Archive for the ‘Sausal Creek watershed’ Category

Upper Castle Canyon

30 August 2021

Sausal Creek is formed by the junction of three streams, two of which are well known: Shephard Creek, which drains Shepherd Canyon, and Palo Seco Creek, which drains the bulk of Joaquin Miller Park. In between them is Cobbledick Creek and its steep-walled watershed, hidden green heart of the Piedmont Pines neighborhood.


Source: Alameda County Flood Control and Water Conservation District

The creek has two branches: the northern one, which I’ll call Beaconsfield Creek after Beaconsfield Canyon at its head, and the eastern one, Castle Creek, which drains Castle Canyon. The easternmost portion of Castle Canyon, a gorge running from the hairpin turn on Mastlands Road, is an 8-acre preserve that’s formally part of Joaquin Miller Park. Over the weekend, that parcel was renamed Dick Spees Canyon, with a bench and plaque, to honor the longtime politician and activist who helped keep the land undeveloped.

The interesting thing about Dick Spees Canyon, and the valley of Cobbledick Creek below it, is that it coincides with the inactive Chabot fault, a deep feature of the East Bay that runs roughly parallel to the Hayward fault. It runs diagonally across this digital elevation model of the area; Dick Spees Canyon is right in the center.


From nationalmap.gov

And the bedrock map of the same area is here. The Chabot fault is the dashed line with the pairs of tick marks on the right-hand (downthrown) side.

The Chabot fault juxtaposes two very different rock units directly across the canyon from each other — the serpentine rock of the Coast Range ophiolite (sp) on the west and the Joaquin Miller Formation (Kjm) of the Great Valley Sequence on the east. Dick Spees Canyon aligns very nicely with the upper part of Palo Seco Creek, forming an unusually good topographic expression of this obscure fault. Leona Canyon is another place it stands out; also in upper Knowland Park. The fault has been traced past Hayward. While it appears to have a long history, it’s very much inactive.

The fault is why I made a point of visiting here in 2019 and returned last Thursday. The parcel is almost completely undeveloped; only a rough footpath runs up the narrow valley floor from a short formerly graded stretch, then zigs up to Castle Drive. (I must advise all visitors not to try walking it downhill until the rainy season firms up the soil there.) The following photos are from both visits.

Here’s the lower end of the property. It’s a steady climb.

Soon the rocks make themselves evident, serpentinite on the right . . .

. . . and mudstone on the left.

Whoever built this fire ring used stones from both sides of the fault.

The valley floor is littered with dead cedar and eucalyptus trunks that need clearing out. And all sides of the canyon are very steep. I don’t expect anyone to cut any trails up them for a long time.

But if they do, visitors might glimpse the views enjoyed by the ridgetop residents who surround this neglected gulch with its interesting geology.

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.

Anomalies of Sausal Creek: The Delta

11 November 2019

This is the last of four posts about Sausal Creek from the hills to the Bay focusing on its odd features, stuff that’s been bugging me like a seed stuck in a tooth. Here I’ll talk about the creek segment below Foothill Boulevard, where the floodplain ends and the delta begins.

A delta is a wedge of sediment, built near a river’s mouth where it deposits most of its muddy load. Streams tend to spread out in their deltas, sending sediment here and there like a Vegas card dealer. While the mouth of a big river like the Mississippi or the Nile fans out in a set of multiple distributaries, little streams like Sausal Creek move their courses every once in a while so that over thousands of years, every part of the delta gets its share of dirt.

Today, Sausal Creek officially meets the Bay in this culvert next to the Fruitvale Bridge . . .

. . . with this monumentation.

But it’s all totally artificial. This body of water is a large canal that was built in the late 1800s as part of the Oakland harbor improvements. Before that, Alameda was not the “Island City” but a town on a wooded peninsula called the Encinal, and the only way to get there on dry land was through here, across the delta of Sausal Creek.

The creek’s delta is unlike the deltas of Oakland’s other major streams. Here’s what I mean. Look at this map of central and east Oakland that shows only young material, whatever is not bedrock.


From USGS map OF 2006-1037

I’ve labeled Sausal Creek, shooting south-southwest straight to the Bay where the Alameda peninsula sits in its way. The dark purple stuff labeled “afem” is all landfill (“artificial fill over estuarine mud”), and the light yellow part marked “Qhf” is young river sediment (“[Quaternary Holocene] alluvial fan deposits”). The three pink blobs labeled “Qds” are areas of old Ice Age sand dunes: one under downtown Oakland, one making up the Encinal, and one under Bay Farm Island.

So before the canal was dug, Sausal Creek, unlike all other Oakland creeks, dumped its load here against a buttress of sand. The willow thickets that gave Sausal Creek its name must have thrived here. The early Anglo settlers were quick to put roads and rail lines through this area, and the brushy marshy creek delta would not stand in their way for long.

Which way did the creek run from here, to the right or the left? The only clues are a few old maps, not all of them trustworthy. The first official map of Alameda County, published in 1857, shows Sausal Creek, at top center, draining to the right into San Antonio Creek, the tidal inlet now known as the Oakland Estuary.

The Haynes map, published in 1878, shows it petering out and not even reaching the Bay. Other Oakland creeks, like Courtland and Upper Elmhurst and Seminary Creeks, were the same before they were diverted into pipes and ditches.

