Archive for the ‘other’ Category

Two lessons about floods

19 October 2015

As we anticipate the strong possibility of heavy El Niño rains, my attention will be on Oakland’s streams this winter. Last week parts of southern California were hit by “thousand-year” rainfall events, cloudbursts that washed thick sheets of mud over roads and properties. We can expect such things here too, in any given thousand-year period.

Arroyo Viejo, the stream that crosses Knowland Park, offers two lessons about floods. The scene below is at the northern edge of the park, looking upstream: a streambed piled with boulders, some as large as sofas. (All photos 800 px)


Notice: these rocks have been tumbled by the stream. How much water would it take to do that? Let’s make a rough, arm-waving estimate.

The rainfall in last week’s cloudburst was almost 4 inches in one hour. Had it fallen on the watershed of Arroyo Viejo above this point — say, half a square kilometer — it would represent an input of roughly 30 cubic meters of rainwater every second.

Picture in your mind that volume of water — no, it would be mud and therefore that much greater — funnelled through this narrow valley. Do a little geometry and it’s easy to see the floodwater would be well above the tops of the boulders.

Hidden in plain sight in this photo, then, is a single hour of tumult that might have happened a thousand years ago or five hundered years ago — or perhaps during the dreadful winter of 1861-62, when it rained for 43 straight days and much of the Central Valley became a lake.

The lesson is that most of geology’s hard work gets done in rare spurts of extraordinary activity.

Okay, the second lesson is hidden in these rocks. All of them, like this boulder as tall as me, are made of conglomerate.


These rocks, assigned to the Knoxville Formation of Late Jurassic to Early Cretaceous age, were laid down by ancient floods in a nearshore or terrestrial setting. I’ll show you three different specimens. Notice the large clasts and the fine-grained matrix that surrounds them.


This boulder displays a wide range of clast sizes. It was probably laid down by what’s called a hyperpycnal flow, a slurry of sediment that carries everything along with it. We’ve watched them happen offshore in Monterey Canyon. Here’s another example.


Instead of an underwater landslide, as seen in the first specimen, this represents something gentler and more organized, like a mudflow, or like the mudslides we saw in the news. The clasts are aligned with the current that carried them here.

The key observation in both cases is that the large clasts are floating in the matrix. In geologist’s terms, they are matrix-supported conglomerates.

Then we have this.


Here’s a beautiful clast-supported conglomerate. It represents a clean bed of well-rounded cobbles, all touching each other, like you’d see in a rushing stream or a rocky beach, nicely infiltrated with clean silt or clay after it was laid down.

None of these stones were made by ordinary sediment wafting down streams during ordinary rainy seasons. They were assembled by floods of all sizes.

Deep time and Deep East

28 September 2015

The deepest part of Deep East Oakland, at the south end of the alphabet streets, is a neighborhood that shows its age. First laid out and developed almost a century ago, it was a desirable locale, with good transportation, fresh air, a warm climate and excellent soil, plus nice views of the hills.


The neighborhood retains modest homes from a wide range of 20th-century styles.


There are also front-yard fences everywhere, signs of a more recent stage of the local culture.


So why do I bring up deep time when I think about Deep East? “Deep time,” the wonderful term first used by John McPhee in Basin and Range, is geology’s great insight that Earth history is essentially infinite. Put another way, by paying careful attention to the geology of the present-day landscape, we can deduce many facts about the deep past. With that knowledge we can visualize ancient worlds with different landscapes, superimposed on our own. From those visions, informed by geologic fact, we can see light shed upon even earlier landscapes and worlds, and there seems to be no limit. This is similar to how astronomers know the universe — deep space — in ever-greater detail as our instruments improve.

We also learn that even while the landscape is far older than the human presence in it, some parts of it are old and some are quite young in geologic terms. The young features took their place by erasing something older. The ongoing processes of geology — uplift, erosion, consolidation, disintegration — lead to a pleasantly mixed landscape just as the ongoing processes of humanity — birth, death, migration, commerce — lead to neighborhoods like Deep East. Landscapes and neighborhoods both are always changing, and each day’s present is a snapshot never to be repeated.

The more I learn how much Oakland has changed and how many ways it can change, the more precious becomes the present. Some day, earthquake or rising sea level or century-long drought will wipe most of our present away. So as I walk around this town I always know that the panorama has a big label on it that reads “Before.”


What the “After” scene will look like is to be determined. We know that by personal initiative and supportive policies, historic properties can be safeguarded for the future, with the hope of maintaining and reviving what’s precious about a neighborhood.

The same is true for Oakland. The same is true for the Earth we live on. Spread the word and enjoy today.

Survivor trees

10 August 2015

I’ve been poking around Oakland’s edges lately. In southernmost East Oakland there’s not a lot of geologic history, but there are some big old trees, survivors of optimistic times, plus nice views of the hills and sky.

This is one of our largest and oldest jacarandas, on 92nd Avenue. (There’s a comparable one on Pippin Street.)


And here’s an even more exotic visitor, an Araucaria from the southern hemisphere (a monkey-puzzle tree, I think) in Sobrante Park.


On 105th Avenue, next to what appear to have been Oakland’s last commercial greenhouses, we have a big old lot where there used to be something but now there’s just a large generic tree. What has it survived?


The ground here is mapped as Bay mud, just past the seaward edge of the alluvial fan of San Leandro Creek. If the soil is still clean, it should be rich stuff.

Down at the end of 105th Avenue, hard by San Leandro Creek, is a cluster of venerable black walnuts.


Unlike the previous trees, these may well be natives. At least the species is right and the riparian (streamside) habitat is correct.

There are surely people in Oakland who remember the stories of these trees.

But I really wanted to talk about another old survivor tree, up in the hills on the south side of Horseshoe Canyon. In fact “Old Survivor” is its name. This last week I had the opportunity to visit it with a small group of people. It’s older than any human memory in these parts.


The group tended to refer to the tree as “the Grandfather Tree.” I thought “Grandmother” was a name just as good. But the DAR, in chiseling “Old Survivor” on this monument by Campus Drive at Carl Munck Elementary School, established priority. The age is subject to revision, though. The 1981 monument put the tree’s age at 425 years, but there’s no birth certificate for this baby. The tree was cored in the late 1960s, and its age at that time was estimated as 415–420 years. Let’s make 1550 its germinal year. So today it would be 465 years old. Here it is.


The Old Survivor is about 100 feet tall and has presence, especially when you think of it as the last old-growth redwood left in its East Bay stronghold. However, the loggers let it be because it was, and is, a scraggly, marginal specimen. Part of its uphill side is bare, fire-scarred wood. It’s flanked by two nuisance trunks that were less than marketable size in the 1850s redwood rush. The slope it sits on is more of a cliff, and had it been toppled it would have shattered.

The Old Survivor is rooted on Leona “rhyolite” rather than the moist mudstone of the Redwood Formation or Oakland Conglomerate that redwood prefers. But it’s fitting that Oakland’s oldest tree grows from Oakland’s oldest rock.


While the tree consists of a triple trunk, the central trunk stands above its siblings pretty well, if I can judge from the photos on oaklandwiki. I took a photo of its crown from the west side.


Old Survivor isn’t pretty, and it’s very hard to visit, but it’s still precious: the last bit of our forest primeval.


Get every new post delivered to your Inbox.

Join 9,537 other followers