GEOLOGIC FORMATIONS IN THE LASSEN PEAK
Composition of the Miocene strata.The Miocene formation embraces sandstones with finer sediments and considerable conglomerate. The detritus of which they were formed was, like that of the Chico group, derived directly from the auriferous slates. In this respect they are strongly contrasted with that of the succeeding deposits, which are composed almost exclusively of volcanic material.
Distribution and relations.Within the Lassen Peak district there is a large area of Miocene deposits lying in the Piedmont region. One of the best exposures is on Little Cow Creek, in Shasta County, at an elevation of about twenty-nine hundred feet. At this point the creek leaves a bold, rugged cañon, cut deep into the light colored sandstone and pebbly beds, and enters upon the irregular, hummocky area that leads to the adobe land on the Chico rocks farther down stream. At the mouth of the cañon, in a fine shaly bed intercalated between the sandstone, a number of fossil leaves were found, which have been identified by Prof. Leo Lesquereux and the results are given in the accompanying list (page 420). With the fossil leaves was found a fresh-water mussel, identified by Dr. R. E. C. Stearns as Anodonta Nuttalliana. The horizontal strata conformably overlying the fossils are chiefly coarse sandstones occasionally containing fine gravel. They are somewhat lighter colored than the members of the Chico group and a thickness of over five hundred feet of them is exposed along the sides of the cañon. The whole series is capped by a lava flow. A few miles to the northeastward, about the head of Montgomery Creek, where the upper portion of the series is exposed, it may be seen that the sandstones pass upward into conglomerates and the volcanic material becomes more abundant near the summit of the series. At this point the strata dip slightly to the eastward away from the shore line and beneath the lavas of the volcanic ridge. A few miles down Little Cow Creek from where the fossil leaves were found, near the Oak Run road bridge, the marine shells of the Chico group were collected, and the difference in elevation of the two exposures, taken in connection with the position of the strata, suggests that there may be a great thickness of finer sandstones and shales between them. The contact between the Chico beds and the Tertiary deposits has not been observed, but their relative positions at adjacent exposures indicate that they are slightly unconformable. The Chico beds extend about a mile up Little Cow Creek from the bridge and dip to the westward. The Tertiary deposits of the hills near by to the northward overlap the Cretaceous and dip gently in the opposite direction, so that there can be scarcely a doubt concerning the discordance of the two formations.
The Miocene formation in the Lassen Peak district, as already remarked, is composed of sandstones and shales below, and conglomerate above. The lower member is least liable to exposure, but its presence is generally indicated by a peculiar, hummocky topography quite unlike that of any other strata in the district. The small hillocks are not always round; they vary in form like the little drumlins of a glacial field and have between them small basins which in the rainy season contain lakes. That such a topographic feature is not the consequence of ordinary erosion has been remarked by many of the mountaineers, who attribute them to land-slides, and this view is supported by the fact that the hummocks are best developed where the general slope has considerable declivity. The finest examples within the district have been seen in the neighborhood of Little Cow Creek, but they were observed also near Lone Rock, south of Light Cañon, in Plumas County, as well as northeast of Coppervale, near the head of Mountain Meadows, where the same formation occurs.
The prominent ridge between Bear Creek and the South Fork of Cow Creek affords an unusually interesting exposure of the upper portion of the Miocene. At the very base of the hill, in the beds of both streams, Chico fossils have been found, so that the hill undoubtedly rests upon the Cretaceous. Above the fossiliferous beds is a considerable thickness of sandstone capped by a heavy layer of conglomerate, and overlying this is usually found a remarkable flow of tufaceous rhyolitic lava. The conglomerate at this point shows apparently a greater development than anywhere else within the district. It has been mined for gold, but without marked success. One of the miners of the region says that it contains enough fine gold to pay from 50 cents to $1 a day. The gravel has only a local development, for it is limited to the slopes of Bear Creek Hill, with a short extension eastward up the North Fork of Bear Creek, where it is mined. This linear extension east and west down the general slope and transverse to the usual strike of the Tertiary strata indicates that the gravel deposit is an old stream bed. Additional evidence in favor of the same view is to be found in the fact that in the immediate vicinity the conglomerate is absent and the overlying tufas (Pliocene) rest directly upon an eroded surface of the sandstone below.