But every other map, of this vintage and later, shows Sausal Creek draining to the left into San Leandro Bay. I have no doubt that the road and railroad builders dug a ditch through the sand to control it (and conveniently mark the boundary between Alameda and Brooklyn Townships). The Thompson map of 1878 is a good example, followed by a map of the same area today. Sausal Creek is at top left.

Is it possible that the creek mouth shifted naturally from the right to the left during those years? Certainly; in fact the notorious winter of 1861-62, the wettest in our recorded history, could have done that by sending a big pulse of mud down the creek as it overspilled its banks. And the 1868 earthquake could have liquefied the ground here. The rarest events make the biggest difference. But in this case I would blame us.

The geologic map of Oakland shows the delta in more detail, outlining especially young wedges of sediment (Qhaf1) that were laid down by the creek in very recent geologic time, probably within the last few thousand years.

It’s even plausible that Peralta Creek, just a few blocks east, joined Sausal Creek off and on over the centuries and contributed to this delta. A prominent example is up in Richmond where Wildcat and San Pablo Creeks form a joint delta, at one point flowing just a stone’s throw from each other. But today Sausal Creek’s mouth is a truncated version of its true self, trapped in culverts for the foreseeable future, a dead delta.

Anomalies of Sausal Creek: The Floodplain

28 October 2019

This is the third of four posts covering the Sausal Creek watershed from hill to Bay. It features the part of the creek between Dimond Canyon and Foothill Boulevard. The early Anglo landowners were quick to establish fruit orchards and berry patches there, giving rise to a settlement and district out east in Brooklyn Township they called Fruit Vale.

That was just the right name for this distinctive locality. Its alluvial land, flat, tractable and naturally well watered, was ideal for planting. I say “planting” rather than farming because within a few decades — the commercial lifespan of an orchard — the farms gave way to grand homes with lush landscaping. That’s the “Fruit” part. The “Vale” part refers, as the word has since the 14th century, to a wide, flat lowland between gentle hills with a stream in it. An 1888 pamphlet described it as “one vast garden,” its roadway “lined with elegant residences and villas surrounded by beautiful lawns and gardens. . . . The soil is easily cultivated and everything planted in it thrives.”

The creek itself is hard to see in this stretch today, where homes and overgrowth block the streambanks. It also runs a good 20 feet below the surrounding land, a typical East Bay arroyo with banks like cliffs. Except for a lovely little fenced-off area on Barry Place, distant glimpses are the closest you can get to the water.

By textbook standards, Sausal Creek has the most well-formed floodplain in Oakland. A floodplain forms wherever a stream runs out of downhill energy and has to drop the sediment it’s carrying. The elevation profile I made for Sausal Creek shows the floodplain portion.

This part of the stream is at grade, like the canyon above it and the delta below. It’s very stable. Neither fully erosive nor fully depositional, a stream in a floodplain has little it can do except rearrange the sediment.

Floodplain streams tend to turn snaky, meandering and migrating like the trickles you’ll see on a rainy windshield. They nip off bits of the bounding hillsides and gradually widen their corridor.


From Phil Stoffer’s geologycafe site

A floodplain is a place where sediment languishes during its slow journey to the delta. A lot of that sediment enters the floodplain from the hills on either side. Here are three different views across the floodplain showing its flat floor and abrupt edges. Fruitvale Avenue at E. 27th Street:

Farther upstream at MacArthur Boulevard:

And still higher at Coloma Street.

For real detail, I always look for a digital elevation model or DEM. The DEM of the upper end of the floodplain shows how flat its floor is and how fragile the west side of the valley is. It also shows how much the stream itself is meandering within the straight floodplain valley.

Unlike the rocky flanks of Dimond Canyon, the hills lining Sausal Creek’s floodplain consist of sediment, ancient gravel beds that I’ve named the Fan. Time to look at the geologic map of the floodplain section.

The material of the Fan, labeled Qpaf on the map (“Quaternary Pleistocene alluvial fan”), is unconsolidated sand and gravel that’s prone to landslides on steep slopes. That scoop out of the hillside just left of center in the DEM image is the notorious McKillop slide, in progress for more than a century. Much of the crumpled land there was smoothed over and preserved as William Wood Park, but the adjoining slope to the south, which failed in 2006, is still a mess.

Two things about this stretch of Sausal Creek strike me as odd. The first is how straight the stream valley is here. Streams tend to curve, so if you find a naturally straight one you look for what’s forcing it to be that way.

My tentative theory, which I alluded to at the end of the previous post, is that the Fan was tilted up, not built up, along with the Piedmont bedrock block thanks to tectonic interactions along the Hayward fault. (In other words, the Fan is not really an alluvial fan but an uplifted relic of an older coastal plain.) I suppose that the stream was incised in a straight line by large flood events early in its history, then widened into the floodplain we see today. It’s easy to imagine big pulses of floodwater coming out of Dimond Canyon like a firehose, most likely after a temporary lake above the canyon, dammed up by an earthquake landslide, gives way. Such floods would rush in a straight line to the sea and set the stage for the floodplain that evolved.

The other odd thing is how closely the stream hugs the west side of the valley. So does Peralta Creek, just east of Sausal Creek. I think this may not be an accident. Glen Echo Creek, on the other side of the Fan, hugs the east side of its valley. Perhaps there is some subtle warping of the crust across central Oakland.

Sausal Creek’s fruited plain may be just a vale, but it’s a vale with some intriguing features.