Archive for the ‘Oakland geology puzzles’ Category

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: Dimond Canyon

14 October 2019

This is the second of four posts about the peculiarities of Sausal Creek, going from its headwaters to the Bay. Here I’ll address Dimond Canyon, the 2-kilometer segment between the Warren Freeway and the flats of Dimond Park. The steep walls of the canyon, which is several hundred feet deep, are entirely hard sandstone of the Franciscan Complex, part of the Piedmont block.

This is the same stone quarried for decades in Rockridge (the Bilger quarry) and the land that would later become Piedmont (the Blair quarries and the Davie Stadium quarry). In fact the Diamond Cañon Quarry was one of two here in the canyon. It’s now occupied by the Zion Lutheran Church, as seen here from across the canyon.

The quarry scar appears on this terrain map as a big round nick in the canyon wall next to Park Boulevard.

A while ago in this space I described Dimond Canyon as a classic water gap — a stream-cut gorge crossing a bedrock ridge that otherwise seems impenetrable.

Geology textbooks will tell you there are two ways for streams to make a water gap. In the first way, the stream was there first (an antecedent stream) and a ridge of resistant rocks rose up around it. In dynamic California, this is a straightforward explanation of our water gaps. In the second, the ridge was there first, inherent in ancient deformed rocks buried under younger strata, and the stream (a superposed stream) cut down to, then into it while stripping off the overlying material. That’s how they explain the Delaware Water Gap and other examples in the gentle Appalachians.

Dimond Canyon is actually a semi-classic water gap. Yes, the ridge it crosses must have risen while the stream was cutting down, but the story is complicated by the fact that the watershed upstream lies across the Hayward fault, and is constantly being moved to the right. This means the canyon has hosted streams from several different watersheds over the past million years or so.

Therefore the streams feeding Sausal Creek today could not have dug the canyon; some predecessor watershed did it. There must have been gaps and surges in the water (and sediments) flowing through this canyon. If we ran things backward a million years, what would it show? The exercise would be blurred by serious uncertainties, but the matter is not beyond all conjecture.

I beg your indulgence as I present some slides from my talk to the Friends of Sausal Creek last month. They’re Google Earth views looking west across the fault. Here’s today, with the fault trace shown in red.

The view may be a bit confusing as I rewind the motion on the fault at about 10 millimeters per year. The far side looks the same because we’re focusing on it while it moves leftward, toward San Leandro. For a long time, Sausal Creek has been carried past small watersheds that, like today’s, could not possibly have carved Dimond Canyon. But about a million years ago, Dimond Canyon would have lined up with the watershed of Arroyo Viejo.

This looks promising because the watershed (the part above the fault) is about twice the size of Sausal Creek’s, giving it roughly twice as much water and cutting power to match.

But to make the canyon, you have to have something pushing up the ridge while the stream across it keeps cutting its way down. There’s nothing obvious that would have been pushing up the bedrock ridge at this time.

Going back a bit further, though, we line up with the great big watershed of San Leandro Creek, a dozen times larger. This stream has plenty of cutting power, evident in the canyon it’s dug where the dam and reservoir sit.

And finally, we have a mechanism here for uplifting the ridge that Dimond Canyon cuts across. The hills of San Leandro consist of a large slab of gabbro so big and strong that it deflects the Hayward fault slightly. Back when the sandstone of Dimond Canyon was grinding past the gabbro of San Leandro, the jostling between these two bodies of rock, caught in a vice by the geometry of the fault (a restraining bend), would have pushed both sides upward because that’s the only way out of the vice. And all the while San Leandro Creek would have been cutting a nice deep water gap as that hard rock rose.

Eventually, inevitably, the fault carried the water gap out of reach, and ever since then Dimond Canyon has housed lesser creeks for episodes of a few hundred thousand years. Sausal Creek trickles down the canyon today not doing much to it, the shrunken tenant of a structure built by a mightier maker.

This story (and that’s all it is really) appeals to me because it would also explain the presence of the Fan — the swath of gold on the geologic map representing Pleistocene sediment.

I’ve always regarded it as a fossil alluvial fan because of its shape on the map, but maybe that’s accidental. Maybe it’s just a chunk of old East Bay land that was lifted along with the Piedmont block, or washed off of it afterward.

I first posted about the problem of Dimond Canyon more than 10 years ago. Takes a while to figure out some things.

Oddball Lake Merritt

16 September 2019

Oakland has several major, permanent streams crossing it from the hills to the Bay. Then it has Lake Merritt, formerly known as San Antonio Slough — an arm of the sea extending more than a mile inland from the shore.

What makes it so exceptional?

I have a theory, based on the last million years or so of geologic history plus some of the latest research.

