from California Geology, July 1973, Vol. 26, No. 7.

PLEISTOCENE LAKE SAN BENITO

By Olaf P. Jenkins

Consulting geologist and retired Chief,

California Division of Mines and Geology

Those of us who have heen working for the California Division of Mines and Geology since Paleozoic time have had an experience modern youth can never match: we worked for Olaf P.Jenkins.

Indefatigable, energetic, often irascible - "OPJ" taught us many things. True, he made all of the decisions and " led " us by a combination of bullying and pushing, but he showed us how to have respect for the proper things: respect for individuals, not " names " respect for a job well done, no matter who did it; respect for ourselves when we deserved it.

Probably his greatest contribution to our lives, and perhaps to science as well, was and is his wealth of superb ideas. His is an original mind, not common in this day when " peers " are more admired than the peerless.

Now in his 85th year, he is still aggressively doing field work and producing new ideas about today's problems, as this paper will attest ........M.R.H.

Map showin9 area covered by Pleistocene Lake San Benito (pink), with the rivers, towns, and highways of today shown in black. The faults in red are active; movement along them was responsible for damming of Pleistocene streams so as to create Lake San Benito and related lakes. The grid in gray shows the areas covered by the U.S.Geological Survey 71/2-minute topographic quadrangles.

During a search for commercial deposits of sand and gravel along the stream courses of the Pajaro-San Benito River basin, extensive silt deposits, obviously left by late Pleistocene lakes, were discovered. They form well-defined siltbed terraces quite different from the stream terraces of sand and gravel. Drilling into the stream channels also revealed lake beds interstratified with the stream deposits.

Undoubtedly, there were many local lakes into which silts were deposited; but only two interrelated large lakes are discussed in this paper because of their particular importance in the effect on the landscape for 10 miles on each side of the San Andreas fault, the movements of which were responsible for the formation of the lakes.

A high-level, 30-mile-long lake, which followed the 400-foot contour, is referred to here as Lake San Benito. A lower-level lake, which occupied the entire San Juan Valley, filling it with silt to the 200-foot level, is herein called Lake San Juan. It was longer lived and left more prominent deposits which have given very fertile agricultural land to the valley. Water-wells have drilled through the silt into blue clay (bearing freshwater shells) and into a gravel aquifer representing the pre-lake San Benito River channel and its alluvial plain, which are cut off sharply by the San Andreas fault half a mile southeast of the town of San Juan Bautista.

The time when the lakes existed is a time not ordinarily given much consideration in geologic history, although it was actually a critical time in the adjustment of the drainage pattern of the entire Pajaro watershed. Movements on the San Andreas fault have been carefully studied immediately along the fault trace by many geologists, as have the displacements of great magnitude-measured in hundreds of miles- which took place during the Tertiary, many millions of years ago. The lakes were present before the reach of archeology and after the deposition of well-known Pleistocene sands and gravels of either the so-called San Benito Gravels southeast of Hollister or the Aromas Red Sands west of the San Andreas fault. There are many puzzling problems which can be answered when consideration is given to the formation, the life, and the destruction of these late Pleistocene lakes, which have left deposits over a very large area.

If more consideration is given to the origin of landscapes of this time in belts both east and west of the San Andreas fault, many new and interesting features may be discovered. The belts should extend at least 10 miles from the fault trace. As examples, at Moss Landing, 10 miles west of the San Andreas fault, during the 1906 earthquake, the railroad station was displaced 15 feet,23 and it is well known that the Salinas River was obliged to change its course behind the dunes along the coast to empty at its present mouth 5 miles farther south.

PRESENT DRAINAGE

The southern end of Santa Clara Valley, together with San Benito Valley, is drained today by the Pajaro River with a watershed of 1,150 square miles. It carries water from the east side of the San Andreas fault to the west side by way of Pajaro Gap (eastern Chittenden Pass), then through western Chittenden Pass, into Pajaro Valley. There is no submarine canyon where the river enters Monterey Bay, but opposite Elkhorn Slough, 3 miles to the south at Moss Landing, is the great Monterey Submarine Canyon.

Pajaro Gap is a very significant feature, both geographically and geologically; as J. Allen' says, it is "the only watergap south of the Golden Gate between the Santa Clara-San Benito trough and the sea." Through it also pass the main line of the Southern Pacific Railroad and Riverside Drive, the road that connects Watsonville and Highway 101. Adjacent to this pass is the huge Logan quarry of the Granite Rock Company which has been in operation since 1900.

