STOP 1: The Alexander Offramp Bus Stop. The roadcut east of Stop
1 is through a ridge of basalt. The core of the rigde is hard and green. The
rock near the original landsurface near the top of the cut is reddish brown.
The cut on the west side of 101 Freeway, northwest of Stop 1, is sandstone.
The core of this roadcut is grey, but near the original surface the color
of the sandstone grades to light tan.
Between the Basalt and the sandstone is chert. Thin-bedded red chert
makes up the southwest flank of the basalt ridge and is exposed on the north
side of the Alexander Av. roadcut, east of the Freeway. The line of grey rocks
going uphill on the east side of the freeway roadcut is a particularly resistant
recemented layer in the chert.
This exposure of chert is Bonnie Murchey's type section for the Marin
Headlands radiolarian fossil zones, chosen as such because it clearly rests
on basalt, has sandstone clearly resting on it, and is not folded or faulted.
However, it is on the freeway, dangerous to study, and cannot be visited with
out a special permit from CalTrans, which is very grudgingly granted.
The grassy patch across the freeway is the site of a persistent landslidewhere
culuvium probably filled a draw eroded into the sandstone. Chert makes the
ridge crest to the west. A major fault hidden beneath the grass and culluviasl
soil extends across the slope, seperating the chert above from the sandstone
below.
Proceed south along the east side of the freeway to the View Plaza.the
road that you follow is the main road going up hill. (You will find toilets
and drinking fountains here.)
STOP 2:South side of the View Plaza. The needle rocks that are visable
along the shore are pinnacles of the chert body we saw across Alexander Av.
at Stop 1. Amazingly, they were the first geologic features in California
to be sketched.
The field stone used in the wall and pillars are boulders of brown basalt
from southern Sonoma county. This basalt erupted as flows on the land. We
contrast it's appearance with the submarine basalt, as you will notice a little
later. Note that it flashes with tiny black crystals, and that some boulders
are full of
Follow the steps down the pedestrian crossing underneath the Freeway
and to the road uphill on the west side.
Stop 3: The first rock exposure on the west side fo the freeway is
sandstone. Study a fresh sample with a hand lens. The translucent gray sand
grains are quartz. The chalky ones are feldspar. The dark ones are minerals
(hornblende, augite, etc.) and volcanic rock fragments. The sandstone breaks
into fragments bound by plane surfaces that join along sharp edges and corners.
The thicket fo the willows on the uphill side of the junction with Conzelman
Road probably marks a spring, for the willows are phreatophtes whose roots have to
penetrate abundant ground-water.
Stop 4: The angular rubble in the roadcut is colluvium, full of angular chert fragments from the chert farther up-slope. The madrone bush on the uphill side is probably close to where the road crosses the fault.
Stop 5: The thin beds of red chert and shale in this excellent outcrop
are tightly folded into folds and 0.5-1 m high and 1-2 m from crest to crest,
with plane sides and sharp crests and troughs. The chert are the hard blocky
beds, and the shale beds are the thin, soft, flaky, beds between them that
weather out, leaving the chert standing in relief. The lines of the crests
and troughs (called Hinge Lines) plunge on the average
about 20° -30° N 80° W. See figure 18-a for a sketch of the folds
of this outcrop.
The folds lean to the north, and the north limbs of anticlines are dhorter than their
south limbs. This configuration of minor folds in the chert is common throughout
the Marin Headlands.
If you stack some sheets of soft paper or cloth between two boards and
slide the top board over the bottom, you will create folds like these; thus
these folds could of been formed if the rock above them had moved to the north.
Since we are dealing with subduction, it was the rock below that moved south.
However, from what we know of subduction along the west coast of North America,
the rock below should have been movign north or northeast. Therefore after
the rocks fo the Marin Headlands wree dubducted and welded together as a single
block, the whole block appears to have been rotated 90°-120° about
a vertical axis.
Independant evidence that this probably happened is found in the magnetic
compass orientaiton frozen into the grains of magnetic minerals in the rocks
presumably at the time of subduction. The inclination of these paleomagnetic
compass needles suggests that their magnetization was acquired 20° farther
south relative to North America than they are now, and also that their present
declination (or horizontal direction) which is S 80° E, can be bought
into conformity with the known position of the poles relative to North America
by a counterclockwise rotation of 120°.
Notice that each end of the outcrop, and over the top, the chert is
covered by rubbly colluvium. This exposure was invisible until the road was
made.
