The Paleocene blob revisited

15 February 2021

It may seem to readers like I’ve been out in the field during the past year, but in fact I’ve been holding back since the lockdown last March, taking my own advice. Last week, on the verge of receiving the Covid vaccine, I decided to formally resume geologizing, and start by giving a small patch of rocks a new, more thorough inspection.

It was thirteen years ago when I first reconnoitered this odd little area of rocks, shown on the geologic map as unit Ta, “unnamed glauconitic sandstone (Paleocene).” It’s described as “coarse-grained, green, glauconite-rich, lithic sandstone with well-preserved coral fossils. Locally interbedded with gray mudstone and hard, fine-grained, mica-bearing quartz sandstone.”

Here’s a closeup in Google Maps with the outline of the “Ta” unit. It manifests as a ridge-forming substrate that is undermined by an active landslide scar (part of which is the notorious Snake Road/Armour Drive landslide) on its northwest end.

In a systematic approach, I sought out the three places marked on the map with strike-and-dip symbols. (I used this same strategy a couple years ago with the overlying Eocene mudstone unit.)

The northernmost site, at the stub end of Armour Drive, is hopeless; it’s been thoroughly disrupted by the Snake Road landslide, and the fortress houses being built on the scar will disturb it more as the owners landscape their grounds. There were no good exposures at all, let alone one showing beds dipping 80 degrees south. But this is what some of the rock looks like: a dark siltstone with a greenish tinge and a bit of clay in it.

The middle locality was where my hopes were highest — an aborted foundation pit on Saroni Drive where the “well-preserved coral fossils” had been documented. In fact, I had asked Russ Graymer, compiler of the geologic map, about this pit. That was in 2009, which by his account was 14 years after he’d visited it (or a good 25 years ago today). He replied that his notes from the site were as follows: “The rock here is massive, black, coarse-grained, glauconitic sandstone and pebbly sandstone. There are many fossils here, including pecten, coral (Paleocene?), shark teeth, and snail. There is also pink-brown siltstone and brown mudstone.”

All I can say is I wish I’d been here 25 years ago.

I gave the site a thorough look, without hammering anything as is my practice. I saw no pebbly mudstone, not even any coarse-grained sand. I noted clayey siltstone and silty shale, hard here and soft there, with fine to massive bedding. On the lefthand side the shale beds were vertical, with the upper side to the east. Nothing that I could possibly interpret as overturned beds with a 60-degree dip.

Elsewhere the rocks had no reliable bedding. Down in front were some crumbling mudstone boulders. One of them had some vague fossil-like shapes that fizzed in acid, but the eyes can be fooled and our rocks commonly have some lime in them. It’s not always meaningful, though I always check for it.

You may wonder how this rock unit was determined to be of Paleocene age, unique in Oakland. As I recall our conversation, Graymer was accompanied that day by Earl Brabb, who said the corals reminded him of Paleocene corals he knew from the Santa Cruz Mountains. In fact I wrote Brabb for more detail and he replied with the location of the roadcut he had in mind. But I never got over there, the email was lost, and Earl Brabb died a few years later. Now I would never gainsay Brabb’s judgment — he was a top-tier field geologist — but that’s the main line of evidence behind this age assignment.

I wish he had been with me at the third site. It’s under a power-line tower north of a bend in Balboa Drive and consists of thin-bedded siltstone, nicely tilted. This spot, at least, is still good.

The roadcut on Balboa Drive was where I hit paydirt. Bedding surfaces were exposed that included sole marks. These occur on the underside of beds, and they indicate that here the rocks are overturned, contrary to what the map shows.

And in the gutter of the curve, buffed by errant car tires, were a couple of these round, laminated objects nestled in situ among the siltstone beds. They responded to acid, indicating that the laminations included calcite. And the rocks nearby displayed a fine vein of solid calcite about 4 millimeters thick.

I would peg these as some sort of fossil, but Earl Brabb might well have said they were just like the Paleocene corals he knew from the Santa Cruz Mountains. The setting could have been a cold seep, such as are known elsewhere in the Great Valley Sequence.