The San Benito River drains a 530-square-mile watershed; Tres Pinos Creek is its major tributary. Pajaro River takes the overflow of San Felipe Lake which is fed by Pacheco and Santa Ana Creeks. Llagas, Carnadero, and Tar creeks, flowing southward, are tributaries of the Pajaro, entering above the mouth of the San Benito, while Pescadero Creek enters the Pajaro a mile farther downstream. Before reaching the mouth of the San Benito, the Pajaro passes between the Sargent Hills on the north and the Lomerias Muertas on the southeast, where large landslides, including the Sargent slide, have probably on occasion dammed the Pajaro to form shallow ephemeral lakes in the Bolsa and San Felipe Lake area.

The San Benito River west and below Hollister cuts through the flat San Juan Valley along its northern side leaving nearly vertical bluffs 20 to 35 feet high on both sides of the stream channel. The bluffs expose lake beds all along their course. The top elevation of silt-filled San Juan Valley is about 200 feet, which was probably about the elevation of Pleistocene Lake San Juan. The bottom of the fill is near sea level.

South of Hollister on both sides of the San Benito River are numerous terraces; some are stream terraces while others are composed of silt deposited by lakes. Beneath the surface of the stream channel, borings encountered silt beds as well as sand and gravel beds.

One prominent lake-bed silt terrace follows the 400-foot contour. Lake San Benito, which deposited this high-level terrace, covered a large part of the basin of San Benito and the southern end of Santa Clara Valley, all of which is now drained by the Pajaro River. To reach Monterey Bay, this river now cuts through a granite ridge on the west side of the San Andreas fault at Pajaro Gap; there granite is in direct contact with Miocene and Oligocene shales on the east side, and there the fault has a recorded history of great activity.

For a distance of 4 miles, from the mouth of San Benito River to Pajaro Gap, the Pajaro River is offset from its southerly course to a northwesterly direction, reflecting an offset of equal distance along the San Andreas fault.

PLEISTOCENE SEQUENCE

In this region, there are deposits representing Plio-Pleistocene, mid-Pleistocene, and late Pleistocene time. However, diagnostic fossils are lacking, and no absolute ages have been determined.

The oldest deposits, regarded as Plio-Pleistocene, are beds of the non-marine San Benito Gravels, locally channeled into the Pliocene Purisima Formation which in places contains many fossil marine shells. The Purisima beds of silt, sand, and gravel are extensive, and are prominent on both sides of the San Andreas fault; whereas the San Benito gravel beds are abundant only on the east side of the Calaveras fault, especially in the Tres Pinos Valley area. Both formations are deformed and in places contain somewhat similar beds of silt, sand, and gravel derived from various older formations. In origin, the Pliocene Purisima Formation is both marine and non-marine.

On the west side of the San Andreas fault, the Aromas Red Sands beds1 are abundant and mantle many of the older formations. They clearly overlie Pliocene Purisima beds in the region of the San Andreas fault and have been assumed to be mid-Pleistocene in age, though no fossils have been found in them. In some places the Aromas Red Sands beds are slightly deformed. They occur in the coastal area of Monterey Bay and inland for some 10 miles. Stripping of the Aromas Red Sands beds indicates the extent of erosion during late Pleistocene and Holocene time on the coast side of the fault. There are some patches of alluvial terraces which thinly cover the Aromas: for example, north and west of Elkhorn Slough the broad terrace shown on the Geologic Map of California actually exposes a bluff of Aromas material 100 feet high on the side of the ancient valley.

Terrace deposits left by old streams as gravel patches are numerous on both sides of the San Andreas fault. Allen1 described the well-developed alluvial terraces along the Elk-horn Valley and gave their elevations as from 60 feet to 500 feet. Although these deposits are probably late Pleistocene in age, they are for the most part earlier than Pleistocene Lake San Benito and Lake San Juan. As Allen said, the alluvial fans are younger than the gravel terraces.

Terraces comprised of silt in the San Benito River area, representing lake beds, may be older or younger than many of the stream terraces that contain gravel. The two kinds of terrace material are often difficult to tell apart, since abundant landslide debris is constantly being added to the deposits in the San Benito River south of Hollister. Lake terraces continue at the same level, whereas stream deposits may be at various levels. Wave-cut terraces give the true height of the shore line of a lake, but where the lake was short-lived it did not leave very well marked shores.

On the published geologic maps of the Pajaro-San Benito area, there has been no differentiation made between these two kinds of terraces, lake and stream. On the 15-minute Hollister quadrangle, N. L. Taliaferro showed "Terraces undifferentiated (stages: Qt1 and Qt6). On the San Juan Bautista 15-minute quadrangle, Allen1 shows "Terrace gravels (1) and fans (2)". Since these maps were published, the U.S. Geological Survey has issued 71/2 minute topographic quadrangles covering the entire area, made with the benefit of air photographs. The detail and expression of the terraces are more easily interpreted on these later topographic sheets. Recent field geologic work in progress on the Hollister 71/2-minute quadrangle by Rogers of the California Division of Mines and Geology differentiates some terrace deposits.