That much of the conglomerate has been washed away is evident from the traces it has left behind. Between the Tamarack road and the North Fork of Cow Creek there is a prominent ridge extending southwesterly from the group of prominent hills about the headwaters of Burney Creek. The western terminus of this ridge is rather abrupt, and it is composed chiefly of hypersthene-andesite overlaid by basalt. High up on its slopes, lying on the andesite, are frequently found well rounded pebbles, often grouped in great abundance, but generally rather sparingly scattered over the surface. They proclaim the former presence of the Miocene conglomerate. Upon the northern side of this ridge, near the head of the cañon of the North Fork of Cow Creek, at an elevation of 3,000 feet, the conglomerate occurs in place resting upon the Miocene sandstone, which has an extensive development further westward on the divide between the North Fork of Cow Creek and Clover Creek. The pebbles on the ridge above referred to reach an elevation of 3,200 feet. Near the head of Oak Run, on the road to Silver Lake, pebbles of metamorphic rocks are scattered quite abundantly over the fresh andesitic lava up to an elevation of nearly four thousand feet. The great elevation at which these pebbles now occur may be due in part perhaps to upheaval since their deposition. These pebbles are almost wholly of quartz and other metamorphic rocks, very unlike the lavas upon which they rest. In places they appear also to overlie tufa, and it is difficult to satisfactorily explain the presence of so small a number of volcanic fragments in a conglomerate so intimately associated with the lavas.
South of Bear Creek the Miocene is not so extensively developed nor so well exposed. On the South Fork of Shingle Creek, near the prominent crater about sixteen miles directly east from Anderson, lying between the fossiliferous Cretaceous and the tufa there is but a thin body of Miocene strata. A better exposure is obtained on Ash Creek, six miles east of Ball's Ferry.
Notwithstanding the thick deposit of tufa (Pliocene) in the Piedmont region, the deep cañons cut across it by the mountain streams occasionally reach the Miocene beds upon which the tufa rests. Along Mill Creek, about twenty miles east of Tehama, where the sandstone is exposed, its upper surface was irregularly eroded before the deposition of the tufa, just as in the case already noted along the South Fork of Cow Creek.
In the cañon of Deer Creek, at an elevation of about two thousand two hundred and fifty feet, there is a remnant of an ancient stream bed of auriferous gravel, which is apparently equivalent to the Miocene conglomerate and has been extensively mined. The gravel is about thirty feet thick and is composed of well rounded pebbles less than four inches in diameter. The pebbles are chiefly of metamorphic rocks, but fragments of lava are also common. The whole is overlaid by tufa as represented in Fig. 16.
Thus far our attention has been given to the Miocene deposits of the Piedmont region within the western portion of the Lassen Peak district. We will now turn our attention to similar deposits found outside of the district southeast of Lassen Peak, about the Mountain Meadows and in the neighborhood of Light Cañon. The formation is apparently continuous from one locality to the other, but the best exposures of it are at the place last mentioned. The northern extremity of Light Cañon opens into a basin-shaped region lying between Lone Rock and the northeastern terminus of Moonlight. This basin was once filled with Miocene lacustrine deposits, of which only a small portion now remains. The sandstone at its base has a thickness of over five hundred feet, and its horizontal beds are well exposed upon the hillsides west of Light Cañon, where they form prominent bluffs. To the east of Light Creek, near Lone Rock, the sandstone is soft and incoherent, so that instead of forming cliffs it gives rise to the peculiar hummocky topography to which reference has already been made. Fossils have not been found in the sandstone of this basin, but at the northern end of Mountain Meadows, near the main road one mile southwest of the point where it crosses the summit towards Susanville, the few fossil leaves which are noted in the list (page 420) have been found. In both localities and at a number of intermediate points connecting the two principal outcrops the sandstone is overlaid by a heavy layer of gravel, which is occasionally cemented so as to form a very firm conglomerate. This gravel has attracted much attention, not only on account of its position, but more particularly because it is auriferous. At both ends of the Mountain Meadows it has been mined, and the work still continues during the rainy season, when sufficient water can be obtained. Within the drainage area of Light Creek the operations have been more extensive and perhaps also more profitable. Considerable money has been spent searching within the lacustrine deposits for old stream beds, which will in all probability never be found. The