Stop 6: The chert here is not especially folded. Behind the lupine
bush are some large knots of chert, into which the shale beds pinch out. This
is evidence that the chert and shale beds were formed, not as layers of pure
silica and clay deposited one on the other on the sea floor, but rather as
segregation's of silica and shale from a formerly more homogeneous ooze that
took place sometime after deposition.
The chert and shale beds appear to be parallel to the original despositional
surfaces. The probable explanation for this parallelism is that there were
cylic variations (with a period of tens of thousands of years) in the proportion
of dust and radiolarian shells that settled to the ocean floor, and that the
silica tended to migrate to where the silica was more concentrated, and the
clay components to where the clay minerals were more concentrated to achieve
the remarkably pure separation that gives chert it's striped or "ribboned"
appearance.
From here we get
a good view of Angel Island, Tiburon, and Raccoon Straits. Angel Island and
Tiburon Peninsula are an entirely different terrain of Franciscan rocks than
the Marin Headlands or any other place in San Francisco. Their rocks contain
abundant minerals characteristic of bluechist metamorphism, so they must once
have been subjected to pressures equal to 20 to 30 km of overlying rock. They
had to have gone down fast, metamorphosed, and returned to the surface quickly,
so that their temperatures did not rise above 300° C. These pressure-
temperature conditions are based on laboratory experiments and thermodynamic
calculations. How this happened we do not yet know, and are still speculating.
Raccoon Strait was the course of the Sacrament- San Joaquin River through
the dry valley that was San Francisco during the ice ages.
Stop 7: The "brick dike" in the road cut at the point hides a cable
that went to fortifications south of the road. Ten feet west of it, the chert
is in contact along a steep, barely perceptible fault, with orange-brown weathered
basalts. At the no-parking sign farther west , this basalt is in contact with
chert to the west, probably also along a fault. Large bulbous masses of chert
with irregular veins of white quartz with crystal-lined cavities are here.
The black coatings on the thin bedded chert are of the mineral psilomelane. This is a common ore
of manganese, and was mined in the Marin Headlands during WWI. Only the lowest
few m of chert have the Magnesium Dioxide staining, which was probably imparted
by circulating hot water close to the spreading center.
Just to the west the chert beds are cut off sharply by a vertical fault.
Except for a small infaulted tongue of chert near the top of the road-cut,
the rock from here nearly to the next point is basalt.
Stop 8: Between the last no-parking sign and the yellow post (marking a culvert) the basalt at the cliff top gradually becomes dark green basalt at road level. The rounded lumps visible in the road cut are pillows. It is usually hard to recognize pillows in weathered basalts, and we will later see better ones on the beach (if the tide is right). At the sharpest point of the inside curve in the draw, the apparent layers of basalt dipping southwest are pillows cut along their long directions.
Stop 9: At this point the lower few meters of the next chert layer
rests on basalt. The lowest beds of chert are thick and pale red. Overlying
the are thin beds of chert black with magnesium dioxide. The beds strike northwest
and dip southwest. A synclinal fold down the face of the chert plunges 30°
SW.
From here we can see west to Kirby Cove and Point Diablo. The valley
of Kirby Cove and the pass at its head are carved in soft sandstone and shale.
The ridgecrest west of Kirby Cove and below the Headlands road is held up
by chert, which makes gray outcrops along its crest. Its lower slopes on this
side are basalt. In the higher hills along the skyline to the west are visible
at least three more repetitions of chert, which we will cross along this walk.
The chert and basalt that we have been on, since crossing the freeway, strike
northwesterly toward the pass and ridgecrest east of it, and are cut off along
a fault on this side of the sandstone.
Stop 10: At Stop 10 we cross the contact between chert and basalt that we crossed just east of Stop 9, and are the basalt we were in before. The chert-basalt contact goes straight sown the hill and up the east side of the grassy spur this side of the eucalyptus forest, which is chert. Sandstone underlies the valley behind that spur.
Stop 11: The bare spoon-shaped hollow above the road is where the colluvium filling a v-shaped hollow failed as a catastrophic mud-slide in 1982 or 1983. The gabions below the road are to repair the damage the mud-slide created. The strips of soft chaparral in the hollows above the road to the north mark the colluvium, which stores moisture through the summer months. Chert bedrock comes to the surface on the crests of the spurs, which are dry through the summer and therefore only support annual grasses.