EXTENT OF LAKE SAN BENITO

Lake beds can readily be seen following the 400-foot contour level, forming a distinct terrace 3 miles south of Hollister along Cienega Road on the east side of the valley at the junction with Hospital Road after it crosses the San Benito River. Here there are well-exposed, flat-lying lake beds of silt, filling the little valley which the road occupies for nearly a mile to the west. The 71/2-minute Hollister quadrangle expresses well this distinct terrace, which continues southward on both sides of the San Benito River valley for 3 miles upstream, near the point where Tres Pinos Creek enters the San Benito River. Tres Pinos Creek bed contains clean gravel and sand, whereas the river bed is contaminated with silt from landslides and Pleistocene lake deposits.

The surrounding Pliocene Purisima Formation, especially on the west side of the Calaveras fault, contains an abundance of silt similar to that of the Pleistocene lake, and no doubt was the source of the lake silt but the Pliocene beds are structurally deformed while the younger lake beds are flat.

On the east side of the San Benito River south of Hollister to the mouth of Tres Pinos Creek is the scarp of the Calaveras fault, which runs northward through the town of Hollister, across the Bolsa and San Felipe Lake. The area just east of the Calaveras fault and south of Hollister is complicated by the presence of other faults and by an abundance of gravel of Pliocene and Pleistocene age. Not only have the movements on the faults been right lateral, but the eastern side south of Hollister appears to have been uplifted. On the west side, the older San Benito gravel beds may have been faulted down and may now lie beneath the San Benito River, as indicated by a water well which reached a gravel aquifer at a depth of 353 feet. At shallower depths, silt deposits were encountered, showing that lake and stream deposits are interbedded beneath the river's channel. On the east side of the Calaveras fault in the region of Tres Pinos and southward a terrace is present at the 500-foot level. It is older than the 400-foot lake terrace, which can be followed throughout this complex area.

Following the course of the 400-foot contour on the 11-minute quadrangles, a terrace may be seen in places which is assumed to be the old shore line of Pleistocene Lake San Benito. In addition to alluvial and stream terraces, both older and younger than the lake terrace, there is a later very prominent 200-foot lake bed deposit completely filling the San Juan Valley. In order to study the 400-foot high lake terrace, and to avoid too much confusion with other terraces, that part of the twelve 7 1/2-minute quadrangles that shows the extent of the 400-foot lake was colored (see map, page 154). Only at the Morgan Hill-Madrone drainage divide (351 feet) and the two divides south of Pajaro Gap was the elevation below the 400-foot contour; in none of these places was it very far below the assumed level of the lake. On the steep hillsides surrounding the Chittenden cul-de-sac, a 400-foot terrace can be seen from the road.

Pleistocene Lake San Benito was certainly well enclosed, ut its maximum height was controlled at 400 feet by the spillovers, one to the north at the Coyote Creek cone, and possibly others west of San Juan Bautista.

In explaining how fishes from both the San Francisco and Monterey watersheds became intermingled, Branner,6 in 1907, called attention to the fact that Coyote Creek could have at times alternated its discharge from one side of the drainage divide to the other, since its cone near Madrone lies directly on this divide. This interchange probably took place prior to the creation of Lake San Benito, when Coyote Creek may have joined the Pajaro drainage to flow down Elkhorn Valley. Today, Coyote Creek for 8 miles hugs the eastern higher side of Santa Clara Valley, which is here about 2 miles wide. If the lake existed today, the spillover would run down the western side of the Coyote Valley. In final draining of the lake, all of the water from the eastern watershed was obliged to pass through Pajaro Gap, which is indeed an important and significant geographic feature.

Northwest of San Juan Bautista, on the west side of the fault, the drainage divide must have moved northwestward with the offset. However, on Highway 101 the divide is today 390 feet in elevation and on Highway 101 lateral the divide is 362. Western drainage over both of these divides is down the Elkhorn Valley. The Pleistocene lakes may have drained both to San Francisco Bay and Monterey Bay.

LAKE SAN JUAN

Pleistocene Lake San Juan probably represents the end product of Lake San Benito when it was drained down to the elevation of 200 feet from the 400-foot level. Lake San Juan, however, appears to have existed for a longer period of time than Lake San Benito because it deposited much more silt-enough to completely fill the valley of San Juan Bautista. The present San Benito River cuts a channel in these silts just south of the Lomerias Muertos, which are hills separating this valley from the southern end of Santa Clara Valley.

