The Merritt sand: A little deeper

3 February 2020

My last post was about the great sand bed that underlies Alameda; now it’s time for a fresh look at the whole geologic unit of which it’s a part: the Merritt sand.

The Merritt sand is mapped in three places: in downtown Oakland, in Alameda and in Bay Farm Island. It’s labeled “Qds” (Quaternary dune sand) on this map of sediment deposits in the Bay area (Open-file Report 2006-1037), largely surrounded by artificial fill on top of Bay mud (afem).

UC Berkeley’s indefatigable Andrew Lawson named the Merritt sand in 1914 “from its occurrence on Lake Merritt, in the city of Oakland.” He considered it a marine deposit, but our understanding has advanced since then. He noted that it was 44 feet thick in a well dug at 665 16th Street (now an apartment complex) near today’s MLK Boulevard. He also mapped it in a long trench on Telegraph Avenue, noting that the top of the sand descended from the surface, just south of today’s 21st Street, to 13 feet deep just north of today’s West Grand Avenue. I’m confident that this is it on the west side of Telegraph at 20th.

The sand ends abruptly to the east of this spot, not appearing at all in the excavation for the upcoming skyscraper on the other side of Telegraph at 20th. That was historically a boggy ground that drained down 20th Street to the lake, and the sediment there is sand and gravel with a good share of clay.

Elsewhere downtown, the eastern edge of the sand is a steep slope, for instance along the lake and at Snow Park.

Since Lawson’s time, the Merritt sand has mainly been of interest to practical geologists concerned with building sites. In the late 1940s, proposals for a second trans-Bay bridge led to a concerted geological investigation of the Bay floor covering two different routes for the bridge. In a 1951 paper, UC Berkeley’s Parker Trask and Jack Rolston reported that the Merritt sand extended across the Bay, reaching a thickness of up to 60 feet. Their cross-section along the route of the Posey Tube shows it well.

Trask and Rolston noted that the sand’s texture “is remarkably uniform” and its grains were typically in the “fine” range, between 1/4 and 1/8 millimeter, although in some places it was extremely fine, forming “material with the characteristics of loess.”

Dorothy Radbruch noted in her 1957 map of the Oakland West quadrangle (USGS I-239) that the Merritt sand reached 65 feet in thickness in a boring where the Crucible sits today. A meticulous worker, she described the material as “Sand, fine-grained, silty, clayey, with lenses of sandy clay and clay. Yellowish-brown to dark yellowish-orange. Grains consist of quartz and feldspar, some magnetite, flakes of white chert from the Claremont [Shale], minor amounts of sandstone, shale, hornblende, pyroxene, biotite. Grains angular to subrounded, frosted. Well-sorted.”

The key word for my purposes is “frosted,” a textbook sign of windblown sand. By 2000, Russ Graymer of the USGS could confidently say that the sand belonged to dunes that “probably began accumulating after the last interglacial high stand of sea level began to recede about 71 ka [thousand years ago], continued to form when sea level dropped to its Wisconsin minimum about 18 ka, and probably ceased to accumulate after sea level reached its present elevation (about 6 ka).” Here “Wisconsin” refers to a formal stage of the North American ice ages.

Large areas of Merritt sand in the Bay, more than 50 feet thick, were exploited by dredgers to build (“reclaim”) land. In one area west of Bay Farm Island, as large as the island itself, some 25 million cubic meters of sand was “borrowed” from the Bay floor and used over the years, most likely, to build up Treasure Island, the Oakland Airport, Bay Farm Island itself and Alameda’s south shore. We owe a lot to this fine sand, and by fine I mean excellent. And as the sea rises, we may need more of this Ice Age resource.

Posted in Oakland sediment and soil | 1 Comment »

Geology of Alameda

20 January 2020

Although these two distinctive cities are right next to each other and were settled at the same time, Alameda is a very different place from Oakland. One way to put it is that when you’re in Oakland, you see Oakland all around you. In Alameda, you see everything but Alameda around you.

And I like both of those things just fine. But in this post I’ll attempt to show the subtle ways Alameda reveals itself.

First a little history. The earliest map showing Alameda in any detail was Captain Beechey’s map of the San Francisco Bay, first published in 1833. Mainly a sailor-centric chart, it focused on the seaward edges of things. It shows Alameda as the peninsula it was until the 1890s, when the tidal canal was completed across its east end making it an island.

Two details are interesting. First, the map shows the seaward edge of the peninsula as an embankment rather than the typical marsh found around most of the Bay. A sandy bluff overlooked the beach and mudflats here. Second, the map used the same tree symbols as it used for the redwoods in the high Oakland hills, and not the round icons used for the encinal oak groves to the north, in West Oakland and downtown. Gary Lenhart, over at alamedainfo.com, suggests that this may mean there were redwoods here. I don’t buy that because the habitat is wrong and because I haven’t seen any mention of redwood groves along the shore, but the possibility is intriguing, especially since Friar Pedro Font, during the Anza expedition in 1776, also sketched the peninsula with a heavy forest (view west).