conglomerate or loose gravel, which has either never been cemented or else resulted from the disintegration of the conglomerate, is widespread, and reaches an elevation of nearly seven thousand feet on the very crest of the eastern escarpment of the Sierras, near Thompson Peak. The lower portion of the gravel is composed almost wholly of pebbles of metamorphic rocks. They are very smooth and well rounded, frequently showing the ellipsoidal form characteristic of beach action.1 In the upper portion the pebbles are larger and those of modern lavas more abundant. Excellent exposures of the upper portion of the gravel occur along the crest a few miles southeast of Diamond Peak, where it forms a prominent, smooth ridge, rising at least one hundred and fifty feet above its base. The pebbles are of all sorts. Those of quartz and of the auriferous slates and granite are generally smaller and much less abundant than those of andesitic lava, which are not infrequently two feet in diameter. A short distance north of the road, where it crosses the summit between Light Cañon and Susanville, at the head of a stream which flows into Susan Creek, the gravel is cemented into a firm conglomerate. It is about three hundred and fifty feet thick and dips slightly to the eastward. High up on the mountain side it rests directly upon the granitic rocks, but at lower levels on both sides of the mountain it is underlaid by the sandstone. Within the conglomerate are occasional thin beds and lenticular masses of shale, which are much indurated, as we shall see further on, by the orographic movements which gave birth to the Sierras. In one of these shales, less than twenty inches in thickness and of limited extent horizontally, numerous fossil leaves have been found.2
The fossils were determined by Prof. Leo Lesquereux, whose results will be found in the appended list (p. 420).
While examining the general structure of the Sierras in the vicinity of Indian and American Valleys I had an opportunity to visit the summit of Spanish Peak and collected some fossils from the auriferous gravels of the Monte Christo mine.1 The gravel mined at this place is doubtless closely related to that at Lot's Diggings, about twelve miles to the northwest and high up on the opposite side of the cañon of the North Fork of Feather River. The gravel is very smooth and round, just like that of the lacustrine deposits east of Indian Valley, but, unlike the latter, it appears to belong to a stream bed. The form of the deposit appears to be linear and confined to a definite channel, which is from one hundred to six hundred feet in width at the surface and about four hundred feet deep. The upper portion of the deposit is chiefly gravel and the lower part distinctly stratified, fine, argillaceous sand or clay (pipe clay), which at some places rests directly upon the granite, at others is underlaid by conglomerate. The fossils were found at a distance of nearly three thousand feet from the mouth of the tunnel and approximately two hundred and seventy-five feet below the surface, in the clayey strata. They occur in great numbers and can be obtained in an excellent state of preservation, but the bed containing them is so soft as to readily disintegrate. Besides the leaves noted in the list of determinations by Professor Lesquereux (p. 420), a fossil fresh-water fish2 was found in the same bed.3 The form and structure of the deposit indicates that it is fluviatile, like the great mass of the auriferous gravels on the western slope of the Sierras described by Prof. J. D. Whitney4a and others; but what relation it holds to the lacustrine deposits to the northeastward has not yet been determined. The character of the pebbles is such as to suggest that they may have been derived from the lacustrine deposits, but there are other considerations which suggest for them a greater antiquity. At Cherokee, for example, in the mine of the Spring Valley Hydraulic Gold Company, the auriferous gravels are overlaid by several hundred feet of fine conglomerate and soft sandstone, which have furnished a few leaf impressions4b to indicate that the deposits belong to the Miocene fresh water lacustrine beds so extensively developed in the Piedmont region. It is evident that the auriferous gravels underlying these deposits belong to an early portion of the Miocene or to the Eocene.
Fossils found in the Miocene srata.The following tabular view gives a list of all the plant remains found in the formation just described. The figures indicate the number of specimens of each species found. All of the determinations were made by Prof. Leo Lesquereux, who has furnished the following statements concerning their geological relations:
"Juglans Bilinica is found along Little Cow Creek and upon Spanish Peak, while Phragmites Oeningensis occurs at both of the other localities. The most abundant form, Persea Dilleri, is related to the living species Persea Carolinensis, variety palustris, of our south Atlantic border, and occurs both along Little Cow Creek and near the summit north of Light Cañon.