The top of this silt-filled valley in its central part follows the 200-foot contour. The bottom of the silt beds, where they terminate in blue clay with fresh-water fossils, extends to nearly sea level. Below the blue clay are the sand and gravel deposits of the buried San Benito River which flowed across the valley before the lakes were in existence. From this valley the drainage crossed the San Andreas fault to enter the upper end of Elkhorn Valley.

The position of the San Benito River prior to the formation of the Pleistocene lakes is indicated by the deeper water wells shown on the map by William C. Ellis" of the U.S. Bureau of Reclamation entitled, "Elevations on top of the blue clay zone, Hollister area." The "O" contours on the map follow these deeper wells and the course of the buried river channel. Subsurface data show that the channel was cut off abruptly by the San Andreas fault about half a mile southeast of the town of San Juan Bautista.

FOSSILS IN THE PLEISTOCENE LAKES

Fresh-water fossils were found in a test drill hole which started at an elevation of 140 feet, located near the mouth of the San Benito River above its bank and that of the Pajaro River. At a depth of 26 1/2 feet to 35 feet "blue clay" was encountered which contained tiny gastropod shells. This fossil bed is, therefore, at an elevation slightly above 100 feet, which would be near the bottom of the drainage.

The fossils were identified by Dr. G Dallas Hanna in a letter to me, dated December 15,1964:

"The four pieces of silt from test drilling near the mouth of San Benito River contain many specimens of small freshwater lake shells. The two species I found in washing one of the pieces are Valvata humerosa Say and Gyraulus parvus Say. Both species are common in lakes today, so I suspect that the age is no earlier than Pleistocene."

Fresh-water shells and redwood logs are reported by Ellis11 to have been found occasionally in the blue clay zone while drilling for water in the Hollister area.

Since these fossils are in the bottom of the blue clay which lies above the pre-lake San Benito River alluvium, they were probably deposited by the oldest of the Pleistocene lakes (Lake San Benito). The upper silts of Lake San Juan have not yet yielded fossils, although they are well exposed on the bluff of the San Benito River in San Juan Valley.

HOLOCENE LAKES

Less extensive lakes have certainly formed by landslides in this region during Holocene time, but they did not reach the 400-foot level. Some very small lakes are located at higher elevations, such as Quarry Lake just south of Pajaro Gap, where the 400-foot contour line surrounds it.

The Sargent landslide undoubtedly at times moved down far enough to block the Pajaro River and to form a broad valley lake in the Bolsa and San Felipe Lake region. There have been swamps in that region, but they may have been associated with movements on the presently active Calaveras fault which passes through San Felipe Lake and Hollister. In 1924, W. O. Clark published maps showing a large artesian basin in the Bolsa region and a small one in San Juan Valley. These lower areas probably had their origin in differential movements, prior to the formation of late Pleistocene lakes.

Soda Lake, a quarter of a mile wide, (also known as Chittenden Lake and Foster Lake) is located on a terraced meander of the old Pajaro River. The lake rests on the terrace gravels which in turn rest on lake-bed silt and blue clay with fresh-water fossils. The elevation of Soda Lake is 140 feet, whereas the floor of the granite under the present river at Pajaro Gap is 75 feet. Soda Lake receives water and seepages from an oil spring on a fault which was mapped by Allen1 as passing under the lake. It is located in a cul-de-sac where local landslides in Pajaro Gap might temporarily form lakes behind the outlet. Soda Lake may have once been a much larger lake, a remnant left at the lower terminus of the Pleistocene lakes. It is now trapped between two landslides.

On the west side of the San Andreas fault, just east of the Zayante-Vergeles fault between Watsonville and Corralitos, there are several little lakes-Pinto, College, Kelly, Drew, and Tynan-that probably had their origin in the down-faulting of the seismically active strip of country between these two great faults.

OFFSETS ON STREAMS AND FAULTS

There are hundreds of offsets on streams crossing the San Andreas fault. They are surface expression of displacement on the fault itself, which invariably is right lateral-the side opposite the observer moved to the right with respect to the other side. In this area are two large faults which are in part parallel to the San Andreas, each 3 miles from it: 1) the Sargent fault on the east side, which branches off of the San Andreas fault to the north and is currently marked by many micro-earthquakes, and 2) the Zayante-Vergeles fault on the west side, which branches off of the San Andreas fault south of San Juan Bautista, and encloses the seismically active area of Corralitos 20 miles to the north.

Where the offset stream crosses the fault is significant in that the stream is offset generally in the same direction as historic offsets measured on the fault, such as those known to have resulted from the 1906 earthquake and also from more recent lesser displacements. In 1903, the railroad bridge at Pajaro Gap was displaced right laterally 31 feet on a branch of the San Andreas fault, a fence near Hecker Pass still shows a horizontal offset of several feet. Today, geologists are taking measurements of continual creeping along the San Andreas fault, as at Vineyard, 17 miles south of Pajaro Gap on Cienega Road. All these offsets are right lateral.