The 1857 Bache map doesn’t cover all of Alameda, but what it does show comports with the Beechey map in depicting a definite edge, not a marshy transition, between land and sea. This segment is from the west end; the Peralta Wharf was where Ballena Bay is today.

This map shows the entire peninsula forested with oaks and labeled “The Encinal,” which is how the first generation of Anglo occupiers knew it.

That’s all long past. The trees went early, turned into firewood and charcoal; the land was farmed, then subdivided for estates and divided again for homes. Underneath it all, the Alameda peninsula is a uniform body of windblown sand dating from glacial times, now surrounded by artificial landfill as seen in the geologic map. Nowhere is the elevation higher than 35 feet.

I should note an exception to that. Once upon a time the shellmound of Mound Street was the tallest thing in town, according to the monument in Lincoln Park. And here let me acknowledge that we live on Ohlone land, and that we don’t deserve the acceptance and welcome the tribes have granted us. Forgive us our trespasses.

Between the natural sand and the artificial fill are Alameda’s lagoons, the city’s most unexpected and hidden feature.

They’re hard to reach, mostly private land. In a few places you can see that the landward side is higher . . .

. . . and the Bay side is lower.

On Willow Street at Alameda Hospital, the transition is plain to see in the roadbed. This is the greatest topographical feature in the whole city.

The main body of the dunes is a very gentle dome, reaching just over 30 feet elevation along Central Avenue. It’s hard to catch in a photo, but charming to see in person. This is looking down Chestnut Street, on the north side of the dome, toward Round Top.

And in the other direction is the top of the dome, such as it is.

It just goes to show that, to a committed geologizer, every place has a there there.

Posted in Other topics | 3 Comments »

Arroyo Viejito

6 January 2020

Some of Oakland’s most interesting land is also its most inaccessible; I’m speaking of our streambeds. And on the whole, the largest remaining stretches of wild streambed belong to Arroyo Viejo. Just to orient you, here’s the Arroyo Viejo watershed, as it’s mapped today by the Alameda County Flood Control District. The red stripe, which I added, represents the Hayward fault. (I’ll return to that.)

Here’s a zoom-in to the lower right corner, showing the upper part of Arroyo Viejo and the valley of a defunct little stream that I’m calling Arroyo Viejito.

The peculiar feature that caught my eye several years ago is how Arroyo Viejito runs parallel to Arroyo Viejo, very close to it, with a distinct rocky ridge between the two streams. Today the two valleys are very different, and a century’s worth of maps hints at what happened. Here’s the 1897 topo map showing the two streams, underneath the word “Viejo.”

In 1915, the area was more accurately mapped, and the two streams are shown as extremely close together at one point.

Everything changed after this. The country club was expanded and the adjoining land was subdivided and developed into the very exclusive Sequoyah district starting in the early 1920s. At that time Golf Links Road was pushed through to what would become the Grass Valley district in the 1950s, and Arroyo Viejito was diverted into the large stream at their closest approach and a sewer line inserted into the abandoned valley. It was very handy for the developers. As of 1947, the little stream had vanished and the land lay open for a new wave of luxury homes.

As of 1980 the buildout around Arroyo Viejito was complete.

The sewer line is accompanied by a maintenance road that is now a nice place for the locals to walk, and it connects with the little-visited creek trail at the north edge of the zoo’s property. I featured this area, in passing, three years ago in Ramble 3.

The reason these two streams ran so close together is related to the Hayward fault. It’s been dragging the lower, western half of Arroyo Viejo north, and for the last few hundred thousand years the stream has stretched out along the fault line before turning toward the Bay. Models of landscape evolution suggest that the headwater streams have been getting squeezed, aligning themselves and crowding together.

The combination of an especially large earthquake and a major flood could cause Arroyo Viejito to break through the narrow waist and join Arroyo Viejo farther upstream, abandoning the stretch with the sewer line and leaving the ridge standing there for a few more thousand years until it erodes away. But impatient developers have short-circuited all of that, and now the little stream is defunct, its former catchment part of a sterile golf course.

As I said, it was the ridge between the two streams that caught my eye and dared me to set foot on it. It’s in the middle of this Google Earth view looking west.

Its sides are very steep; it’s like an island. One day I found that it has a tiny trail running along its top, and signs of an old road and excavations. My guess is that the ridge was dug up for fill material when the sewer line was put in. The high-resolution lidar data acquired along the Hayward fault a few years back covers the west half of the ridge, and the resulting digital elevation model (with the trees and buildings stripped away) shows these features plainly.

Lately I’ve visited this ridge and the stream valleys of both Arroyos, in search of access and ultimately in search of rocks. Access beyond what I’ve already mentioned is difficult, and I have paid dearly for it in poison-oak rash. But I shall return.

