Mr. Goodyear read the following paper:
Salt Spring Valley and the adjacent region in Calaveras County.
BY W. A. GOODYEAR.
Having spent some time during the past summer in Copperopolis, and the region lying west and northwest from it, I offer the following observations respecting its topography and geology. I will first notice the
TOPOGRAPHY.
For a general description of the topography, etc., of Calaveras County, including the main features of the region in question, reference may be made to Prof. J. D. Whitney’s Report upon the Geology of California, Vol. I, p. 253. In addition, however, to what is there stated, I will say that Copperopolis lies at the southwestern base of Bear Mountain, the summits of which rise to an altitude of something more than 2,000 feet above the sea. The Gopher Hills, also mentioned in the report, form a well defined and connected, though subordinate range, lying to the southwest of, and nearly parallel with the general course of Bear Mountain. This range forms a prominent feature in the topography of the region for a distance of at least fifteen or eighteen miles southeasterly from the Calaveras river. Its summits are probably 1,400 feet above the sea, and the lowest break or gap within the distance named is that through which Rock Creek finds its way to the plains below. The valley or depression between the Gopher Hills and Bear Mountain, whose average width is four to six miles, has received the name of Salt Spring Valley. Its general altitude is little less than 1,000 feet above the sea, that of the town of Copperopolis being nine hundred feet according to H. P. Handy’s survey of a railroad route from Copperopolis to Stockton. I should mention that for several miles northwesterly from Copperopolis, Bear Mountain has an outlier along its southwestern base, in the form of a low but tolerably well marked hilly ridge, between which and the base of the mountain is a narrow but continuous valley; and it is in this valley that the copper-bearing belt of Copperopolis is found. Southwest of this outlier, and for a distance of three or four miles northwesterly from Copperopolis, Salt Spring Valley consists mainly of a region of low hills, traversed by a net-work of steep and narrow gulches. Farther northwest the surface of the valley for three or four miles is more uniform, and here we find the nearly level area of “Tower’s Ranch,” and the gently sloping basin of the “Salt Spring Valley reservoir.” Beyond this, the country is again hilly to the Calaveras river. Southeast and south of Copperopolis, the surface is everywhere hilly. The slope of the Gopher Hills towards the southwest is rapid until we reach the low rolling country which forms the border of the San Joaquin Valley.
Black Creek debouches from Bear Mountain a mile or so southeast of Copperopolis, and flows to the Stanislaus. Littlejohn’s Creek takes its rise in the hilly regions of the valley west of Copperopolis, and flowing southwesterly, finds its way through the hills into Rock Creek. The latter rises in Bear Mountain, five or six miles northwesterly from Copperopolis, and flowing southwest across Salt Spring Valley, breaks through the Gopher Hills, and continues its course through the lower country to French Camp Slough, a branch of the San Joaquin. All these creeks become dry in the summer, though in winter they often carry very large volumes of water. At the point where Rock Creek breaks through the Gopher Hills is the substantial dam of the Salt Spring Valley Reservoir.
GEOLOGY.
The strike and dip of the rocks are more or less variable; but, so far as my observations extend in the region described, they have everywhere the same general northwesterly trend and high northeasterly dip which characterize so large a portion of the gold-bearing slates of central California. The strike is usually from N. 50° W. to N. 70° W., (magnetic) and the dip from 50° northeast to vertical. I have seen no case here of a decided southwesterly dip, nor of a low one to the northeast. It is somewhat remarkable, by the way, that this high northeasterly dip should be so general as it is in the great mass of auriferous slates which forms the southwestern flank of the Sierra Nevada. It is towards the granite axis of the chain, instead of from it, as would seem more natural. The causes of this are by no means as yet fully explained. It is a circumstance, however, which would lose none of its interest in the future, if, as certain facts mentioned in the Geological Report, Vol. I, p. 286, might possibly seem to indicate, further explorations should prove it to be in general a great inversion of the strata—their upper portions having been “forced back by immense pressure from above, producing a condition of things similar to that so often observed in the Alps, which is known as the ‘fan structure,’ and has so much perplexed geologists.” When we take into account the enormous denudation, amounting to thousands of feet in perpendicular depth, which is known to have taken place in the Sierras within the most recent geological periods, and the whole of which, in this case, must also have belonged to the inverted portion of the strata—unless indeed the inversion were produced by a peculiar sliding and bending of the strata by their own weight, the upper flexure having been since entirely removed—and when, in addition to this we consider the hundreds of miles in length, and the great thickness of the strata in question, we can perhaps begin to appreciate the magnitude of the movements and forces which would be involved in producing such an effect. It would indeed, if true, be a striking illustration of the grandeur of the scale upon which many of the physical features of this country have been cast, as compared with those of other and better known regions. But it is hardly worth while to speculate further upon probabilities like this in the present state of our knowledge, and I return to my subject.
In Salt Spring Valley, the rocks consist almost entirely of slates, with little variety of character, generally thin-bedded, fine-grained and argillaceous, sometimes magnesian or chloritic, and often splitting with facility into very thin sheets. The thinnest bedded varieties are usually fragile, and the structure is often wavy; but sometimes the cleavage is regular and thin enough, and the rock possesses sufficient strength to furnish a tolerable material for roofing purposes; although no attempts have been made, so far as I know, to thus apply it;—and, in fact, the expense attendant upon its excavation and transportation would preclude any extensive use of it, even if its quality were unsurpassed, which it is not.