First of all, we need to ignore the Lake Merritt we know today:

. . . and think of Oakland as it originally existed. This is an excerpt from the “Bache map” of 1857, a survey of the waters surrounding the newborn city of Oakland and its neighboring town of Brooklyn painstakingly made by the U.S. Coast Survey. It covers the same area as the Google Earth clip above. It’s a fat 1200-pixel image worth zooming in on (or study the full-size scan from Wikipedia).

“San Antonio Creek” was the inlet that led to the existing landing at Brooklyn. It had a central channel, just a couple hundred yards wide, that was deep enough for ships, and the rest was tidal mudflats or treacherous shallows. The slough extending to the north — today’s Lake Merritt — had strong tidal currents and a very shallow mouth. Small craft could use it when the tide was high, and duck hunters were a common presence there, but for serious commerce it was useless, and Oakland’s landing at the foot of Broadway was little better.

Back then, San Antonio Slough had a wider mouth lined with wetlands, with terraces roughly 25 feet high on either side. Later the mouth got filled in leaving the narrow passage we know today . . .

. . . but if you look for it, for instance down 10th Street past the museum and auditorium, you can get a sense of its original width.

My theory starts with taking the mind back into recent geologic history — the dozens of ice ages that have occurred regularly for the last 2-plus million years. When the ice caps were at their largest, the sea sat hundreds of feet lower than today. Except for the Golden Gate itself, the whole Bay was dry land, and all of our creeks ran out far beyond today’s shoreline to join the combined Sacramento-San Joaquin River. Today’s Lake Merritt, then, is a drowned stream valley — a term east coast geologists know well, but seldom used around here.

For clarity’s sake I will use the name Merritt Creek for the stream that occupied that valley during glacial times. Glen Echo Creek ran into Merritt Creek down a swale where the north arm of Lake Merritt sits today.

The eastern arm of today’s lake was where three creeks joined: Pleasant Valley, Wildwood and Indian Gulch (Trestle Glen) Creeks. You know, let’s call the drowned valley Pleasant Valley, because it surely was one. The late Pleistocene creatures and vegetation there I will leave to your imagination.

Three more smaller streams also drained into Merritt Creek: “Kaiser Creek” at 20th Street, “Adams Point Creek” at Perkins Street and Park Boulevard Creek at the E. 18th Street landing.

Here they all are on the watershed maps from the Alameda Country Flood Control District.

And if you adjust this map in your mind by subtracting the sea, Merritt Creek also received input from 14th Avenue and 23rd Avenue Creeks (that is, the rest of San Antonio Creek).

My argument is that Merritt Creek is a drowned valley today, instead of an ordinary creek like the rest of Oakland’s streams, because it cut down deeper than other creeks. I can cite three reasons for that.

First, Merritt Creek had the largest watershed between San Pablo and San Leandro, thus it had the greatest water-gathering power in the area — especially during glacial times. And as the watershed map shows, the stream network is well organized, capable of delivering stormwater in a big flush. It didn’t dribble across a wide coastal plain like Temescal and Sausal Creeks on either side. Whereas those creeks spread out their floodwaters on the plain and slowed their flows (depositing their sediment across the landscape), Merritt Creek was confined between elevated banks and couldn’t slow down. It was better equipped to cut into the exposed floor of the Bay.

Second, Merritt Creek drained a large area of hard bedrock: the Franciscan sandstone, shown in blue on the geologic map, that underlies the hills of Piedmont. I argue that this substrate didn’t generate as much mud or clay as its neighbors and made the stream network less prone to clogging.

Third, unlike Oakland’s other major streams, Merritt Creek’s watershed didn’t cross the Hayward fault and was not affected by it. This is an intricate subject I plan to address in future posts as well as my book. Briefly, the fault messes with streams as its sides slip past each other. Headwaters in the hills get slowly cut off from their downstream reaches. Streams get stretched and snap, interrupting their natural evolution into well-organized networks like Merritt Creek’s. The head of one stream gets grafted onto the stem of another stream, and the transportation of sediment from hill to bay — the basic function of streams — is stymied and randomized.

Maybe this argument is easier to read in a simple image, a shaded digital elevation model of central Oakland. The fault line is obvious, as is the integrity of Merritt Creek. Temescal and Sausal Creeks reach around Merritt Creek’s drainage, like hands holding a bowl, and cross the fault with disruptions you can explore on the AC Flood Control District site.

Another more scientifically phrased argument was just published in the journal Earth Surface Processes and Landforms. The paper is based on the example of the Dead Sea, where human intervention has been lowering the world’s saltiest lake. A team of geologists took that as an analog of the glacial cycle and asked how the streams feeding the Dead Sea have responded. The bigger, wetter streams cut down into the land, keeping up their deliveries of sediment as the water recedes, while the piddly streams give up and stay behind. Reading the abstract, I immediately thought of our creeks and the exceptional one whose drowned valley is, for the moment, our little mediterranean sea, our miniature San Francisco Bay, named Lake Merritt.

