LINDQEEN.] GRAVELS OF LOWER MOORE CREEK VALLEY. 659 



preserved on the east side of the creek, sometimes on the west side, 

 which makes it necessary to well establish the course of the channel 

 before commencing extensive work. The old gravel is found at ele- 

 vations of from 25 to 75 feet above the creek, and has been exposed 

 by a number of tunnels. The gold is fairly coarse, the particles 

 being about the size of mustard seeds, but it can not be said that 

 these old placers have been shown to be very rich. They have not 

 as yet been extensively worked. At a place 1 mile above Half Way 

 House, on the east side of the creek, 6 feet of granitic gravel, covered 

 by 3 feet of sand, is exposed below 30 feet of basalt. One-half mile 

 farther up the old channel is well exposed by two tunnels on the 

 western side of the creek. The old bed rock lies 25 feet above the 

 stream, and the deposit has been developed by 175 feet of tunnels. 

 Above the bed rock lies 8 feet of coarse gravel with a streak of sand 

 in the middle; 70 feet of solid basalt covers this. The gravel is 

 reported to contain about 65 cents per cubic yard. As the gravel is 

 not extremely compact, and as the basalt forms an excellent roof, it 

 might be possible to profitably exploit many of these small stretches 

 of old channels by underground hydraulic operations. 



High gravels. Though the lower part of Moore Creek has not been 

 thoroughly examined, it has been shown that gravels exist on some of 

 the high ridges within this drainage. The study of these high gravels, 

 which are probably of Tertiary age, is of the highest importance, as 

 it enables us to trace more fully the geological history of the region. 

 The largest of these high gravels was found on the summit of a high 

 and narrow ridge between the two forks of Thorn Creek, at an eleva- 

 tion of 4,500 feet. This deposit, which consists of extremely well- 

 washed gravel of granitic and quartzose character, has been worked 

 to some extent by the hydraulic process and found to contain gold in 

 paying quantities. The extent from east to west is only a few hundred 

 feet. The depth of gravel is probably considerable, although difficult 

 to estimate on account of slides on the steep slopes. Small patches of 

 gravel are also reported to exist on the neighboring ridges. It is 

 probable that during the highest stand of the Neocene lake which 

 occupied the whole lower basin of the Snake River gravel deposits 

 filled the upper parts of Moore Creek and the Boise River. The 

 occurrence of this high gravel will be discussed again later in con- 

 nection with the geological history of the Idaho Basin. 



UPPER MOORE CREEK VALLEY. 



Configuration of the valley. Five miles below Idaho City the form 

 of the valley suddenty changes. From a level bottom the slopes rise 

 gradually, numerous creeks branch, and the whole forms a broad, 

 basin-like depression. The general character is well shown in PL 

 XCIII, looking southwest from Idaho City. Three miles above Idaho 

 City a narrow canyon begins again. Seen from some elevation, the 