Several left-lateral faults trending northeast-southwest are shown on Allen's1 geologic map between the Vergeles and San Andreas faults.

North of Pajaro Gap, Allen1 measured 15 right-lateral offsets on streams, 14 of which had a shift of about 3500 feet. There is now no way of telling definitely which side of a fault moves or how far away from the main fault trace the adjoining country has shifted with the fault.

For those offsets that occurred in prehistoric time, there is no way of telling exactly when the movements took place. The movement on the fault which caused these particular 14 offsets on streams just north of Pajaro Gap is also recognized just south of the Gap, as Allen has indicated. About half a mile south is an offset in the crossing ridge and a distinct bend in the river, which is also opposite the last loop in the 200-foot meander left in the terrace that encloses Soda Lake. Since the 200-foot Lake San Juan may be related to these early meanders and to Soda Lake, we may have sufficient correlation to encourage an actual age determination. The 400-foot lake, however, is earlier than this, and so is the greater offset of the Pajaro River.

South of Pajaro Gap, the Pajaro River flows in the fault zone for Ii miles. This reflects an offset which may have occurred at the same time as the 1-mile offset on Bird Creek, 15 miles farther south.

Sulphur Canyon appears to have once been the continuation of Bird Creek; this would make an earlier offset of 41 miles on Bird Creek. Farther south (20 miles southeast of Pajaro Gap), Pescadero Creek, in crossing the San Andreas fault, is offset 4 miles. It is quite conceivable that the 4-mile offsets on Pajaro River, Bird Creek, and Pescadero Creek to the south were contemporaneous. Chittenden, San Juan Bautista, Hollister, Mt. Harlan, and Paicines 7 1/2-minute quadrangles show these three offsets.

U.S. Highway 101 crosses the San Andreas fault 4 miles south of Pajaro Gap. The highway goes through a pass, only 200 feet in elevation, in the eastern Purisima range of hills. The hills continue southward at elevations less than 400 feet until they disappear half a mile north of the town of San Juan Bautista; but they rise to the north to more than 600 feet in elevation. If the Pajaro River had continued in its southerly course from Santa Clara Valley, it would have reached this area. In order to continue to flow west to Elk-horn Slough, it must have entered a deep notch in the granite ridge, like Pajaro Gap. As the notch was carried northwestward by right-lateral movement, the entire drainage would have been blocked when the notch passed behind the higher range of Purisima hills just east of the fault. In this area, the Anzar rift, landslides slid into and blocked the outlet, forming the dam which produced Lake San Benito. How quickly it was dammed is a question, but the trap was nearly 2 miles long. The dam may have persisted for about 50,000 years. There is no notch on the east side through these higher Purisima hills.

Along the west side of the San Andreas fault, from just north of the town of San Juan Bautista to Pajaro Gap, the belt of basement rocks on which sediments rest is 6 miles long. At the southern end, the basement rock is schist, representing the oldest rocks of the Coast Ranges, the Sur Series. The rest of the ridge is made up of hard quartz hornblende diorite, the same type of granitic rock mined at the Logan quarry. It was this belt of old hard rocks that the river system had to cut through to make its way to Monterey Bay. The deep outlet notch was made through this granitic rock when it was on the west side of the fault from the pre-lake San Benito River. Then the fault moved the notch north-westward, breaking its connection with the river. According to the water-well records, this was half a mile southeast of San Juan Bautista. The notch was then moved by the San Andreas fault 7 miles northwest to its present position at Pajaro Gap. The drainage was blocked by slides halfway on its journey.

If the fault moved then at its present rate of about 2 inches per year, it would have taken more than 200,000 years for the notch to move the total distance, while the dam, which trapped the lakes, was formed less than 100,000 years ago. Therefore, these lakes could not have existed in early Pleistocene and Pliocene time, which was millions of years ago.

Between the southern bend of Vergeles fault and the straight San Andreas fault is an area of extremely complex graben structure in Tertiary rocks. It is a down-dropped block, measured in thousands of feet. Granite on the west side of the Vergeles fault is in contact with Miocene volcanic rock on the east side. A well near Dumbarton road, on the east side of the fault, bottomed in Eocene at a depth of 7136 feet. )Occidental Petroleum Corp. Bingaman No. l, sec. 34, T.12S., R.3E.) The major structural disturbances that accomplished these changes probably took place millions, rather than thousands, of years prior to the horizontal offsets on the San Andreas fault that formed Lake San Benito and Lake San Juan.