The rocks of the Oakland Hills are poorly organized and poorly exposed, and hence not really well mapped. They’ve been overturned and broken and shuffled around. Whenever I try to make sense of them I doubt my senses; that’s the way the Earth just is here. A geologic map is as much an exercise in imagination as in observation. The pros are certainly better mappers than I am, but they aren’t superhuman and their work can be interrogated; the rocks can speak differently with each visit. The outline on the map, as far as I can tell by checking around its edges, is fairly correct — you’ll notice that every line is dashed, meaning it’s inferred, not firmly nailed down.

The “Ta” rock unit hasn’t revealed itself to me as a coarse-grained green lithic sandstone, more like a fine-grained sorta greenish lithic siltstone. Geologists train themselves and have tools to specify rock colors, but to me green is always suspect; our woods favor mosses and algae, and our weathering environment favors rusty colors.

The rock here is definitely something other than the Redwood Canyon Formation to the south and the Eocene mudstone to the north. It’s a little piece of somewhere different.

Mapping rocks never ends

1 February 2021

A few days ago I took part in the latest monthly meeting of my local geological society — we do it via Zoom these days — and our speaker, Christie Rowe of McGill University, reported on three research projects her grad students are doing in the Bay area, specifically the Franciscan Complex. The Franciscan is a scramble of different rocks that has challenged geologists since they first came to California.

Fifty years ago Stanford’s Gary Ernst recognized that the Franciscan represents the mess of material that gathers around a subduction zone, where oceanic crust (a now-extinct neighbor of the Pacific plate, in our case) slides beneath continental crust (the North America plate). So now we know what it is — the tectonic equivalent of the dirt in a bulldozer’s blade — and prompted by that knowledge we can try to unscramble the mixed-up pieces and learn what they might tell us about California’s geologic history or what happens in subduction zones.

Rowe is a Marin County native who’s been working since her PhD days on the latter problem, in the Franciscan rocks of her home ground. Specifically, she’s been looking for preserved bits of ancient earthquake faults. Normally these are buried deep underground, but they’re important because subduction-related earthquakes, so-called megathrust events, are the largest on the planet. Think of the 2011 Tohoku earthquake in Japan, the magnitude-9 monster whose tenth anniversary is coming up on 11 March. The Marin Headlands are full of them, broken in pieces.

Rare bits of the Franciscan have survived being subducted deeper than 25 kilometers and then returned to the surface, without totally wiping out what happened to them down there. The work requires dogged persistence. You have to look hard to find these “high-grade blocks” in the first place, then put your face close to them, magnifiers out, detect signs of slippage, then bring samples to the lab and determine what that slippage means — whether it happened on the way down, on the way up or afterward as the San Andreas fault system wrenched it all sideways.

Heart Rock, at Jenner Beach up the Sonoma coast, is small enough to fit inside a living room. One of Rowe’s grad students is mapping it at centimeter scale, spending a master’s thesis worth of effort on this one outcrop looking at rocks like this.

Seeing all this during Rowe’s talk took my mind, among other places, out to the rocks of Shepherd Canyon and Redwood Peak. The last person to give those strata a PhD-level scrutiny, using all available tools of the time, was a Berkeley grad student named Jim Case around 1960. Yes, 1960, a time when researchers were stuck in a mental framework of now-forgotten concepts and plate tectonics was still years in the future, when optical microscopes, brass seives, fossil correlations and test-tube chemistry were the best tools we had.

Case got his PhD, demonstrating that he’d mastered these tools as well as the literature, but he didn’t accomplish much more than correct a couple of ideas from earlier studies, establish a few new rock units on the map and tentatively correlate them with other units scattered around the East Bay. He put his little brick into the Wall of Science, then went on to a long research and teaching career doing other things.

