not unfrequently I noticed 
in a section of the bank a thin stratum of clay which, when it formed the surface layer, had 
been exposed to the sun’s rays, and had cracked ‘and contracted till its fragments were widely 
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