"Two species, Magnolia Hilgardiana and Quercus Moorei, are identified in the Eocene of the Mississippi; and two others, Aralia Lasseniana and Oreodaphne litsœœformis, are related to the Eocene (Sézanne and Gelinden) of France and Belgium.
"Seven species are identified in the Miocene of North America, viz: Platanus dissecta, Laurus Californica, Cornus hyperborea, Pterospermites spectabilis, Quercus Olafseni, Laurus socialis, Juglans rugosa.
"Five species are identified in the Miocene of Europe: Myrica Ungeri, Juglans Bilinica, Phragmites Oeningensis, Oreodaphne Heeri, and Cinnamomum Scheuchzeri.
"Three species have been identified with those of the auriferous gravels of California, viz: Ficus microphylla, Magnolia Californica, and Platanus dissecta. These three, with Cornus Kelloggi as closely related to Cornus hyperborea and to the living Cornus Nuttalli of California, represent the recent forms of plants described above. The relation is therefore evidently to the Miocene."
Prof. Lester F. Ward, who not only saw the fossils but also examined the list of species, determined by Professor Lesquereux, expressed the opinion that the strata from which they were taken, as well as the auriferous gravels, the upper portion of the John Day group, with those at Corral Hollow and perhaps the Bellingham Bay beds, all belong to the same series, and he agrees with Professor. Lesquereux that they are Miocene, most likely Upper Miocene.
List of fossil leaves determined by Prof. Leo Lesquereux
Hypsographic and climatic, conditions during the Miocene.During the Miocene the northern portion of the Sacramento Valley was occupied by an extensive fresh-water lake, which stretched far to the northeastward through Lassen Strait, that marks the limit between the northern terminus of the Sierras and the Coast Range and is now occupied by the volcanic ridge of Lassen Peak. A similar body of water existed northeast of Indian Valley, in the country now occupied by the very crest of the Sierras, northwest of Honey Lake, at an elevation of nearly seven thousand feet. From the fact that the lacustrine deposits on both sides of Lassen Peak pass beneath its lavas, it is believed that they are continuous and were all laid down in the same lake, which at that time covered a large portion of what is now the northern end of the Sierras and extended from the Sacramento Valley far into Oregon. Mr. Clarence1 has called attention to the wide distribution of Miocene lacustrine deposits in that region, extending from beyond the Columbia River south, through Oregon into Nevada and California, and it may now be added that they pass through the gap separating the north end of the Sierras from the Coast Range into the northern portion of the Sacramento Valley. To the large body of fresh water in which these sediments were found King gave the name Piute Lake. The Piute Lake deposits reach much farther up on the flanks of the Sierras than the littoral deposits of the Chico epoch, indicating clearly that between the close of the Chico epoch and the beginning of the Miocene there was a change in the relative elevation of the Sacramento Valley and the Sierra region. This change was effected by the elevation of at least part of the region of the Coast Range, and apparently also of the Cascade Range, so that the oceanic waters were excluded and the formation of Piute Lake was rendered possible. That this elevation occurred at the close of the Cretaceous is rendered altogether probable by the fact that the Tejon group has not yet been recognized within the Piute Lake region. Perhaps we may yet find in the earlier deposits of that lake the fresh-water equivalents of the Tejon group of western Oregon and southern California.
The distribution of the lacustrine deposits in the Monntain Meadows region can not be well illustrated until the cartographic work of that district has been completed. Their relations, however, appear to fully justify the statement that at the time they were deposited the country to the eastward was higher than the region now occupied by the Sierras, at least in the latitude of Diamond Peak. Furthermore, if these deposits were, as we suppose, all laid down in the same lake, it is evident that during the Miocene the sediments which now form the eastern crest of the Sierras near Diamond Peak must have been on about the same level with those of the Sacramento Valley.
In answer to my question concerning the climatic condition of that region during the Miocene, as indicated by its flora, Professor Lesquereux states that "by the presence of a large number of Laurineae the flora becomes related in its general characters to that of a region analogous in atmospheric circumstances to Florida." With this view Prof. Lester F. Ward also fully agrees, and it is in complete harmony with the inference drawn from structural relations, viz, that during the Miocene that country was a broad platform, with gentle relief, and but moderately elevated above the sea; or, in other words, the surface of the region was but little above its base level of erosion.
Last Updated: 28-Nov-2006