Stop 12: The controlled chert in this road-cut has undergone two periods of folding. Fold hinge lines plunge west and SW down the cut, and also trend SE parallel to the road. I do not know where the second (SE- trending) fold occurred, or how, or why. This makes a spectacular picture in the afternoon sun. Beyond the curve ( and sharp draw with elaborate culvert) the chert is tightly folded similar to Stop 5.
Stop 13: In the middle of this road-cut on the west side of the draw, an orange- weathering band of basalt about 50 feet thick goes straight up the cliff. The great mass of basalt we were on between stops 8 and 9, and which makes the cliff west of the bridge, has thinned down to this. Its north contact has to be a fault, and, so it appears, is its south contact.
Stop 14: We cross the basalt again here.
Stop 15: At the yellow post just east of the brown road sign, we
cross a fault onto yellow-weathering sandstone and shale. An active landslide
marks the fault. This has to be a fault, because the basalt butts into it
and is cut off a few tens of feet down the hill to the south. The fault trends
diagonally to the northwest, and crosses the skyline just west of the large
chert outcrop on the skyline.
West of the road junction in the pass, you can see a large landslide
on the east side of the road down to Rodeo Valley, also in the sandstone.
This landslide has not changed its appearance since the 1950's.
Stop 16: Weathered basalt is exposed in the road-cut by the white gate. Beyond it the road-cut is entirely in rubbly colluvium with angular fragments of chert from the hill above, and the next bedrock in the cut, about 100 feet north of the twisty road sign, is of chert
Stop 17: The tightly folded chert in the road-cut along here is illustrated
in fig. 18-b, (on pg. 17 of this article). Enormous gullies were carved in
the hillside below the roads by runoff from the road, due to the ineffectiveness
of the culverts improperly placed by the Army.
Stop 18: Here is a photograph of the folded chert.
Stop 19: A few feet of yellow-weathered sandstone rests on the chert
in a gentle anticlinal fold at this spur, with a syncline in the draw to the
north. Bonnie Murchey and I have not been able to collect the youngest radiolarian
fossil zones from the top of the chert here. Zone MH-5 seems to be the youngest
present. Some of the chert may have been eroded by the submarine currents
which deposited the sandstone.
A massive chert outcrop can be seen on the hilltop to the north. Below
the road are cliffs and ledges of massive gray-weathering chert, which can
be traced across the slopes to the south to the north end of the ridge on
the west side of Kirby Cove. These are outcrops of a particularly massive
recrystallized zone on the chert that, according to Bonnie Murchey, marks
the gap of 15m wide (called hiatus) when no chert was deposited.
The 15 of 20ft. of sandstone is overlain by about 40ft. of basalt. Their
contact is intensely sheared shale and must be a thrust fault. Chert, locally
manganese stained and therefore of the oldest radiolarian fossil zone, rests
on the basalt.
Stop 20: The well exposed contact between basalt and chert here must also be a fault. There is sheared purple shale along it. It dips about 45° S. From here to the next point, the basalt is nearly continuously exposed in the road-cut. You can make out the outlines of the pillows in the swirly lines on the road-cut.
Stop 21: From here we can look back to the north and can see the
enormous gullies carved by runoff from the road. Below us to the east is the
ridge on the west side of Kirby cove. The smooth rock outcrops on the west
slope of this ridge are bedding surfaces on the chert. This slope, parallel
to the bedding, is called a dip slope.
Right where the road bends sharply around the point, the oldest, MnO2-
bearing chert, rest on the basalt. Bonnie Murchey and I have prepared a guidebook
to be published commemorating the centennial of the Geological Society of
America. We needed a place where paleontologist could collect chert harmlessly
to study radiolarians. We chose this road-cut, and found that it has 3 of
the fossil zones, MH-1, MH-3, and MH-4.
Stop 22: There is a pit toilet in the southern of the two tunnels.
The basalt we saw on the east side of Stop 21 underlies the canyon to the
south, and the chert we are on underlies the lower half of the west side of
the canyon.
We will take the one way road down hill. It's all down hill from here
to the beach, so we'll stop dawdling over the geology, except to point out
that the round summit (Locality 23) between the road and the Golden Gate bridge
is held up by still another body of the same sequence of chert; basalt crosses
the saddle between the road and that summit.
Stop: 24 On the west side of the first saddle passed by the road is a thin wedge of manganiferous chert, between bodies of basalt. This chert thickens westward and pinches out eastward. The capping of the chert on the hilltop to the west is part of the same fault slice of chert that makes the summit Locality 23.