However, the landscape between the Vergeles and San Andreas faults must have been drastically changed, because well-developed Elkhorn Valley does not continue eastward to the San Andreas fault, although the Pajaro-San Benito River drainage once crossed the fault into Elkhorn Slough, now an abandoned river valley. It is herein suggested that horizontal movement carried this section, or a part of it, northward in late Pleistocene or more recent time.

Another interesting feature is the alluvial cone just below Pajaro Gap and north of the town of Aromas. Crossing the San Andreas fault, the river flows over granite bedrock at elevation 75 feet. No gravel or boulders are now being contributed to the cone, although it contains quite a thickness of sand, gravel, and boulders, as well as clay and silt. During the drainage of the Pleistocene lakes, the water must have been clear. This means that the coarse material was deposited before the outlet was moved northward, and before the lakes were formed.

A well, 550 feet deep, which was drilled 6,500 feet downstream from the Gap currently produces adequate water for the plant of the Granite Rock Company. The driller's log indicates that 226 feet of the hole was in alluvium and the rest was probably in Pliocene Purisima sediments. Since the elevation of the collar of the well is 80 feet, the elevation of the bottom of the old channel must be 146 feet below sea level.

It is conceivable that this part of the Pajaro River with its cone (nearly 2 miles long by about a mile wide) was once located below the pre-lake San Benito-Pajaro River, half a mile southeast of San Juan Bautista, where it was cut off by the San Andreas fault. Here the "blue clay zone", contoured by Ellis "is at zero elevation where the fault terminated it. The surface elevation at this point is 200 feet; the depth to gravel is about 250 feet, which would mean that the river cone on the west side of the fault was about the same as the present cone elevation of the Pajaro River north of Aromas. Subsidence may have caused the minus sea level elevations or change in sea level during the Ice Age.

It is herein suggested that the 4-mile offset on the San Andreas fault affected principally the graben-strip lying between the Zayante-Vergeles and San Andreas faults.

GEOMORPHIC CHANGES ON THE WEST

SIDE OF THE FAULT

The changes in the physical features on the west side of the San Andreas fault were even greater than those on the east side after the 4-mile offset.

First, there may have been a general drying up of country that had previously supported a good river system; but when Pleistocene Lake San Benito started to spill over its rim, some water again flowed down Elkhorn Valley. When the lake finally broke through the dam and started pouring through the new outlet at Pajaro Gap, the lake was lowered to the 200-foot level, leaving Elkhorn Valley nearly dry. Escaping water from the east side of Lake San Benito formed temporary Pleistocene Lake Aromitas on the west side in the little basin, or cul-de-sac, around the present town of Aromas.

Then the flood water broke through the second Chittenden Pass which lies a few miles west of Pajaro Gap, but only after partly escaping over two hanging spillways on the south rim of Lake Aromitas. Next, Pajaro-Corralitos Valley was filled with another lake, Pleistocene Lake Pajaro. Before Lake San Benito had time to break through a coastal barrier of hills, it spilled over a range of hills lying to the south, comprised of Aromas Red Sands. Several spillway valleys were formed, all of which led into the abandoned Elkhorn Valley.

This spillover of the lake on its south side left interesting little hanging spillways, each of which is now occupied by a road. The first of these on the east is the highest in elevation, 290 feet, while below its head is the town of Aromas, elevation 100 feet. This hanging spillway is now occupied by Carpenteria Road; its drainage ran into Elkhorn Valley, and at the crossing of the Zayante-Vergeles fault, it shows a right lateral offset of less than half a mile.

The second of the hanging spillways is occupied by San Juan Road; its highest elevation is 190 feet. It starts at the Aromas cul-de-sac and Pajaro Valley, and runs into Elkhorn Valley.

The third hanging spillway, at elevation of 100 feet, is occupied by San Miguel Canyon Road, where the escarpment is apparently offset 2 miles right laterally on the Zayante-Vergeles fault. All of these little spillway valleys drain toward Elkhorn Valley, except for a short way at their north ends, where recent erosion has cut them back. They now contain so very little drainage that in some places swamps and lakes have formed; the sides have washed down and have no flowing streams to flush them out. At the mouths of these spill-way valleys, there are alluvial fans.

There is a fourth hanging spillway, occupied by Vega Road, which is 150 feet in elevation. It flows in a southwesterly direction and connects with a very short hanging spillway, 90 feet in elevation, through which Garin Road runs. This is the fifth hanging spillway; the sixth connects with these last two and is occupied by the main line of the Southern Pacific Railroad and also Elkhorn Road (the original county road connecting Watsonville and Pajaro with Salinas). It is reported that in modern times, during a high flood in Pajaro Valley, water runs through this little valley (present elevation 50 feet) and into the head of Elkhorn Slough, where ships used to go from the coast before the 1906 earthquake caused the deep mud of the slough to well-up, blocking it to such traffic.

