In search of lime

25 May 2020

I like to brag that Oakland contains more rock types within its boundaries than any other city in America. But alas, it’s missing one of America’s most widespread rock types: limestone. Yes, there are pods of dolomite limestone in the Claremont Shale, but that’s a far cry from what Oakland’s first outside settlers hankered for, which was real calcite limestone.

Limestone is essential for civilization because it’s the default industrial source of lime (calcium oxide or hydroxide), without which you can’t manufacture any kind of decent mortar, or plaster, or cement. Lime has been made for thousands of years by simply roasting limestone, which consists of the mineral calcite (calcium carbonate). This was traditionally done in a lime kiln, a stone furnace that was loaded with rock and firewood.

The Bay area does have bodies of limestone, and these were staked out early. But the nearby firewood supplies (i.e. forests) were soon depleted, and the industry was fitful and limited until reliable supplies of coal and, still later, oil became available.

One alternative to limestone in Oakland was oyster shells, which we once had in abundance. Unfortunately, many of the old Native shellmounds had oyster shells in abundance too, and this led to the end of them.

A few years ago I had the chance to visit the remains of an old-fashioned lime kiln in the Santa Cruz Mountains.

Another old site was on the coast near Rockaway Beach, south of Pacifica. I wrote a piece about it for KQED a while back that explains where most of our local limestone comes from, tectonically speaking.

This kind of industry lasted only a few decades. Around the turn of the last century, big manufacturers started to mine massive bodies of limestone. One was on the Santa Cruz County coast at Davenport; it supported a company town out there for a whole century. Another was in the South Bay near Cupertino, based in a quarry on Permanente Creek. One of Henry J. Kaiser’s early companies operated it, and when Kaiser eventually became head of an industrial giant, during World War II he took the name Permanente for the name of his innovative company health-care plan. The quarry is still digging, almost 80 years later.


By Steve Jurvetson from Menlo Park, USA – Flickr.com – image description page, CC BY 2.0, from Wikipedia commons

Another big limestone quarry was in American Canyon, in Napa County. But the one that fascinates me most is the former Cowell lime quarry, which sits between Walnut Creek and Concord, on Lime Ridge. For about 40 years, starting in 1908, this quarry exploited a thick deposit of travertine — hot-spring limestone. It supported a company town too. Now some of the workings are in Lime Ridge Open Space, where you can scrutinize these unusual deposits. They aren’t like anything else around.

This post was inspired by my maniac brother Steve, who explores the woods around his Lyme, New Hampshire home to locate old cellar holes. Now he’s branched into old lime kilns.

In other news, I’m happy to report that the latest Covid-19 guidelines now allow the use of public transit for outdoor recreation. This means that all of my geology walks and rambles can be undertaken again, and I’m looking forward to getting out there, taking due care of course.

Decorative blueschist

11 May 2020

Since the shutdown I’ve been scrupulous about going out only on essential business. Yes, my idea of essential is wandering around with one eye on the ground, the other on the landscape and my mind in the clouds, but it’s not really essential. Basic, baseline exercise is essential, and I can get that in with a walk of a mile or two.

That said, a walk of a mile or two can take you to lots of interesting places no matter where you live. The upper part of Fairmount Avenue is within that perimeter, and that’s where this stone wall caught my eye. Oakland’s houses of a certain age occasionally feature our local blueschist, and this is a particularly striking example.

Just a few feet farther, the scope of the work emerges.

The entire front of the property, plus the long stairway to the entrance including the risers, employs this stone to dignified effect.

Blueschist can be garish, but here it complements both the concrete and the Craftsman house in color and texture. I took a close look at it to compare with its appearance at various Oakland locations. The stone is unusually sound and was apparently quarried for this purpose. I don’t think rock yards carry this kind of unfashionable stone today — it’s hard to source large, consistent quantities, and artisanal stonemasons are no longer as plentiful as they were a century ago.

I don’t know any commercial sources of rock like this, although it’s conceivable the old Hutchinson quarry in El Cerrito, where the recycling center is today, used to yield some from its upper level. I suspect it came from a pit in the Crestmont/Serpentine Prairie area. This big exposure on Old Redwood Road shows the general quality of the rock up there.

Some of the rock in Crestmont is a pretty close match, but in general it’s more fractured and pocked with carbonates.

I suspect, but have no way of proving at present, that this old pit at the back edge of Serpentine Prairie was the source. Our serpentinite/blueschist isn’t useful for crushed stone, and although “a few tons” of poor-quality asbestos was once mined up here, this particular rock has no sign of containing tempting amounts of chrysotile. So that leaves decorative stone as the most likely product.

And that reminds me that I need to visit the meadow up there soon, for the exercise.