Stop 25: Where the road crosses to the north side of the ridge, we turn left through the parking lot, take the left-hand (east) fork. If you are not wearing long pants and a long-sleeved shirt, you are now going to be sorry, for there is a lot of poison-oak growing along the trail.
Stop 26: At the foot of the trail, walk first to the west end of the beach. The bluff behind is in gray sandstone. Several large landslides have poured onto the beach during winter.
Stop 27: Near the west end of the beach, some large blocks have fallen
from a point of rock to litter the beach. If you look up at the point, you
can see that part of it consists of thin-bedded chert. The chert rests on
sandstone with a fault contact. Notice how sheared the sandstone and shale
are at the base of the point. The blocks fell from the cliff just west of
and above the chert. They consist of two kinds of rock: a dark green rock
which is pillow basalt; and (2) a pink to white rock with white veins, which
can be scratched with a knife. This is limestone (CaCO3) which we can demonstrate
by causing it to fizz with a drop of dilute hydrochloric acid.
This limestone must have accumulated at fairly shallow depths in the
ocean, much shallower than the depths at which accumulated the chert we have
seen so far, as the following paragraph makes clear. Together with other evidence
from the basalt in exposures south of here, and at the south end of Rodeo
cove, we interpret the rocks between this limestone and Point Bonita to have
accumulated on a sea-mount, and not at a deep mid-ocean ridge.
Except for a strip of warmer water along the equator, the deep ocean
water is so cold and CO2 rich that all small carbonate shells have been dissolved
by the time they reach a certain depth, usually 2 or 3 km, which is called
the carbonate-compensation depth (or CCD for short). Only seamounts that rise
into shallower waters accumulated in very deep ocean water, and is generally
free of calcium carbonate to have preserved on this basalt, the basalt must
have been above the CCD, hence on a seamount.
Stop 28: If we can get onto the platform on the south side of the
sharp crested point, we will see some of the finest exposures fo pillow basalt
anywhere on land. We can tell which way the pillows flowed when they erupted
on the sea-floor, which way was up, and what was a horizontal plane at the
time the pillows erupted, using criteria I will demonstrate. Samples were
collected from this locality for measurement of paleomagnetism, and you can
see the holes were they were drilled.
° The large pile of boulders at the west end of the beach, just north
of the sharp- crested point, fell from the cliff during the storms of February
1986, and unfortunately buried some exciting and important geology. Luckily
we had mapped, photographed, and collected from this area just two weeks prior
to the storm, and can report on it-but no- one can now check up on us.
Stop 29: Return east along the beach. About 200ft. east of the access
trail, the sandstone ends and green red chert forms the sea-cliff. The chert
within a few m of the sandstone is the youngest chert of the sequence (Zones
MH-5,6,and 7). Stop 29 is at the east end of its outcrop, where we can get
fresh pieces of chert, in which to see the radiolarians,. At Stop 29, also,
the sandstone can be seen to be in depositional contact on the chert (the
contact has obviously been tilted, and now dips 70° S), and fragments
of the chert can be seen to be embedded in the sandstone. They were probably
ripped up from the ocean-floor by the turbitiy currents that deposited the
sandstone, and indicate that the chert was already segregated, and probably
pretty hard, by the time the sandstone was deposited.
This is a great place to eat lunch and have a sun bath.
The sandstone here is in thick turbidite beds separated by thin beds
of shale. At the next reentrant to the east, the sandstone is in angular blocks,
broken and drawn out between masses of sheared shale. This is typical broken
formation and melange matrix. Some green blocks in this mess are fragments
of chert. Others are thought to be submarine volcanic ash. The rock is so
badly sheared that it makes landslides, which in February 1986, covered some
important outcrops here that we had just photographed and mapped.
At the east end of the beach we are back at the fault zone we saw at
the west end of the beach, which has been running just offshore.
Stop 30: The deep notches here have been worn into the cliff by landslide
along belts of gooey melange matrix in the fault zone. Huge blocks of greenstone
and sandstone within the fault zone stand out as giant rocks. Within the melange
are smaller blocks of serpentine, chert, and green basalt. The cliff to the
south and the point west of Pt. Diablo, as well as Pt. Diablo itself, are
mainly pillow basalt, the same unit that we saw at the west end of the beach,
and excellent pillows can be seen on them. The little cove between the two
points exposes the fault zone, together with a narrow sliver of chert, which
is the main locality for zone MH-2.