The elevation of Pajaro Valley now ranges from sea level to 60 feet. There is a 200-foot-high escarpment on its south side. Old meanders on the Pajaro River cut the banks at 100 feet elevation. Lake terraces at different levels are now visible. The broad, even flatness of the valley indicates that it was once a lake bed, and was so recognized by W. H. Brewer as early as 1864 ("an old lake filled in as is shown by the terraces around its sides.") Lake beds are exposed on Garin Road overlying the Aromas Red Sands. The outlet of Pajaro River now lies between coastal hills that are over 100 feet in elevation. The river opened a passageway through them that is now 14 miles wide. It enters Monterey Bay without a submarine canyon opposite its mouth; whereas one of the deepest canyons in the world-Monterey Submarine Canyon-lies directly opposite the mouth of Elkhorn Slough. The famous Monterey Submarine Canyon, however, was not formed entirely during the late Pleistocene time. It is quite old, dating back to the Miocene Epoch, according to some authorities. The drowning of Elkhorn Slough was probably more recent -likely taking place when the valley's previous river cut it during the Pleistocene Epoch while the sea level was lower than it is today. The earliest lakes described herein were all younger than this.

It is sometimes more difficult to visualize what was happening in very late Pleistocene time or early Holocene (Recent) than to explain the events of much earlier geologic times. We live in a period of great seismic activity responsible for dams, lake, floods, numerous landslides, and the changing of the courses of major rivers.

Acknowledgments

The geologic map and report on the San Juan Bautista quadrangle by John Eliot Allen served as a background and inspiration for the present discussion. Sub-surface information was supplied from ground-water reports by William Ellis, the U.S. Bureau of Reclamation, and Chabot Kilburn of the U.S. Geological Survey. Recognition of the former existence of Pleistocene lakes by their silt deposits came about through investigation and drilling for sand and gravel for the Granite Rock Company, Bruce Woolpert, President. Valuable suggestions were contributed by Oliver Bowen and Gordon Oakeshott during the course of the study. Manuscript reading, editing, and invaluable criticisms by Joseph Clark were used in the final preparation of this paper. Photographic contributions were made by Mary Hill. The maps were drawn by Mrs. Adrienne Morgan. To all of these friends, I am very grateful.

References and Maps

Note: With the exception of papers dealing with recent movements on faults, most of the geologic reports listed here concern times earlier than the very late Pleistocene when the lakes herein described were formed.

1. Allen, John Eliot, 1946, Geology of the San Juan Bautista quadrangle, California: California Division of Mines Bulletin 133, 112 p., 12 pl., 10 fig.

2. Baldwin, T. A., 1967, Morphologic clues to geologic history of the northern Salinas Valley and San Andreas fault: in Guidebook to Gabilan Range and adjacent Son Andreas fault: American Association of Petroleum Geologists, Pacific Section, and Society of Economic Paleontologists and Mineralogists, Pacific Section, 1967, p.

92-93.

3. Beard, C. N., 1941, Drainage development in the vicinity of Monterey Bay, California. (abst.) PhD thesis, University of Illinois.

4. Bishop, Charles C., and Chapman, Rodger H., 1967, Bouguer gravity map of California, Santa Cruz Sheet: California Division of Mines and Geology.

5. Brabb, Earl E., Maddock, Marshall E., and Wallace, Robert E., 1966, Field Trip, San Andreas fault from San Francisco to Hollister: California Division of Mines and Geology Bulletin 190, p. 463-4.

6. Branner, J. C., 1907, A drainage peculiarity of the Santa Clara Valley affecting fresh-water faunas: Journal of Geology, v. XV., no. 1, 10 p.

7. Brown, R. D., Jr., and lee, W. K., 1971, Active faults and preliminary earthquake epicenters (1969-1970) in the southern part of the San Francisco Bay region: Basic Data Contribution 30, to accompany mop MF407.

8. Clark, Joseph C., 1970, Preliminary geologic and gravity maps of the Santa Cruz-San Juan Bautista area, Santa Cruz, Santa Clara, Monterey, and San Benito Counties, California: U.S. Geological Survey Open-file Map scale 1:125,000.

9. Clark, William O., 1924, Ground water in Santa Clara Valley, California: U.S. Geological Survey Water-Supply Paper 519, 290 p., XiX pi., 20 fig.

10. Derieth, Charles Jr., 1907, The destruction extent of the California earthquake . . .: in, Jordan, D. S., The California Earthquake of 1906: p. 191-196.

11. Ellis, William C., 1952. Elevations on the top of the blue clay zone, Hollister area: U.S. Bureau of Reclamation, unpublished map.

12. Farquhar, f. P., 1930, Up and dawn California in 1860-1864. The journal of William H. Brewer. Princeton, Yale University Press, p. 152,289.

