Archive for the ‘Oakland geology views’ Category

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.

Return to Sugarloaf Hill

13 May 2019

It’s been almost four years since my last visit, and no locality, even the wildest, ever stays the same. Sugarloaf Hill, that iconic bump in the ridges of East Oakland, is one of the city’s wildest places. It helps being part of the Leona Canyon Open Space Reserve, an odd holding of the East Bay Regional Park District away from the usual watershed lands and coastal strips.

Sugarloaf Hill is the highest point underlain by the Leona volcanics. The drainage is sharp enough to discourage trees, and the EBRPD considers it a good example of grassland that still includes a lot of native species. Last week the peak, like most of the hills, was nearing the end of the green season and starting to turn summer gold.

The loose stones on the peak have been moved around since my last visit. Then, they were arranged in a rectangle, like the outline of a small building. Now they’re piled in a cairn that displays them nicely. The same energetic person or people who did that also brought up a chair, which I found very welcome after scrambling around the steep slopes.

This hilltop deserves a real bench, and a decent path to reach it. The existing trail is steep enough to be tricky footing, and the poison oak keeps edging closer on all sides.

On this visit I made a concerted attempt to find another trail to the top, both from the bottom up and from the top down. And there are some faint paths on the lower slopes. One of them led me past this old city benchmark, undoubtedly recorded on some obscure list but not relevant for quite a while.

This wild place did not start that way. Its wildness is not a primordial state or a static climax; it’s a temporary illusion created by depopulation — in Oakland’s case, the depopulation of genocide, followed by its softer sibling gentrification — leading to “parkification” or managed neglect. Untended, the hilltop will become impenetrable chaparral, the most dangerously fire-prone habitat we have.

For centuries, perhaps millennia, this hill was maintained as grassland by its native caretakers. They did controlled burns to do that, and the deer and the antelope helped keep it grazed. When the Franciscan priests of New Spain captured and enslaved the natives, the abandoned land made its way into the hands of the Realty Syndicate. Cattle grazing kept it in a simulacrum of the aboriginal flower fields.

In the 1970s the developers of Caballo Hills sought to divide this rangeland into premium country estates: nine large parcels of 40 to 50 acres. Someone would surely have stuck a private castle up here. The city of Oakland just wanted to start harvesting property taxes instead of a few steers. Instead, after neighborhood opposition, the developers deeded it to the EBRPD and went on to subdivide the ridgetop of Campus Drive into one-acre lots.

Nowadays what threatens the meadows of Sugarloaf Hill is the relentless growth of brush and chaparral. As decades pass, the ground cover rises, alien broom sprouts without hindrance, poison oak burgeons. Footpaths devolve into deer trails or disappear altogether. Eventually the most intrepid hikers give up, until a well-funded crew can reclaim the way. The EBRPD is committed to monitor the plants and animals in the park, so it’s up to that agency.

A rugged jeep trail used to be here, running up from the north end and circling the peak.

Bits of it are still accessible, but most is heavily overgrown. If EBRPD restores the road, the land would be ready for controlled burns again. The hill is a perfect site — isolated on all sides, yet accessible. The park’s planning document envisages controlled burns here, along with fuel reduction and similar half-measures.

Sugarloaf Hill could be a showcase for this deeply traditional land-management technique. For Merritt College students who already study the park, the rejuvenated hill would enhance their educational resource. It would be kept prime habitat for the Alameda whipsnake and other precarious species. And the views would remain fantastic in all directions.

Next, the park district could advance another item in its planning document: bringing back the historic York Trail. The old right-of-way, still visible in Google Maps, runs along the north side of Sugarloaf Hill, then up to Skyline Boulevard near Brandy Rock Way.

It would open a much-needed connection to Anthony Chabot Regional Park over the Parkridge land bridge.

Geology of King Estate Open Space

4 March 2019

After tramping all over Oakland, I still find its landscape full of uncertainty and mystery. The alluring hills of King Estate Open Space Park have brought me here time and again, sometimes to lead walks, sometimes to just stop and smell the flowers. Last month I came back yet again, this time to look harder at it.