After lunch we return to the beach access, climb back to the road, and
head north for Sausalito. At the top of the ridge, cross the road and take
the path down the draw north road.
Stop 31: The head of the willows on the draw probably marks where the ground- water table is close to the surface. There may be flowing water here.
Stop 32: Where the path ends at a deeply rutted dirt road along the south side of the equestrian field, turn left and follow the rutted road to the highway. Follow the highway about a 100ft. west, look for a road turning north off the highway. Cross highway and follow the brown sign to the Rodeo Valley, etc., trails. The sign here about ticks is instructive.
Stop 33: Look for bridge across creek. Cross bridge and immediately
turn right on path through reeds to the Rodeo Valley trail. Turn right-east
- up valley on this trail.
The ridge to the north is crossed by belts of chert and basalt we saw
just west of the pass between Stops 15 and 16. Chert makes the peaks and the
prominent spurs, and basalt generally underlies their swales.
Stop 34: Take the right hand trail. The valley and saddle here are
eroded into the sandstone and melange that make the pass at the road triangle
between Stops 15 and 16. The prominent chert outcrop on the left hand side
of the trail ahead is probably a block of chert embedded in the clay-rich
melange matrix. Abundant tules (Scirpus, the dark green reeds) growing in
and along the trail, gray and yellow clay in the roadside ditch, and the water
trickling in the ditch, all attest to the imperviousness of the melange, which
has forced ground-water seeping through the pores of the chert hillside to
the east and north onto the surface, making the spring.
Where the trail bends sharply to the right and starts to climb the ridge,
we probably cross onto chert.
Stop 35: There are good exposures of chert as the trail goes around the nose of the hill. This is an old farm road. There was a dairy farm in upper Rodeo Valley as late as the 1950's. the chert is part of a fault slice that underlies the chert we first saw west of the freeway. This slice pinches out southeastward in the hill on the other side of the Rodeo Beach road, and the fault slice we saw earlier pinches out near the cluster of military housing in the valley south of us.
Stop 36: The road crosses from chert onto sandstone. This is the
great fault we crosses at Stop4. From here to the hilltop we will be on sandstone,
the same sandstone we saw at Stop 2. Part of it appears crushes and clay-rich,
so I mapped it as melange matrix. It is deeply weathered, pale yellow, and
soft and crumbly.
The high steep ridge east of us, Wolfback Ridge, consists of chert of
the type section we saw at Stop 1. The low divide between Wolfback Ridge and
the 950ft. hill west of our bus stop, is where the large body of sandstone
deposited onto the chert of the type section, which sandstone we are now on,
crosses the drainage divide of Rodeo Valley. If you look up to the southeast,
you can visualize how the west face of Wolfback Ridge is a dip slope on chert,
how the ridges make the chert bodies, and how the rocks dip southeasterly
and dip to the southwest.
The 1111ft. hill to the northwest of us is capped by chert, probably
part of the same body we were in between stops 35 and 36. I had much trouble
mapping the geology around this hill , because of poor exposures and confusing
sandstone-chert relationships.
Stop 37: As we approach Stop 37 we are almost at the top of the type section of chert, which makes a low ridge at the head of the valley. Take the path to the left (north) to join the road to the TV towers at a colorful metal pipe structure; turn right (east) on the road for about 100ft., then turn left (north) onto a road with a barricade that leads along the contour into the eucalyptus forest on the east side of the 1111ft hill. (You have to go around to the right of the barricade). DO NOT go east-down hill; this is a private road.
Stop 38: Near the north edge of the forest, look for a brown Park Service sign indicating the Morning Sun Trail. Take the trail downhill to the freeway. The upper slopes are underlain by the body of chert at the type section, and near the bottom the trail crosses a fault (invisible here) to a lower slice of chert.
Stop 39: Cross the access road and walk along the north side of the tunnel beneath the freeway. Cross at the pedestrian lane 50ft. north of the east end of the tunnel to the east side of the frontage road, and walk right (south) along the frontage road to the gravel parking lot and the fire station.
Stop 40: Walk single file along the left side of Spencer Avenue downhill to the left, staying as close as you can to the curb, and be prepared to jump into the bushes. The Sausalitians drive their Porches and Ferraris like they are on a Gran Prix and cut the insides of bends closely. The safety of pedestrians is not a concern of theirs.