Since then, other distinguished geologists have been over this territory. Case collaborated with Dorothy Radbruch of the USGS, a sharp and able field geologist. And in the late 1990s when Russ Graymer was putting together the East Bay geologic map that I rely on, he tramped the area with the late Earl Brabb and was ably advised by the late David Jones. Each of these workers found new things and revised their predecessors’ achievements. It always paid to reinspect the rocks. Nevertheless, none of them pulled out all the stops and pioneered a new in-depth reassessment of this interesting area.

We could do much better today. Every tool has advanced. The jigsaw puzzle of ancient California is far enough finished that any piece, if studied closely enough, can be placed on the table near — or even exactly on — its correct position and joined to other pieces. This would be more satisfying than what Case could accomplish in his time. We just need another grad student to take it on, another local who has imprinted on his or her home ground.

Mapping never ends, and geologic mapping always improves. New bits of rock are being exposed all the time. Fresh eyes see new things, and persistence furthers.

Anza and the Fan

18 January 2021

After Pedro Fages came through the East Bay in 1770 and 1772, no one from New Spain appears to have visited the land on which Oakland sits until Juan Bautista de Anza led an exploring expedition here in the spring of 1776. Scholars seem to be quite sure of where the group went, but when I examine the record I find more and more room for interpretation and inference.

I’m not going to pretend I’ve examined the manuscripts or know 18th-century Spanish. I’ve accessed translations of those manuscripts and, well, here’s some of the questionable things and wiggle room I see in the record.

  • Apparently no one in the group, neither Anza nor the group’s diarist, Father Pedro Font, had been with Fages in the earlier visits, so they had only a slim written record to compare against the countryside they saw. We can’t rely on their interpretation, especially as it appears they took a different route than Fages.
  • We can’t rely on their directions. The East Bay from Berkeley south is persistently slanted 33 degrees west of north (thanks to the Hayward fault and the plate boundary of which it’s part) making it hard to eyeball true directions; magnetic north was apparently 12 degrees east of true north at the time (thanks, NOAA); and Font complained about the poor quality of his compass, so the explorers’ impressions are suspect. The men were not experienced sailors either, people I might trust, but army soldiers. Moreover, I wonder about the transcriptions. There seem to be too many instances of “northwest” (noroeste) and not enough of “northeast” (noreste) to fit the written route on modern maps. And the scholars, whom I trust on this topic, point out that very discrepancy between different copies of these documents, all of which were made by hand.
  • We can’t rely on their distances. No one had odometers. Font did his best, prefacing his notes with a discussion of the length of the Spanish league (approximately 2.6 miles, apparently), but even so the men were on horseback whereas Fages had been on foot. On the day they came through Oakland, Anza wrote down that they went “about ten leagues” while Font put it at “some fourteen leagues.” The day’s ride was long, from Hayward to Pinole, and the group was in a hurry, with their goal still ahead of them. (And just as Fages had complained in 1772, the Anza party was beset with mosquitoes all that day.)

Long story short, I think that whereas Fages mostly skirted the Fan as he sought (and failed to find) an easy level route through Oakland, Anza rode pretty straight through it.

Anza’s group set out from their camp on San Lorenzo Creek, in present-day Hayward, that morning at 7 and rode along the foot of the hills, with a jog upstream to get across the deep arroyo of San Leandro Creek, and then on to the edge of the Fan, the hill of Pleistocene gravel occupied by Evergreen Cemetery, in the lower right corner of this digital elevation model.

Where Fages turned left to stay on the flats, Anza headed the horses straight, toward a promising gap in the hills,

and into the Allendale flat. Almost fifty years later, Luís Maria Peralta’s family would put the first hacienda on their huge East Bay land grant there, just across Peralta Creek. “About two leagues” after crossing San Leandro Creek into Oakland territory, Font wrote that they “crossed a small arroyo without water and almost without trees,” which I think was Peralta Creek. “Then a little further on we ascended a hill which is on a straight line with the mainland and the plain which runs toward a very thick grove of oaks and live oaks on the banks of the estuary,” where he sketched the view toward the Golden Gate. That is this drawing, showing the Alameda peninsula flanked by San Leandro Bay on the left and the Estuary on the right:

In my interpretation, he would have been sitting on the ridge where Patten University sits today but higher up, maybe where Lincoln Avenue starts today. It also could have been down on Carrington hill; both ridges line up with the Alameda peninsula, which was a large encinal (live-oak grove) at the time.

