54 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
In the vicinity of Leveridges Ferry, just above the mouth of Rattle- 
snake Creek, the canyon of Snake River is 6 or 8 miles broad, with a 
steep wall capped with basalt on the northern side and a less precipitous 
slope formed entirely of soft sediments on the southern side. As is 
commonly the case throughout the course of Snake River below Glenns 
Ferry, the plain bordering the canyon on the north rises gently north- 
ward, and is covered with fine, yellowish-white soil. In the precipi- 
tous and in part vertical escarpment forming the northern side of the 
canyon the following section was obtained, but the details change in 
a conspicuous manner, owing principally to variations in the number 
of sheets of basalt present, when the cut edges of the strata are fol- 
lowed even short distances, either up or down stream: 
Section of Snake River Canyon near mouth of Bruneau River. 
Feet 
1. Columnar basalt, compact, contains numerous flattened steam holes an inch 
in length, but is not glassy, and does not rest on a bed of lapilli or breccia. . 25 
2. Horizontally and even-bedded sandy clay, in part clean, loose sand, largely 
concealed beneath talus from basalt cliff above 210 
Terrace about 1,000 feet wide, covered with well-rounded quartzite cobbles 
and sand; surface irregular. This layer of cobbles is from 5 to 15 feet 
thick, but is not a part of the section, having been deposited during the 
excavation of the canyon. 
3. Fine, grayish-white, evenly laminated clay 35 
4. Parting of white sand, unconsolidated; can be traced along the face of the 
bluffs for a mile or more \ 
5. Fine, grayish-white clay, like No. 4 20 
6. Fine, evenly laminated pinkish clay 30 
7. Clean, well-rounded pebbles 1 
8. White sandy clay or silt 4-5 
Terrace, 200 to 300 feet wide, covered with cobbles. 
9. Basalt, highly scoriaceous, glassy ( lower rim rock) 40 
10. Fine, unconsolidated sandy clay, base concealed 35 
11. Old flood plain, surface level, composed of fine silt 35 
12. Modern flood plain, overflowed during high water 15 
Total, about 400 
The conspicuous variations which occur in the number and thick- 
ness of the lava sheets exposed near the mouth of Rattlesnake Creek 
will be noted later, as well as the significance of the striking differ- 
ences between the basalt which forms the two rim rocks present where 
this section was observed. The numerous minor variations exhibited 
by the sedimentary beds in this same region might be shown by many 
sections, but space will permit of but one more. 
On the south side of Snake River Canyon rj at a locality termed Shell 
Mountain, about 10 miles below Glenns Ferry and nearly opposite the 
mouth of Bennett Creek, the bold bluffs immediately bordering the 
river are composed almost entirely of fine, unconsolidated, nearly 
