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.

Anomalies of Sausal Creek: The Headwaters

30 September 2019

The Sausal Creek watershed is full of anomalies and questions from its headwaters to its San Francisco Bay outlet. Here I’ll look at the top end of Sausal Creek — the part that isn’t Sausal Creek. The creek originates where three different tributaries join: Shephard Creek, Cobbledick Creek and Palo Seco Creek.

The divide between Shephard and Cobbledick runs up Chelton Drive, then Darnby and Carisbrook Drives, up to Skyline. The divide between Cobbledick and Palo Seco runs mostly up Castle Drive to Skyline, so if you know the area these are easy to visualize.

That map, from the Alameda County Flood Control and Water Conservation District, has Sausal Creek proper beginning at the junction of Shephard and Cobbledick, which is also where the Hayward fault crosses the creek (more about that below). But because that’s in a culvert deep beneath the Warren Freeway, for purposes of this post I prefer to put the origin a little farther down, in an easy-to-miss opening east of the parking lot of the Montclair Golf Course driving range at the head of Dimond Canyon. That’s where Palo Seco Creek, the third tributary, comes in from the redwood-filled canyon of Joaquin Miller Park.

So with that settled, let’s look at the headwaters on the geologic map. This area includes a wide variety of rock units — the Sausal Creek watershed touches more different rock types than any other Oakland stream.

Don’t worry, I won’t go into the rocks, although there are a lot of them and they’re interesting . . .

I’ve added the Hayward fault to the map, as a thick red line, just to show how different the rocks upstream and downstream are. That’s because motion on the fault has been dragging the west side to the north for a few million years. That explains two major peculiarities of Sausal Creek, the first being its lumpy longitudinal profile.

I made this stream profile by walking down the creek from the top of Eastwood Court to the Bay, recording elevations with my smartphone altimeter.

A normal stream profile describes a nearly smooth listric curve — steep at the top and level at the bottom. The bottom of the curve represents what’s called the base level for the whole stream, sea level in this case. A stream with a nearly perfect curve is said to be at grade. Two basic things will put kinks in that curve: rocks that are especially hard or soft, and changes in base level. For instance, ice ages lower the sea level by hundreds of feet, and streams have to adjust during that time by digging down their beds toward the new base level. (I alluded to this in my last post with respect to Lake Merritt.)

There’s a big discontinuity in this curve right where the Hayward fault crosses, just above the 3 kilometer mark. The lower half, Sausal Creek, is at grade, even though it crosses hard sandstone in Dimond Canyon and young sediment farther down. But the fault has ripped its head off and put on another head — the Shephard-Cobbledick-Palo Seco system. It’s a Frankenstein creek. I think this head transplant has happened more than once.

Sausal Creek, over the last million years or so, has done fine even with its head ripped off and replaced. The worst that might have happened is that it had less water in it for a while. But in the upper creek system, these changes have drastically affected its base level.

Picture it: the two sides of the Hayward fault are moving past each other at about 10 millimeters a year, or a kilometer every 100,000 years. So for a good long time, Shephard Creek flowed down against the high rocky ridge of the Piedmont hills. Very likely it turned north for a long ways, the way Temescal Creek does today, before flowing around the north end of the ridge. As its route to the Bay got longer and longer, the slope of the stream grew gentler as it remained at grade. The effective base level, that is to say, was up around the elevation of the Thornhill district.

Then along came Dimond Canyon, moseying up on the far side of the fault, and at some point Shephard Creek switched over to that route. All of a sudden, it had a lower base level. It was not at grade. So it started eroding downward into its streambed and eroding uphill, trying to re-establish that nice listric profile.

When that happens to streams, what geologists call a knickpoint appears in the profile. The extreme case of a knickpoint is a waterfall, but more often they’re just rapids. In Shephard Creek, there appear to be two knickpoints.

Pay attention next time you ride down Shepherd Canyon Road. There’s a nearly level stretch in the road between Saroni and Escher Drives, where the railroad trail meets the road, then a steep “rapids” below. The other knickpoint is under the landfill of Shepherd Canyon Park, where the creek is buried in a culvert. A more carefully made profile would show it better, but that may not be possible.

I could conjecture a story that accounts for these features, but there’s a lot I don’t know so it would just be armwaving. For instance, the stream profile is based on the elevations of the roadway rather than the actual streambed except for the part between Shepherd Canyon Park and Mountain Boulevard (which is so thickly wooded I don’t recommend you visit, even though I did) and the one data point at the golf club. For another, the history of vertical movements along the Hayward fault is almost completely unknown, other than that the hills on the east side are rising today at about a millimeter per year. So enough about that.

I mentioned that the creek has two major peculiarities, and here’s the second one. The upper part of the Sausal Creek watershed is not a pretty, textbook stream network shaped like a nice tree — what geologists call a dendritic drainage pattern. It’s more like a bush in a gale, and all of the streams that cross the fault are warped. Here’s how it looks in the set of stream maps on the Oakland Museum website:

And for comparison here’s Temescal Creek:

And Arroyo Viejo, the weirdest of all.