The best resource on the park’s history and vision is on the Oak Knoll Neighborhood Improvement Association site at oknia.org. The aspirational Site Plan has the following concept for the park: “The winds sweep my imagination across the horizon. We move over the hills exploring the wilds and oaks embrace us. Here, in this place for everyday we cultivate community.”

King Estate Park is a grassy ridge at the south end of the Millsmont-Eastmont hills, an apparent pressure ridge that stretches along the Hayward fault from Mills College to the Oakland Zoo area, between Seminary Creek and Arroyo Viejo. What drew me here as a geologist was the geologic map (USGS MF-2342) that depicts the area as a peculiar ancient gravel (Qpoaf on the map), the largest piece of this material in Oakland.

The attractive thing is that according to the map key, these deposits “locally contain freshwater mollusks and extinct Pleistocene vertebrate fossils.” The odd thing is that they don’t match another criterion: that this dense gravel “can be related to modern stream courses.”

I explored the portion of the park north of Fontaine Street. Here’s the street map, marked with the three locations I’ll be showing photos of.

And here’s the aerial view, from Google Earth.

And just for fun, here’s the digital elevation model, with buildings and trees removed.

OK! Location 1 is on the steep western slope at the north end of the park, which I climbed. Halfway up is a sizable area of rock rubble consisting entirely of Leona volcanics, the same bedrock shown in pink labeled “Jsv” on the geologic map. The near-outcrop is in the lower left corner of this shot.

And the rock looks like this.

Down at location 2, there’s a spiral labyrinth that people have made in the last few years; I don’t remember seeing it before. But on the assumption that it’s made of stones from the immediate surroundings, I infer that it’s Leona volcanics over here too.

The Leona is generally light-colored with some greenish bits and some red-orange coatings where it’s weathered, hard to describe in detail but distinctive once you’re familiar with it. Once a range of underwater volcanoes that subsequently underwent a lot of alteration, it offers up a variety of intriguing bits that lacking a petrographic lab I can only scratch my head at.

Anyway I’m looking all over, and every bit of gravel on this hill is Leona volcanics. Now a river gravel, which is what the map describes and what I expected (indeed, what I actually perceived on previous visits!), consists of rounded clasts and a variety of rock types from the stream’s catchment. Other gravels in this town are just that way, but not this. The whole time I’m there I’m muttering to myself, “this stuff is colluvium” — raw rock rubble, mixed with soil, that hasn’t moved from its birthplace except maybe in landslides.

In location 3, we have proper bedrock. It show up where the soil has been scraped bare . . .

. . . and farther down the slope in genuine outcrops, which I always cherish.

So in sum, the whole north half of the park, far as I can tell, is either bedrock or colluvium of the same stuff: Leona volcanics. How did it get to be mapped as Pleistocene river gravel? The MF-2342 geologic map was published in 2000. There are two previous serious maps of this area. Dorothy Radbruch mapped it in the 1960s for map GQ-769, and there the area looks like this.

She called it “Qg”, “gravel, sand and clay” and noted that it contains pebbles of Leona Rhyolite (what I call Leona volcanics). She also said this: “Contains molluscs of probable early Pleistocene (Irvingtonian) age.” They were at the locality marked by the triangle with “22133” next to it; that number refers to a “report filed at Washington, D.C.” which is probably gathering dust deep in a back room. She also referred to reports on two boreholes, numbered 95 and 96 on the map, which recorded various kinds of gravel down as deep as 45 feet. That could’ve been deeply weathered Leona as easily as anything else.

But you know what? I’m going to go with Andrew Lawson’s original map of the area from 1914 (Folio 193), in which everything is just straight Leona.

Even though he thought the Leona was very young (hence the name “Tln” meaning Tertiary Leona), he could tell what the ground was saying. At least, he and I agree. I’m sure, though, that he scratched his head as much as Dorothy Radbruch and I did. And they must have enjoyed the view as much as me.

I’ll just have to poke around here some more.