“Then, descending the hill, we crossed another arroyo almost without trees and with some little pools of water which did not run. This appears to be the arroyo which Father Crespi called the Arroyo del Bosque and which empties into the extremity of one arm of the estuary.” He refers to Sausal Creek as described by the Fages party in 1772.

“We continued the journey over hills and plains, crossing two more arroyos with little water, deep beds, and a heavy growth of trees, the second one having more than the other, and both of them flowing into a bay which the arm of the estuary forms on this side.” These I interpret as Indian Gulch and Pleasant Valley Creeks, which clearly both drain into Lake Merritt, which was then a narrow inlet with wide mudflats. I don’t think the group went near the Lake but instead were higher up the valleys — they were on horseback, after all.

“Afterward we entered a plain in which we crossed two small arroyos without water.” Finally they had left the hills of the Fan and were back on the East Bay plain. That would make these two streams Temescal and Strawberry Creeks. “From this plain we clearly descried the mouth of the port, and when the point of the red cliff on the inside was in line with the outer point of the mouth, I observed the direction in which they ran, and saw that it was to the west with some declination to the south.” That matches the view of the Golden Gate as seen from Berkeley.

Oakland remained something of a distant place until the San Francisco and San Jose missions had captured or driven off the native people and established their own purposes on the territory: food and fodder cultivation in the Richmond area and cattle range in the flats from Fremont to East Oakland.

I want to note that Fages and Anza did not come as conquerors. Their assignment, at the dawn of Alta California, was to establish friendly relations with the natives. They exchanged gifts with every group they met: glass beads, generally; and a surprising number of native groups gave them duck decoys in addition to food. The situation did not last. The priests could not even perceive the fine-tuned ecosystem in which the Ohlone were the keystone species, only naked children of nature who must be trained in their own god’s image.

Pedro Fages and the Oakland Fan

4 January 2021

Oakland was visited soon after the missionaries of New Spain established their capital port city at Monterey, in 1769. The area’s unusual terrain gave them trouble, and part of the reason was the set of treeless hills of sand and gravel in its midsection that I call the Fan. The Fan interrupts the nice clean plain that otherwise extends the whole East Bay. Here’s how it looks in a digital elevation model and the geologic map. (As always, click to enlarge.)

The first Europeans to have stood where Oakland sits were four scouts under the leadership of Spanish-born army captain Pedro Fages, who came from Monterey to the East Bay in November of 1770 with a party of six soldiers and a muleteer. It was a short reconnaissance visit that Fages, a seasoned leader who earlier that year had been left in charge of Alta California, made on his own initiative.

In the first part of his trip, Fages pioneered the route inland from Monterey that is traced today by U.S. 101. He then made his way up the East Bay, making camp at San Lorenzo Creek in present-day Hayward, near where scouts from the Portola expedition had apparently stopped the previous year. Fages’ notes are vague; this is the entire record of the day:

November 28. — Four soldiers set out to explore the country, and at night returned saying that they had travelled about seven leagues to the north; that the country was very good and level; that they had climbed to the top of a hill but had not been able to see the end of an estuary which lay before them and which communicated with the one which lay at our feet; that they had seen many tracks of cloven hoofs which they thought were of buffalo; that close to the hills which they passed at the right there were some springs of water; and that they had crossed two small streams of it. They said, also, that they had seen the mouth of the estuary, which they thought to be the one which entered through the bay of the port of San Francisco. This I confirm through having seen it.

(Some explanation: The Spanish league is a little over 2-1/2 miles. The “estuary which lay before them” refers to San Pablo Bay and the one “at our feet” refers to San Francisco Bay. The “port of San Francisco” means Drakes Bay, the “bay of the port of San Francisco” is the bight between Point San Pedro and Point Reyes, and the “mouth of the estuary” is the Golden Gate.)