You know how the Pleistocene was, full of large beasts that have slouched off into extinction: mastodons, giant ground sloths, sabertooth cats, dire wolves and so on. There were monsters around then.

And today we have three monsters around the lake. Have you seen them? The newest one is named Makkeweks, inspired by Ohlone stories, and lives in Snow Park.

Makkeweks joins the newly restored Mid-Century Monster (here as seen in 2005) . . .

. . . and the original, the one and only Fairyland Dragon.

Think of the Pleistocene when you visit them.

Geology of King Estate Open Space

4 March 2019

After tramping all over Oakland, I still find its landscape full of uncertainty and mystery. The alluring hills of King Estate Open Space Park have brought me here time and again, sometimes to lead walks, sometimes to just stop and smell the flowers. Last month I came back yet again, this time to look harder at it.

The best resource on the park’s history and vision is on the Oak Knoll Neighborhood Improvement Association site at oknia.org. The aspirational Site Plan has the following concept for the park: “The winds sweep my imagination across the horizon. We move over the hills exploring the wilds and oaks embrace us. Here, in this place for everyday we cultivate community.”

King Estate Park is a grassy ridge at the south end of the Millsmont-Eastmont hills, an apparent pressure ridge that stretches along the Hayward fault from Mills College to the Oakland Zoo area, between Seminary Creek and Arroyo Viejo. What drew me here as a geologist was the geologic map (USGS MF-2342) that depicts the area as a peculiar ancient gravel (Qpoaf on the map), the largest piece of this material in Oakland.

The attractive thing is that according to the map key, these deposits “locally contain freshwater mollusks and extinct Pleistocene vertebrate fossils.” The odd thing is that they don’t match another criterion: that this dense gravel “can be related to modern stream courses.”

I explored the portion of the park north of Fontaine Street. Here’s the street map, marked with the three locations I’ll be showing photos of.

And here’s the aerial view, from Google Earth.

And just for fun, here’s the digital elevation model, with buildings and trees removed.

OK! Location 1 is on the steep western slope at the north end of the park, which I climbed. Halfway up is a sizable area of rock rubble consisting entirely of Leona volcanics, the same bedrock shown in pink labeled “Jsv” on the geologic map. The near-outcrop is in the lower left corner of this shot.

And the rock looks like this.

Down at location 2, there’s a spiral labyrinth that people have made in the last few years; I don’t remember seeing it before. But on the assumption that it’s made of stones from the immediate surroundings, I infer that it’s Leona volcanics over here too.

The Leona is generally light-colored with some greenish bits and some red-orange coatings where it’s weathered, hard to describe in detail but distinctive once you’re familiar with it. Once a range of underwater volcanoes that subsequently underwent a lot of alteration, it offers up a variety of intriguing bits that lacking a petrographic lab I can only scratch my head at.

Anyway I’m looking all over, and every bit of gravel on this hill is Leona volcanics. Now a river gravel, which is what the map describes and what I expected (indeed, what I actually perceived on previous visits!), consists of rounded clasts and a variety of rock types from the stream’s catchment. Other gravels in this town are just that way, but not this. The whole time I’m there I’m muttering to myself, “this stuff is colluvium” — raw rock rubble, mixed with soil, that hasn’t moved from its birthplace except maybe in landslides.

In location 3, we have proper bedrock. It show up where the soil has been scraped bare . . .

. . . and farther down the slope in genuine outcrops, which I always cherish.

So in sum, the whole north half of the park, far as I can tell, is either bedrock or colluvium of the same stuff: Leona volcanics. How did it get to be mapped as Pleistocene river gravel? The MF-2342 geologic map was published in 2000. There are two previous serious maps of this area. Dorothy Radbruch mapped it in the 1960s for map GQ-769, and there the area looks like this.

She called it “Qg”, “gravel, sand and clay” and noted that it contains pebbles of Leona Rhyolite (what I call Leona volcanics). She also said this: “Contains molluscs of probable early Pleistocene (Irvingtonian) age.” They were at the locality marked by the triangle with “22133” next to it; that number refers to a “report filed at Washington, D.C.” which is probably gathering dust deep in a back room. She also referred to reports on two boreholes, numbered 95 and 96 on the map, which recorded various kinds of gravel down as deep as 45 feet. That could’ve been deeply weathered Leona as easily as anything else.

But you know what? I’m going to go with Andrew Lawson’s original map of the area from 1914 (Folio 193), in which everything is just straight Leona.

Even though he thought the Leona was very young (hence the name “Tln” meaning Tertiary Leona), he could tell what the ground was saying. At least, he and I agree. I’m sure, though, that he scratched his head as much as Dorothy Radbruch and I did. And they must have enjoyed the view as much as me.

I’ll just have to poke around here some more.