The bedrock map looks like this, but I am suspicious of all of it given the difficulty of access and the paucity of outcrops. One big goal of mine has been to inspect the stream bed where bedrock might be exposed, for some real ground truth. I suspect that geologists, while doing their best, have resorted to drawing lines based on the topography.

The green zone marked KJk is shale and conglomerate of the Knoxville Formation, and that’s what I’ve always found in the eastern chunk of it. This shale is just west of Golf Links road where it crosses the creek.

And the conglomerate is abundant as loose boulders (not bedrock) downstream. It’s beautiful stuff.

But I have found none of it yet in the western section. Instead, everywhere I’ve looked the rock is either coarse sandstone shot with calcite veins, interpreted as the very oldest part of the Knoxville . . .

. . . or familiar rocks of the Leona volcanics (Jsv).

This includes up on the little ridge and down in the Arroyo Viejo streambed.

I still have a good bit of territory to visit, though. The streambed will have to wait until dry season, when I can poke around this weird-ass lime-cemented breccia.

And there’s more ridge to check out. Outcrops like this are so crusted with lichen that I might need to bring a rock hammer for some very careful, unobtrusive chipping.

There are some other charms in this northernmost stretch of Knowland Park. Every time I’ve visited there are fresh deer bones, indicating a mountain lion’s sphere of influence. And the cries of exotic animals occasionally drift down from the zoo’s hilltop center.

No other place in the world exactly like that.

Posted in Leona volcanics, Oakland conglomerate, The Hayward fault | 5 Comments »

Lake Chabot’s north shore

23 December 2019

Oakland has a lot of ground to cover, and it can take a while for me to return to places I’ve been before. In this case, it’s been four years since I traversed the trails on the north side of Lake Chabot, between the reservoir and the golf course.

The land is much the same (though I’ll point out some differences). It’s my frame of reference that’s changed.

Here’s the Google Earth view of the area. My walk started near the dam and went up the trail (the Bass Cove Trail) along the west edge of the lake, then back down on the unmarked, unsigned trail running just west of all the chaparral.

The latter trail is basically an access road for the power line that runs through here over the hills. The land ownership is mixed, but there are no barriers.

For reference, here’s the bedrock map of the same area. (It also shows the access road snaking along the power line.)

Jsv is the Leona volcanics, KJk the Knoxville Formation, and the slightly darker green on the right is the Joaquin Miller Formation. The blue section labeled KJfm is mapped as Franciscan melange. I’ll show you some of these. What’s changed in my frame of reference is that I’ve learned there are conflicting interpretations of that blue section. I found no smoking-gun evidence in my visit, so you’ll be spared the details in this post, but my eyes were peeled in a way they weren’t before.

I like a lot of things about this time of year. For purposes of geologizing, the footing is firm and quiet even off the trail. Also, the rain has washed the outcrops clean.

Of course, rocks like this are still covered with lichens so you can’t be quite sure what they are. The best-exposed rocks are in the streambeds, like this blueschist boulder.

All three of these are in the melange unit. For purposes of my enlightenment, it’s the matrix between these blocks that’s crucial, but none of that was visible. Just have to keep looking.

The sandstone of the Joaquin Miller Formation is nicely exposed in the rain-washed roadways.

It’s a pretty pure sandstone; there are spots in the trail where rainwater has washed the eroding stone downhill into sandy drifts.

Sand is nearly eternal. It can be recycled time and again in the rock cycle for hundreds of millions of years. But that’s another post.

And off the trail I was pleased to find some excellent examples of the conglomerate at the base of the Knoxville Formation, which is otherwise mostly shale.

The rounded cobbles in this exposure are largely composed of the Leona volcanics, proof of the genetic link between the two bodies of rock and a clue to the geography of ancient California during the Late Jurassic.

Enough bedrock. Other things I like about this time of year are that it’s cool, the air is clean and fragrant, the colors are distinctive and the light is interesting. It’s a primo time, if you ask me, to walk the high hills, and this part of town offers good views of Fairmont Ridge and the lake.

Even a peek at the Hayward hills and Mission Peak beyond.

The parks district has been visibly sprucing things up in the park. And along the power line, it’s obvious that PG&E has been at work too, taking the fire risk seriously by reducing the fuel load.

They’ll probably shred this plant material and leave it on the ground, but I would favor some good old-fashioned controlled burning here in the fire-friendly chaparral. They’re even making a little headway against the eucalyptus, which besides its fire hazard tends to shed limbs. Eucs make fine specimen trees, like the one across the way, but lousy forests when they aren’t well tended — take a look at Australia these days.

Lake Chabot and the surrounding parks are a special part of Oakland. Get yourself out there; let your mind roam free.

Even at the bottom of the year, there’s a lot of good light.

Posted in Oakland blueschist, Oakland conglomerate, Oakland geology puzzles | Leave a Comment »

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