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.

Oakland geology ramble 8: Piedmont Ridge

19 August 2019

I don’t always care about rocks; geology is about more than rocks. I don’t even always care about geology; sometimes I just want a vigorous, geographically arbitrary hike. Ramble 8 is one of those — a traverse of the ridge crest above Piedmont, our highest ground west of the Hayward fault. It goes from the Rockridge BART station to the 33 line bus stop at the Leimert Bridge, on the lip of Dimond Canyon, about five miles end to end. Nevertheless, it has geology and rocks along the way. Here’s the route. There’s a map with more detail at the end of the post, where I also have some announcements.

The BART station features the “Rockridge” destination signage, mounted above a selection of Sierran boulders. This time, some ten years after it was installed, I noticed that the lettering design is quirky, contrasting “rock” and “ridge.”

You could climb Keith Avenue and barge up upper Broadway to attain the heights at Contra Costa Road, but it’s more interesting to circle behind the College Preparatory School grounds on Brookside Avenue. The school nestles in a steep little stream valley — unlike many similar places, this is not a former quarry but, apparently, a former turn-of-the-last-century park that had a short existence.

Once you get up to Contra Costa Road via Eustice and Buena Vista avenues, the walking is pretty and level. While you’re here in this remote part of town, check out the blueschist outcrop at 6063, vacant since the 1991 fire. The entire ridge on this hike is in Franciscan melange, a mudstone matrix containing odd lumps of other rock types. You won’t see much of it.

At the very end of the street is Erba Path, a steep set of stairs down to the saddle in the ridge where Broadway Terrace peaks on its way past the south entrance to Lake Temescal and points east. Cross that busy road and head right back up again on Sheridan Road, visible here at upper left.

Turn right off Sheridan at Agnes Street, unless you want to explore the little-trod path that joins Sheridan’s two halves. (Everyone should at least once.) Take high-flying Cochrane Avenue, where views east of Thornhill Canyon and Glen Highland’s settled slopes beckon.

Then jog right again up the saddle of Florence Avenue (where I sent you in the last post) to Proctor Avenue. This is pretty steep, but short. By this point you’re well above 700 feet. You’ll go higher later, but first you clamber a hundred feet down to the next saddle where Moraga Avenue crosses the ridge. Take the well-marked pedestrian crossing, but beware — this is the most dangerous road crossing of the hike.

Proceed on Estates Drive, which climbs nearly to the 800 foot contour. Up here are two curious reservoirs that date from Oakland’s water wars, when private water companies struggled to supply the fast-growing region from local sources while they vied with each other in deadly capitalist strife. (A ruinous series of bankruptcies and mergers ended in the 1920s with the formation of EBMUD.)

The Dingee and Estates reservoirs were constructed, fast and furiously, at the highest point of the Piedmont hills to provide good water pressure. EBMUD is upgrading these old concrete bathtubs to proper steel tanks, built to withstand big earthquakes on the Hayward fault just a few hundred yards east.

From here you head down again to La Salle Avenue, which takes advantage of yet another saddle in the ridge. If you’re ready to quit at this point, go left on La Salle, left again on Bruns Court and cross that high pedestrian bridge over the Warren Freeway to Montclair Park (because you can!) and catch the 33 bus at La Salle and Moraga. Otherwise, cross La Salle and stay on Estates, which is a little to the right.

Estates climbs again, not so far this time. Because this walk hits the highest spots, take Dawes Street up the hill and over, where you simply must visit the south end of Pershing Drive and admire Oakland’s best outcrop of Franciscan chert.

From here on it’s all downhill. Dawes rejoins Estates Drive here, and as you start down Estates you can see across Dimond Canyon.

Geologically and geomorphically speaking, the other side is also part of Piedmont Ridge, but the large water gap of Dimond Canyon is impassable without a long detour. So, down you go to the bus stop at the Leimert Bridge.

Along the way are two more highlights. First, at the Piedmont line the road passes the head of the former Diamond Cañon Quarry, which today houses the Zion Lutheran Church. Recent foundation work here has exposed fresh rock; maybe you’ll see some too.

And second, enjoy this wonderful volcanic breccia used for the landscaping at 170 Estates Drive. There are whole walls of it.

The house itself is something to see, too.

And as promised, here’s the detailed route map (1126 X 1126 pixels), followed by some announcements.

The excellent, out-of-print book Stories in Stone: Travels Through Urban Geology by David B. Williams is being reissued in paperback by the University of Washington Press. If I didn’t have the hardcover already I’d buy this classic. More information on David’s website.

My own book manuscript is making the rounds of a publisher, and while I await a yea or nay I’m trying to get my arms around a whole lot of scientific literature pertaining to Oakland’s rocks. It’s a ridge walk of the intellect, but I want to make Chapter 5 as good as humanly possible. I hope to buttonhole some of the real experts next month at the Geological Society of America’s annual meeting in Phoenix.

But first I’ll be giving a talk to the Friends of Sausal Creek, on 18 September at the Dimond Library, titled “Sausal Creek: The Last Million Years or So.” You read it here first (unless you follow me on Twitter, @aboutgeology), and I’ll repeat the announcement in the Q&A thread soon.

Oakland’s wild rail path

5 August 2019

The seasons are changing now, if you follow the pagan calendar. This weekend marks the turning point between astronomical pagan summer (6 May to 6 August) and pagan autumn (6 August to 6 November), or as I think of them, High Season and Waning. They are offset exactly half a season from the conventional astronomical seasons. High Season consists of long days, and Waning consists of shortening days. (Likewise, Low Season consists of short days, and Quickening consists of lengthening days.)

Nature is acutely aware of these seasons. The belladonna lily (Amaryllis belladonna) sends up its naked-lady flowers at this time. The strawberry tree (Arbutus unedo) ripens its rich little fruits (I can understand why Pliny the Elder named them “eat-only-one” because they’re so satisfying).

And of course the blackberries are in full swing.

I returned last week after eight years to the “secret street” at the south end of Florence Avenue, where it meets the old railbed of the Sacramento Northern Railway (also known as the Oakland, Antioch, and Eastern Railway). Unlike that first visit, when I was busy and could only gaze up the path, this time I had the leisure to walk its whole length, up to Broadway Terrace where it’s fenced off.

The path has geology up at its north end, but it’s worthy just as woods. Even right next to the Warren Freeway, it’s as secluded as any place in Oakland.

It’s shown as the dashed route on this map. You can see that Florence Avenue, heading over a saddle in the ridge above Piedmont, used to connect with Florence Terrace once upon a time. That’s the Lake Temescal park at the top.

There are lots of blackberries growing here, so don’t wait. The first ones are the best. Near the north end is a landslide scar that was repaired with much labor to protect some homes on Sheridan Road. The work was finished with dark shotcrete, but it doesn’t really blend in.

If you look close you’ll see little splotches of white. Those mark cracks where lime-bearing groundwater has seeped through and deposited calcite as it evaporates.

I can foresee these growing into falls of travertine in a few years. Beyond the landslide is a high cut into the hillside, made decades ago when the rail line was first pushed through. And the bedrock here is mapped as classic Franciscan melange, the big blue field on the geologic map — the edge of which happens to correspond to the Hayward fault.

I half expected the rock exposed here to be fault gouge, the fine-ground, mealy stuff that fills many of California’s active faults (for instance at the London Road slide). It’s real close to it: highly weathered mudstone that’s likely to come down hard in our next big quake. Whether the railbed will be cleared again afterward can only be conjectured. I’ll look at this cut again more thoroughly next time I’m here, whenever that might be.

On your way back, look again for blackberries. I know I didn’t get them all.