The men apparently hiked a total of about 16 miles, or 8 miles each way, as estimated by walking. If that whole route was “very good and level” ground, they must have stayed on the flats, and they must not have reached San Antonio Slough (present-day Lake Merritt), which they would have noted because it blocks the way pretty definitively. They were passing the hills overlooking San Leandro and the lower hills of the Fan “at the right” and crossed “two small streams.” These would have been San Leandro and Sausal Creeks, at their lowest flow early in the rainy season.

The hill they climbed, I think, was the hill of San Antonio Park: it was near where they must have turned back, and from its top can be glimpsed parts of San Pablo Bay and the gap of the Golden Gate, although the actual water passage is not visible. Fages “confirmed” the sighting because he could see the gap, though less clearly, from where he spent the day in Hayward. The presence of the Golden Gate was known from the 1734 voyage of Cabrera Bueno, but it had not yet been charted. Here’s the inferred route.

And on the Bache map of 1857 I show the hill’s location with an X. At that time it was a lookout station for the Brooklyn harbor; today it’s a park that still has good views.

And here’s the topography in detail.

Fages returned in the spring of 1772 to push farther north. This time it was a proper expedition that included 12 soldiers, a muleteer, an Indian guide and a priest, Juan Crespi, whose diary served as a formal record. Their primary goal was to fix the position of the Golden Gate in preparation for founding the San Francisco mission; secondarily they sought to find a way around San Pablo Bay. Oakland, with the hilly maze of the Fan and the inconvenient slough in its midst, was in the way.

Leaving the familiar campsite in Hayward on 26 March, they hiked four leagues. Crespi recorded that they crossed five creeks, “three of them medium-sized and two very large, their banks grown with alders, cottonwoods, live oaks, and some laurels.” By my count these would be San Leandro Creek (large), Arroyo Viejo (small), Lion Creek (small), Peralta Creek (small) and Sausal Creek (large).

At the end of four leagues we halted not very far from an arm of the estuary [Oakland Estuary/San Antonio Creek], which forms with another [San Leandro Bay] something of a peninsula [Alameda], where there is a grove of live oaks, which one can enter only from the mainland side. The camping place is about four leagues from the principal estuary or arm of the sea, which we made out from this place very well, and it looks like a sea to us.

I place the camp’s location near Foothill Boulevard west of Sausal Creek and not, as others have done, up near Mills College. Crespi observed, “The site is very suitable for a good settlement; for on account of the proximity of the forest they could provide themselves with timber and firewood. This place was called Arroyo del Bosque.”

The next day they set out early heading north-northwest, but then ran into the wide tidal channel of Lake Merritt: “Because of the estuary [San Antonio Slough] which surrounds the wood [Alameda] and penetrates into the land about four or five leagues until it heads in a mountain range, we were compelled to travel about a league and a half by some ranges of hills, which, although they are all treeless and grass-covered, annoyed us very much with their ascents and descents.”

This part of town is still up and down. The land seen by the Fages party differed from modern Oakland in that the creeks feeding Lake Merritt — Indian Gulch, Wildwood and Pleasant Valley Creeks — were not culverted and paved over with nice asphalt streets, but wide marshy tracts extending well away from the Lake, especially at this time of year. The detours were surely tedious. The mosquitoes, too, drove them all nuts.

After three leagues of trudging they came out on the other side of the Fan. My idea of their route was up Grand Avenue, over the ridge at Linda Avenue, across the valley of Glen Echo Creek and over the last low ridge of the Fan on 41st Street. At this point the party “entered an immense plain,” and they saw for the first time the Golden Gate itself.

They stopped for a while to observe it closely, and Crespi noted its features accurately. We can approximate this view today from the crest of 40th Street Way. They went on to the northwest that day for another league and into history, leaving the territory of Oakland on good terms. They didn’t return this way but instead walked up toward the Delta and looped back south through present-day Walnut Creek and Pleasanton. The next visitors to Oakland were the Anza party in 1776.

DEM images from the National Map site, route maps from gmap-pedometer.com