13. Fowle, Royal E., 1946, Operations of the Granite Rock Company quarry and plant at logan, San Benito County: California Division of Mines Bulletin 133, pp. 77-78.

14. Greene, H. G., 1970, Geology of southern Monterey Bay and its relationship to the ground water basin and salt water intrusion: U.S. Geological Survey Open-file report.

15. Griffin, William 1., 1967, Provenance, deposition, and deformation of the San Benito Graveix, California: in Guidebook to Cabian Range and adjacent San Andreas fault. American Association of Petroleum Geologists, Pacific Section, and Society of Economic Paleontologists and Mineralogists, Pacific Section, 1967, p. 61-73. 16. Jenkins, Olaf P., 1938, Geologic mop of California: California Division of Mines, scale 1:500,000.

17. Jennings, Charles W., and Strand, Rudolph G., 1965, Geologic Map of California, Olaf P. Jenkins edition, SanIa Cruz Sheet, California Division of Mines and Geology, scale 1:250,000.

18. Jones, William F., 1911, The geology of the Sargent oil field: Uni. versity of California Publications, Bulletin, Department of Geology, v. 6, no. 3, p. 55-78, pi. 13-18.

19. Kilburn, Chabot, 1970, Ground-water hydrology of the Hollister and San Juan Valleys, San Benito County, California 1913-68: U.S. Geological Survey, advance copy.

20. Maddock, Marshall E., and Hudson, Travis l., 1968, implications of Franciscan rocks near Palaro Gap regarding movement along the San Andreas fault: Proceedings of conference on geologic problems of the San Andreas fault system. Stanford University Publications, Geological Sciences, v. XI.

21. Martin, B. D., and Emery, K. O., 1967, Geology of the Monterey Canyon, California: American Association of Petroleum Geologists Bulletin, v. 51, no. 11, p. 2281-2304.

22. Oakeshott, Gordon B., 1966, San Andreas fault in the California Coast Range province: California Division of Mines and Geology, Bulletin 190, p. 370.

23. Omori, F., 1907, Preliminary note on the cause of the California earthquake of 1906: in Jordan, D. S., The California earthquake of 1906, p. 299.

24. Rogers, Thomas H., 1967, Active faulting in the Hollister Area, In Guidebook to Gabilan Range and adjacent San Andreas fault: American Association of Petroleum Geologists, Pacific Section, and Society of Economic Paleontologists and Mineralogists, Pacific Section, 1967, p. 102-04.

25. Rogers, Thomas H., 1972, Geologic map of the Hollister quadrangle, 1:24,000: California Division of Mines and Geology, in progress.

26. Ross, Donald C., 1970, Quartz gabbro and anorthositic gabbro: markers of offset along the San Andreas fault in the California Coast Ranges: Geological Society of America Bulletin, v. 81, p. 3647-3662.

27. Ross, D. C., Wentworth, C. M., McKee, E. H., 1973; Cretaceous mafic conglomerate near Gualala offset 350 miles by San Andreas fault from oceanic crustal source near Eagle Rest Peak, California: U.S. Geological Survey, Journal of Research, v. 1, no. 1, January-February 1973, p. 45-52.

28. Starke, G. W., and Howard, A. D., 1968, Polygenetic origin of Monterey Submarine Canyon: Geological Society of America, Bulletin, v. 79, no. 7, p. 813-826.

29. Taliaferro, N. L., 1948, Geology of the Hollister quadrangle, California: California Division of Mines, Bulletin 143 (map only), scale 1:62,500.

30. Tocher, Don, and Nason, Robert, 1967, Fault creep at the AlmadenCienega Winery, San Benito County: In Guidebook to Gabilan Range and adjacent San Andreas fault: American Association of Petroleum Geologists, Pacific Section, and Society of Economic Paleontologists and Mineralogists, Pacific Section, 1967, p. 9-10.

31. Woodford, A. O., 1951, Stream gradients and Monterey sea valley: Geological Society of America Bulletin, v. 62, no. 7, p. 799-852.

U. S. Department of Agriculture Soil Survey Maps Scale: 1:62,500

Soil Survey of the Salinas area, California: no. 11, Series 1925, operations 1925. Map covers Elkhorn Slough and shows its outlet with that of Salinas River, one mile north of Mass landing; also shows Pajaro Valley Consolidated Railroad, since removed. indicates "marine or freshwater shells" in "Elkhorn sand" near coast.

Soil Survey of the Santa Cruz area, California: no. 25, Series 1925, issued Jan. 1, 1944. Map shows Soda lake, Pajaro Gap, meanders of Pajaro River on terrace, and hanging valleys, with lakes in them.