Geological Survey Bulletin 614 Guidebook of the Western United States: Part D. The Shasta Route and Coast Line

SHEET No. 4A (click on image for an enlargement in a new window)

On the way up Pismo Creek beyond Pismo (see sheet 4A, p. 120) the first rocks to be seen on the east are those of the Santa Margarita formation (Miocene), the equivalent of the lower part of the Fernando formation farther south. South of Edna the creek crosses a large area of these rocks, which in places are heavily charged with asphalt.

At the crossing of the Pacific Coast Railway just south of Edna are bold bluff exposures of asphalt-impregnated sandstones of the Pismo formation, which is probably of the same age as the Santa Margarita formation. They dip south at moderately low angles and are underlain by more steeply dipping shale beds of the Monterey group. These sandstones were formerly mined for asphalt, but the production of asphalt as a by-product from the refining of oil has for the present rendered them of no value.

Some oil wells have been drilled on the west side of the track south of Edna. One or two of these wells encountered oil in commercial quantities, but they have never been operated continuously, owing to mechanical difficulties caused by the thickness of the oil. The dark-colored asphaltic rock is cavernous in places and is eroded into grotesque forms. East of Edna are some well-developed stream terraces.

At Edna the railroad enters San Luis Valley, which extends northwestward for 20 miles. On the northeast the valley is bounded by the Santa Lucia Range and on the southwest by the San Luis Range. At the northwest end it opens to the ocean. This valley, unlike most valleys in the Coast Ranges, has no stream flowing lengthwise through it. Pismo and San Luis creeks, which drain most of it, enter from the east and cut through the San Luis Range to the sea.

Edna is built in part on the Paso Robles (ro'blace) formation, of Pleistocene, Pliocene, and late Miocene age. The rocks composing this formation, however, are much better exposed farther north.

From a point 2 miles north of Edna to a point 1 mile beyond the town of San Luis Obispo a set of rocks not hitherto seen on this route, the Franciscan group, is almost continuously exposed.¹

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¹The rocks of the Franciscan group are older than the oil-bearing rocks over which the route has thus far lain and are probably of Jurassic age. They comprise sandstones, conglomerates, shale, and local masses of varicolored thin-bedded flinty rocks. The flinty rocks consist largely of the siliceous skeletons of minute marine animals, low in the scale of life, known as Radiolaria, and on this account they are called radiolarian cherts. All the rocks mentioned have been intruded here and there by dark igneous rocks (diabase, peridotite, etc.), which have in large part undergone a chemical and mineralogic change into the rock known as serpentine. Closely associated with most of the serpentine are masses of crystalline laminated rock consisting largely of the beautiful blue mineral glaucophane and for that reason called glaucophane schist. Schist of this character is known in comparatively few parts of the world, but is very characteristic of the Franciscan group. It has been formed from other rocks through contact metamorphism—that is, through the thermal and chemical action set up by adjacent freshly intruded igneous rocks. The Franciscan group is one of the most widespread and interesting assemblages of rocks in the Coast Range.

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The most prominent topographic feature in the vicinity of San Luis Obispo (Spanish for St. Louis the bishop) is the row of conical hills that begins with Islay (ees-lie') Hill, on the right (east), a little over 2 miles southeast of San Luis Obispo, and extending to Cerro Romauldo (ro-mowl'do), about 4 miles northwest of the town. There are of eight these hills (Spanish cerros), the four larger northwest and the four smaller southeast of the city. These hills, of which The Bishop (1,502 feet) is the highest, are composed of igneous rock and are the cores of small volcanoes which broke through the Franciscan sedimentary rocks. The eastern part of Islay Hill consists of a surface flow of basaltic lava.

From San Luis Obispo, which is a division point on the railroad, the Pacific Coast Railway runs to Port San Luis on the coast, passing through the resort of Sycamore Warm Sulphur Springs. At Port San Luis is the end of the Producers' Transportation Co.'s pipe line from the San Joaquin Valley oil fields.

San Luis Obispo is one of the old Spanish towns of California. A mission was founded here in 1772 by Father Junipero Serra, and the church building is still standing.

The most important metallic mineral resource in the San Luis region is chromite, which occurs in the serpentine of the San Luis and Santa Lucia ranges.

On leaving San Luis Obispo the train crosses San Luis Obispo Creek, which emerges from a canyon in the Santa Lucia Range, on the right (east). On the left is Cerro San Luis Obispo, one of the old volcanic necks. A mile and a half beyond the station, on the right, is the California Polytechnic School and in the foreground a ledge of rocks characteristic of the Franciscan.

