Collection: Books and Periodicals

About This Item:

Title: The Tertiary Gravels of the Sierra Nevada of California by Waldemar Lindgren (1911) Collection Type: Books and Periodicals Source(s): Google Books # of Pages: 301 Download PDF: User Login Required Download OCR Text: User Login Required

Date Scanned: 2024-07-14 06:26:22 Item Sourced From: PDF First Created: 2025-03-17 05:53:20 Last Updated: 2025-03-17 05:53:20

Original Held At: Searls Historical Library Location: 161 Nevada City Hwy, Nevada City, CA 95959

Page: of 301  

DOWNIEVILLE QUADRANGLE, 107 reported that the channel was found to widen out to such an extent as to considerably lower the tenor of the gravel breasted. A study of the grades of this channel develops many points of the greatest interest. As shown above, the grade from Poverty Hill up to La Porte was fairly even and averaged 100 feet to the mile. Above La Porte evidences of very serious disturbances are present on every hand. A fault zone which is at least 1 mile in width and which has a general northwest direction crosses the channel at La Porte, the total downthrow on the northeast side being probably 520 feet. The presence of faulting was recognized by both Pettee and Turner, although the full extent of the dislocation was scarcely realized. The first fault is seen in the exposed bedrock in the upper end of the La Porte diggings; the downthrow on the east side is 55 feet, and the gravel beds are bent over the fault scarp, which is nearly perpendicular. Further evidence is seen at the Spanish diggings, a detached body of gravel three-quarters of a mile southeast of La Porte, which is abnormally depressed 200 feet below the level of the old channel. The general direction of the channel was at this point parallel to that of the fault lines, and several slices have evidently been cut off and differentially dropped or elevated. The downthrow is 200 feet to the northeast, but there are at least two benches at intermediate elevations and a higher ridge or “‘horse’”’ of slate separating the area of greater downthrow from that of smaller downthrow in the La Porte diggings. The Clay Bank tunnei has been driven for nearly 3,000 feet in north-northwest direction to open a supposed channel in this vicinity. The elevation of its portal is about 4,800 feet. No gravels of value had been found in 1901, and it is indeed probable that only a fragment of the northeast rim, cut off by a fault, exists here. Possibly if the tunnel were continued several thousand feet farther it might encounter the other ae itis thrown portion of the channel, as described in the following paragraph: From the upper end of the La Porte gravel area, at the Dutch diggings, the channel was drifted northwestward for 500 feet, but at that point the gravel was cut off by a “wall” of ‘“‘lava,”’ which probably means andesitic tuff. These relations would suggest that a downthrow of the northeast side of at least 130 feet had been encountered, this being the distance from bedrock to volcanic covering at this point. There is little doubt that this is the continuation of the same fault zone which depressed the vicinity of the Spanish diggings at the southeast end of the gravel area. The channel has next been traced at the Halsey bore hole, about 2 miles north of La Porte, in a gulch draining into Little Grass Valley at an elevation of 5,370 feet. The section (p. 105) indicates that the boring has penetrated into the deepest part of the channel; the bedrock elevation is 4,938 feet, or 118 feet below that at the Dutch diggings, about a mile distant. About a mile farther up the ridge, in an east-northeast direction, the Feather Fork Gold Gravel Co. has opened the channel by a long tunnel, the elevation of which was determined on information derived from borings. The elevation of the bedrock here is 4,780 feet or 158 feet lower than at the Halsey bore hole. The channel has been successfully drifted for some distance upstream to a point where it widened out greatly, this spreading being accompanied by a lowering of the tenor of the gravel. Two miles farther up the ridge in a northeast direction is the Thistle shaft, which was sunk some 20 years ago by the same company. The deepest bedrock at the shaft has an elevation of 5,030 feet, or almost exactly the same as at the Dutch diggings. From tnis point the channel has been mined upstream for some distance and proved very rich. The’ grade for a mile from the shaft, up to the line of the adjoining property, was 200 feet, the channel being reported as normal. From this point up to Hepsidam the channel is easily accessible by tunnels, and has been almost continuously mined, the grade being throughout about 200 feet to the mile. The property of the Niagara Consolidated Mining Co. is located 2 miles above Gibsonville, at the head of the North Fork of Slate Creek, and extends across the lava-capped ridge for about a mile in a northeust direction; the channel has been mined continuously underneath