Pill Hill

27 April 2020

Pill Hill, an odd outlier of ancient alluvial gravel in North Oakland with a long history, rises almost 50 feet above its flat surroundings. Today it’s thoroughly encrusted with buildings, as seen here looking up Broadway from the YMCA gym building and down Broadway from the Kaiser hospital parking structure.

Beginning in the 1860s it was known as Academy Hill or College Hill for its private schools in gracious settings, which included St. Mary’s College, the Pacific Theological Seminary (now the Pacific School of Religion on Berkeley’s Holy Hill), the California Military Academy, the Pacific Female College, Hopkins Academy (where publisher J.R. Knowland started his first newspaper as a student), and other long-gone institutions.

Although the hill started out as bare grassland, after a few decades of landscaping the location was described in 1885 as “healthful, retired, and beautiful” and was served by horsecar lines on both Telegraph Avenue and Broadway.

Anthony Chabot built the first reservoir of his Contra Costa Water Company here in 1868, near today’s Summit Street and Hawthorne Avenue at the hill’s highest point. It held a million gallons of Temescal Creek water, and Academy Hill institutions may have been early customers supplementing their own wells. See its location on the 1878 Dingee Map:

and here’s the spot today.

The 1949 USGS topographic map shows the hill lovingly outlined in 5-foot contours. Given all the construction and digging done here since then, I think not even a modern lidar survey will ever match the fidelity of this map.

You can see that all the academies were gone by then (except for Grant Senior High, now the Zapata Street Academy), replaced by hospitals. Hence today’s name of Pill Hill. Probably the availability of large land parcels, the subsequent improvement of the water supply, the great street access, the advantages of a concentrated healthcare district (including the original Samuel Merritt College) and the attractive setting favored this change. When Pill Hill’s three big hospitals combined in 1992 to form Summit Medical Center, it was the hill they shared that inspired the name. The views from the hill, especially from the higher hospital floors, remain excellent.

Pill Hill is part of the widespread body of old Pleistocene-age gravel that I call the Fan, specifically lobe 2. Here it is on the geologic map.

Where this material extends across 27th Street, there’s a low hump in the road that the builders didn’t bother to flatten.

And at its north end, the construction of I-580 wiped out the hill, but a little bit still extends into Mosswood Park. This cut in the low rise at the park’s south edge may expose the gravel, but of course I can’t dig into it.

The rest of the hill’s periphery is an abrupt edge; this view west from Broadway down 30th Street is typical. I showed a few more views of the hill in this post from 2011.

The gravel of the Fan is considerably older and more consolidated than the alluvial plains around it. Exposures are very difficult to find, which is maybe why I’m a bit obsessed. The official description of this map unit (Qpaf, Quaternary (Pleistocene) alluvial fan) includes the tantalizing bit that they “locally contain fresh water mollusks and extinct late Pleistocene vertebrate fossils.” I did present one good (short-lived) exposure here in 2015.

The Oakland Hills

13 April 2020

These days we’re all sheltering in place: doctor’s orders. What better thing to do than rummage through those thousands of camera images on our hard drives? This phase of our lives is going to last a while, and I have little sense of what I’ll be writing here instead of reports from the field, my tried-and-true resort. I’ll start anyway, with a dozen shots of Oakland’s incomparable defining feature: its backdrop of hills. From near, from far, from every part of town, the Oakland Hills make up our civic stage set.

What they do: The early boosters in the 1800s wrote florid panegyrics about both the beauty of the hills and their salubrious effect on our local climate. They elevate the Pacific winds so that San Francisco’s notorious ground-hugging summer fog never bothers us, and they gather orographic rainfall that keeps the streams flowing. The hills give our topography a concave shape, like a skateboard ramp, that makes every place in the city visible from every other place.

How they look: The hills shift their colors and shadows throughout the day and around the year. Right now they stand out in crisp focus because the air, almost free of traffic pollution and seasonal haze, is so clear. Look at them on a hazy or misty day and they resolve into a set of ridges, arranged in depth. They turn the most prosaic places pretty, and they set off our best buildings to advantage. They peek from behind every photo, a welcome intruder. Without them, this would not be Oakland.

What they are: The Oakland Hills are tectonic hills. They arise along the far side of the Hayward fault because there is a modest amount of compression across the fault in addition to its dominant sideways displacement. They’re not the product of thrust faulting, bulldozed up like the San Gabriels or the Rockies or the Himalaya; not the product of crustal stretching like the basins and ranges of Nevada; not erosional eminences like the Appalachians. They’ve been sort of smeared up. They have a wide variety of rocks in them, arranged in a pre-existing order that isn’t closely related to their shape.

These images are presented from north to south. As usual, click them for the 800-pixel version.

I offer these images not to encourage you to visit the hills (unless you live among them), but to inspire you to hang in there until we can again. I hunger to return to them, but at least I can see them whenever I walk outside.