There’s a struggle going on between the stream’s innate tendency, driven by gravity, to dig itself a home with an optimal shape and the motion of the ground beneath, driven by tectonics, that keeps messing it up. The shapes of the streams, and the landscape they live in, are a snapshot of that struggle. They remind me of the shapes of trees high on windy mountains — although one case involves organisms and the other purely physical systems, the similarities are tantalizing.

Geologists are starting to explore this topic with computer models. A recent paper in Geophysical Research Letters, with the dry title “The Role of Near‐Fault Relief Elements in Creating and Maintaining a Strike‐Slip Landscape,” has some state-of-the-art animations that address the exact situation of Oakland’s fault-crossing streams. You don’t need to understand all the modeling details — I certainly don’t — to enjoy the illustrations and the movies.

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.

The great Tunnel Road cut

2 September 2019

The land on the south side of Hiller Highlands is far from its native state: it’s been extensively quarried for many years, and what’s left is a rocky, weed-choked waste. But the roadcut is also a geological treasure.

It’s one thing to look at a hillside and determine what it’s made of, another to study it carefully enough to determine what formation it belongs to. These are worthy accomplishments to be sure, but a more precious one is to find and study a place where different rock units come in contact.

Here’s where I bring out one of my favorite quotes from the history of geology, in the early 1800s when people were beginning to work out what the rocks were telling them. A party of geologists including the eminent Sir James Hall took the Rev. William Richardson, a notorious opponent of the newfangled Scottish school (now textbook orthodoxy), to the Salisbury Crags in the heart of Edinburgh. There they showed him a contact between traprock (a basalt lava flow) and sandstone, pointing in particular to bits of sandstone that were enclosed within the traprock. This contact was proof positive that the basalt was (1) a formerly molten rock that (2) had intruded into the sandstone long after the sandstone had formed:

“When Sir James had finished his lecture, the Doctor did not attempt to explain the facts before him on any principle of his own; nor did he recur to the shallow evasion of regarding the enclosed sandstone as contemporaneous with the trap; but he burst out into the strongest expressions of contemptuous surprise, that a theory of the earth should be founded on such small and trivial appearances! He had been accustomed, he said, to look at nature in her grandest aspects, and to trace her hand in the gigantic cliffs of the Irish coast; and he could not conceive how opinions thus formed could be shaken by such minute irregularities as those which had been shown to him.”

Contacts among Oakland’s rock units are hard to find because our rocks are poorly exposed to begin with. And even when you do find contacts, they may not preserve the small and trivial appearances that might tell you the most. But the Tunnel Road cut exposes a large area of rock, as seen in this aerial view from Google Maps. The slope is interrupted by several cutbacks that serve to stop runaway boulders and allow access for maintenance — most recently by the herds of goats that helpfully cleared away most of the French broom — and visitors like me last week.

This big cut exposes a significant contact right along the road, just west of the Gateway Emergency Preparedness Exhibit Center (under the word “Hiller”). The geologic map shows the spot as the contact between the Leona volcanics, in pink, and a teeny splinter of olive-green Knoxville Formation directly above the “o” in “Substation.”

The contact today is somewhat obscured by vegetation, so let me show it to you first as it appeared from Tunnel Road in December 2007, when the state last cleaned it up.

From lower left to upper right, the rocks gradually give way from highly altered volcanic rocks, containing some shaly beds, to dark-brown shale. Some of the experts heartily disagree on what exactly is happening here, but it’s widely taken to represent the very top of the Leona volcanics and the very base of the Knoxville Formation, lowest member of the highly sedimentary Great Valley Group.

Here’s how it looked in July 2019. You can tell in both photos that the shale is crumbling down the slope almost as fast as it’s exposed whereas the volcanics stand sturdier.

Along with a few other localities scattered around California, here’s proof that the two rock groups started out as peaceful neighbors, the shale laid gently down upon the volcanics under the Late Jurassic or Early Cretaceous sea. Although later tectonic events wrenched and stretched and broke all of these rocks as California gradually became its present self, this spot remained untouched for geologists to argue over. As an example, I captured Cliff Hopson pressing a point to the late Eldridge Moores at this very outcrop in 2005, no doubt discussing some minute irregularity.

It was a pleasure to stop here in July with John Wakabayashi, leader of that 2005 field trip. He noted how important it is to revisit outcrops: “When you come out, you notice things you didn’t notice before.” And he pointed out features of the volcanics I hadn’t picked up on my own. For instance, the locality is unusual in featuring fairly fresh volcanic glass, which can yield more faithful geochemical data than the altered rocks around it.

He also found something he hadn’t seen before: carbonate veins with shapes that reminded us of soft-sediment deformation. This suggested to him that they may have been original constituents of the rocks and not later alterations, in which case they might preserve microfossils.

Familiar features of the Leona volcanics are well displayed here, including its lumpy and fractured texture, a reminder that the unit is mostly not lava flows but ash beds and landslide deposits, fused and altered by hydrothermal springs.

Slickensides — polished fracture surfaces — testify to much later activity related to the Hayward fault and the rise of the Coast Range.

And I always take pleasure in spotting the green devitrified ash whose color is attributed to celadonite, possibly with other green secondary minerals like prehnite, chlorite and epidote.

Familiar places can still reveal new things, if you keep your eye on seemingly small and trivial appearances.