The railroad curve known as "the horseshoe" begins at Goldtree, 4 miles from San Luis Obispo, with a grade of 2.2 per cent. The ascent continues through shallow cuts in sandstone belonging to the Franciscan group, but at the end of the horseshoe, above Goldtree, the road swings to the left and crosses a belt of serpentine to the open hilly country of the overlying Monterey shale. Near Serrano (elevation 941 feet) Franciscan rocks again appear in complex association with serpentine and dark intrusive rocks. The railroad continues on these rocks principally for 4 miles, passing by six tunnels through ridges of diabase. The longest tunnel, 3,616 feet long, is farthest north and pierces the divide into Salinas Valley, nearly 600 feet beneath the summit.

The black earthy shales seen between the northernmost tunnel and Cuesta (239 miles from San Francisco) are of Lower Cretaceous (Knoxville) age. A little of the same shale was crossed between the first and second tunnels. The shale overlies the Franciscan rocks, but the two formations were not deposited continuously. The Franciscan rocks were raised above the sea and eroded, then submerged again and covered by the muds now consolidated as the Knoxville shale. The geologist expresses these facts briefly by saying that the Knoxville lies unconformably on the Franciscan. On the right (east) at Cuesta is an area of dark intrusive rock (olivine diabase). Beyond this the railroad passes across a small area of light-colored shale (Monterey), traverses another area of dark Knoxville shale, and enters upon a belt of sandstone, which with some conglomerate and shale composes the characteristic Chico (Upper Cretaceous) formation. The sandstones of the Chico weather into large blocks. Near Santa Margarita the sandstone is covered by recent alluvium.

The general trend or strike of the formations here, as elsewhere in this part of California, is northwest, in accord with the general alignment of ridges and valleys.

Santa Margarita is on the southwest side of a northwest-southeast depression which is followed for most of its length by Salinas (sa-lee'nas) River. The valley extends for 8 or 9 miles southeast of the town and is followed by the railroad for about 10 miles to the northwest. It owes its form partly to geologic structure and partly to erosion, as it is occupied by comparatively soft Tertiary formations, while the hills on both sides are composed mainly of harder, older rocks. Along the southwest side of the valley, to the left of the railroad, are Franciscan and Cretaceous beds. On the northeast, to the right, is a large area of granite, probably of pre-Jurassic. age. The valley represents a long, narrow trough or syncline along which the Tertiary beds have been bent down into the older rocks. Faulting along the sides took part in the formation of the syncline. Erosion afterward removed the Tertiary beds on each side of the down-folded and down-faulted strip, exposing the older rocks of the present hills. The Tertiary rocks of the present valley were protected to some extent by being inlaid, as it were, into the older rocks, but, being soft, they were hollowed out into a valley. At some stage in the erosion and hollowing process Salinas River several miles above Santa Margarita became deflected to the granite on the northeast side of the valley. Gradually the valley as a whole became deeper and wider, but the river had deepened the channel that it had begun in the granite and was unable to leave it. Consequently the river to this day leaves the belt of soft Tertiary rocks, turns northeastward into the hard granite and after flowing for 5 or 6 miles through that rock, returns to the main valley northwest of Santa Margarita. This is a case of what geologists call "super-imposed drainage."

The Santa Margarita region is one of rolling hills well covered with the wild oat (Avena fatua)¹ and dotted with oaks. It is an excellent stock country. Growing on the bottom land of the adjacent valley are some of the finest white oaks (Quercus lobata) to be seen in California.

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¹The wild oat, one of the most abundant and characteristic grasses of California, was brought to this country from Europe.

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Half a mile beyond Santa Margarita, on the right at milepost 235, is the Producers Transportation Co.'s pump and ten oil tanks with an aggregate capacity of 75,000 barrels.

The type section and locality of the Santa Margarita formation is just north of the town, where the soft, more or less limy coarse sandstone of the formation occurs along the railroad for nearly a mile. These beds carry large fossil oyster shells which are characteristic of the Santa Margarita formation throughout the Salinas Valley region.

Near Eaglet, on the left (west), there are several abrupt changes from Monterey shales to the coarse beds of the Santa Margarita and vice versa. The hills on the right (east), across the river, are composed of granite. A bed of Vaqueros (va-kay'ros) sandstone that can be seen lapping up over the granite along the base of the hills probably once arched completely over the granite, but part of it has been removed by erosion. The sandstone in turn is overlain by the Monterey shale.

The low hills on the east side of the river opposite Atascadero (a-tas-ca-day'ro, Spanish for a deep miry place) are composed of the Paso Robles formation, already seen at Edna. The formation in general consists of gravels whose pebbles are composed of shale of the Monterey group. It ranges in age from late Miocene to Pleistocene. Just north of Atascadero station, on the west side of the track, is a good exposure of the white fossiliferous Santa Margarita sandstone, dipping north and overlain unconformably by the light-colored Paso Robles gravel.

At Templeton the railroad turns northward, leaving the valley of the upper Salinas and passing through Paso Robles Canyon toward the lower or main Salinas Valley. Well-developed river terraces are to be seen in this vicinity.

Just before reaching Paso Robles station the train passes a gravel pit in which a tooth of an extinct elephant has been found. The gravel is of stream origin.

Paso Robles (Paso de los Robles, Spanish for pass of the oaks), with its hot sulphur springs and mud baths, is widely known as a pleasure and health resort. The springs have a temperature of 95° to 110° F. and the mud baths of 140° F. The Camino Real (ca-mee'no ray-ahl', Spanish for king's highway), at first a trail but later a road connecting the missions from Mexico to San Francisco, passes near by and is now used by automobile tourists. The nearer hills on both sides of the railroad from Paso Robles to San Miguel are composed entirely of the Paso Robles formation.

At milepost 214, 2 miles north of Paso Robles, near the railroad and the river, is the new hot mud bathhouse of the Paso Robles Hotel. The Camino Real is on the left, and beyond it are attractive parklike fields and hills with scattered oaks.

On the left as the train enters the little village of San Miguel (me-gale') is the old mission, built in 1797, of adobe brick, with its bell in front and remnants of the inclosing walls on the right. San Miguel is near the head of the main Salinas Valley, which extends northwestward in an almost straight line nearly for 100 miles to Monterey Bay. It is one of the best examples of the narrow northwest-southeast valleys, owing their form to erosion controlled by geologic structure, that are characteristic of the Coast Range between San Francisco and Los Angeles. From the vicinity of San Miguel the northeast slope of Salinas Valley, toward the Gabilan (ga-vee-lahn') Range, is suggestive of a great plain gently tilted toward the observer and furrowed by streams flowing straight down the slope. It is perhaps a tilted block of the earth's crust with a fault along its northeast edge and probably another along its southwest edge, under the valley. The surface of this slope is covered with the Paso Robles formation, through which the stronger streams have cut into the underlying Santa Margarita formation.

The Paso Robles formation forms the near hills on both sides of the river between San Miguel and Bradley. Just north of the mouth of Indian Valley, at Chanslor (205.5 miles from San Francisco) there is an excellent example of one of the landslides which frequently occur in the soft Paso Robles formation when it has been undermined by stream erosion.

From McKay a branch railroad runs to the Stone Canyon coal mine (24 miles), now worked only in a small way to supply local demand. The coal of Stone Canyon occurs in the lower part of the Vaqueros sandstone and is the best in California. In places the bed is 16 feet thick, and if it were not for the abundant supply of oil in the State the coal would probably again be mined for shipment.

Near the mouth of Nacimiento River (on the left) the railroad bears to the right and crosses Salinas River on a steel bridge that was partly washed away in 1913. Two of the old steel spans may be seen lying in the river. Very little water flows over the sandy river bed in summer, but there is a large flow beneath the surface.

Bradley, though a small town, is the center of a considerable cattle industry. The alluvial flat and terrace of Salinas River in this vicinity are shown in Plate XXIX, A. Beginning about 2-1/2 miles northwest of Bradley there are indications of a line of disturbance, apparently a great fault, along the west side of Salinas Valley. A little farther on, at 192.5 miles from San Francisco, the prominent bluffs on the right (east) are of the Paso Robles formation and the bills on the left, across the river, are composed largely of the Monterey shale. Inasmuch as the Paso Robles normally overlies the Monterey, their occurrence so close together at nearly the same level suggests that there is an extensive fault between them, along the river. That a large fault occurs along the west border of the Salinas Valley is indicated also by the presence of a line of springs in that region. Many of these springs are hot, and some of them, as at Paso Robles and Paraiso (pa-ra-ee'so, Spanish for paradise), have been improved as health resorts. Along this fault line the great earth block previously referred to as sloping up toward the crest of the Gabilan Range on the northeast side of the Salinas Valley must have dropped down, bringing the Paso Robles gravels into juxtaposition with the Monterey and Santa Margarita (Miocene) beds and providing the structural conditions for the subsequent erosion of the long, straight valley.

PLATE XXIX.—A. ALLUVIAL FLAT AND TERRACE OF SALINAS RIVER NEAR BRADLEY, CAL. B. SAN CARLOS CHURCH, MONTEREY.

About 35 miles almost due west of Bradley is the Mansfield mining district, where some coarse gold has been recovered from stream beds. The gold is probably derived from quartz veins in igneous rocks of the Franciscan group. Some nuggets weighing several ounces are said to have come from this district. Farther southeast, toward San Luis, a number of deposits of quicksilver and chromite occur.