Archive for the ‘the Fan’ 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.

Anomalies of Sausal Creek: Dimond Canyon

14 October 2019

This is the second of four posts about the peculiarities of Sausal Creek, going from its headwaters to the Bay. Here I’ll address Dimond Canyon, the 2-kilometer segment between the Warren Freeway and the flats of Dimond Park. The steep walls of the canyon, which is several hundred feet deep, are entirely hard sandstone of the Franciscan Complex, part of the Piedmont block.

This is the same stone quarried for decades in Rockridge (the Bilger quarry) and the land that would later become Piedmont (the Blair quarries and the Davie Stadium quarry). In fact the Diamond Cañon Quarry was one of two here in the canyon. It’s now occupied by the Zion Lutheran Church, as seen here from across the canyon.

The quarry scar appears on this terrain map as a big round nick in the canyon wall next to Park Boulevard.

A while ago in this space I described Dimond Canyon as a classic water gap — a stream-cut gorge crossing a bedrock ridge that otherwise seems impenetrable.

Geology textbooks will tell you there are two ways for streams to make a water gap. In the first way, the stream was there first (an antecedent stream) and a ridge of resistant rocks rose up around it. In dynamic California, this is a straightforward explanation of our water gaps. In the second, the ridge was there first, inherent in ancient deformed rocks buried under younger strata, and the stream (a superposed stream) cut down to, then into it while stripping off the overlying material. That’s how they explain the Delaware Water Gap and other examples in the gentle Appalachians.

Dimond Canyon is actually a semi-classic water gap. Yes, the ridge it crosses must have risen while the stream was cutting down, but the story is complicated by the fact that the watershed upstream lies across the Hayward fault, and is constantly being moved to the right. This means the canyon has hosted streams from several different watersheds over the past million years or so.

Therefore the streams feeding Sausal Creek today could not have dug the canyon; some predecessor watershed did it. There must have been gaps and surges in the water (and sediments) flowing through this canyon. If we ran things backward a million years, what would it show? The exercise would be blurred by serious uncertainties, but the matter is not beyond all conjecture.

I beg your indulgence as I present some slides from my talk to the Friends of Sausal Creek last month. They’re Google Earth views looking west across the fault. Here’s today, with the fault trace shown in red.

The view may be a bit confusing as I rewind the motion on the fault at about 10 millimeters per year. The far side looks the same because we’re focusing on it while it moves leftward, toward San Leandro. For a long time, Sausal Creek has been carried past small watersheds that, like today’s, could not possibly have carved Dimond Canyon. But about a million years ago, Dimond Canyon would have lined up with the watershed of Arroyo Viejo.

This looks promising because the watershed (the part above the fault) is about twice the size of Sausal Creek’s, giving it roughly twice as much water and cutting power to match.

But to make the canyon, you have to have something pushing up the ridge while the stream across it keeps cutting its way down. There’s nothing obvious that would have been pushing up the bedrock ridge at this time.

Going back a bit further, though, we line up with the great big watershed of San Leandro Creek, a dozen times larger. This stream has plenty of cutting power, evident in the canyon it’s dug where the dam and reservoir sit.

And finally, we have a mechanism here for uplifting the ridge that Dimond Canyon cuts across. The hills of San Leandro consist of a large slab of gabbro so big and strong that it deflects the Hayward fault slightly. Back when the sandstone of Dimond Canyon was grinding past the gabbro of San Leandro, the jostling between these two bodies of rock, caught in a vice by the geometry of the fault (a restraining bend), would have pushed both sides upward because that’s the only way out of the vice. And all the while San Leandro Creek would have been cutting a nice deep water gap as that hard rock rose.

Eventually, inevitably, the fault carried the water gap out of reach, and ever since then Dimond Canyon has housed lesser creeks for episodes of a few hundred thousand years. Sausal Creek trickles down the canyon today not doing much to it, the shrunken tenant of a structure built by a mightier maker.

This story (and that’s all it is really) appeals to me because it would also explain the presence of the Fan — the swath of gold on the geologic map representing Pleistocene sediment.

I’ve always regarded it as a fossil alluvial fan because of its shape on the map, but maybe that’s accidental. Maybe it’s just a chunk of old East Bay land that was lifted along with the Piedmont block, or washed off of it afterward.

I first posted about the problem of Dimond Canyon more than 10 years ago. Takes a while to figure out some things.

Land and rocks of westernmost Piedmont

2 April 2018

The western end of Piedmont includes the headwaters of Pleasant Valley Creek, which is tucked under Grand Avenue. But the slopes and gullies of the valley can’t be hidden, and what may seem like a scramble of streets is a nice place to walk around. The topography I’m talking about is north of Oakland Avenue, between Grand and the former quarry of Dracena Park.

None of it’s bedrock; nothing wrong with that. It’s the uppermost part of lobe 3 of the Fan, that big arc of ancient alluvium hills across central Oakland. The bedrock part, farther uphill, is much steeper walking, and the rock (the blue field labeled “Kfn” on the geologic map) doesn’t show itself either.

I should make an exception to that statement — Dracena Park is a great place to see the bedrock. But for the purposes of this post it’s just a pretty place, either in the former main pit:

or in the valley on the north side, now full of redwoods.

This is the most dramatic bit of stream valley in the area. Elsewhere, it’s easy to trace the drainage lines; in fact during our recent batch of rain, you could see water coming out of the ground just like in the old days before people lived up here.

This slopy bit of suburbia is criss-crossed with hidden history. The Key System streetcars used to serve the neighborhood. Indeed, a railroad syndicate once planned a major route through here that was going to run through upper Fruitvale and on to San Jose. The right-of-way appears in this 1927 map (courtesy of the Oakland Library History Room), long after the plans were abandoned.

The part of it running as far as Oakland Avenue did end up with rails as part of the Key System’s C line. If you look closely at the area in Google Maps, the lot lines give the route away.

The transit routes, road and rail, took advantage of the saddle in the ridge of lobe 3, at this spot where Pleasant Valley Road eases over the hill and becomes Grand Avenue. The view is west across the saddle toward Rose Street.

OK, enough of that. There are indeed rocks in this neighborhood. See that clever stone wall in the foreground in the previous shot? Here’s a closeup.

Other rocks are more laid back, understated but cool.

Still others, the only word for them is homey.

They’re all mostly landscapers’ stone, purchased at a commercial yard. But here and there you’ll see genuine domestic Oakland rocks. This little wall is made of serpentine/blueschist from around here, probably the pit at Serpentine Prairie or nearby.

And the Franciscan red chert is very likely from Piedmont itself, which sits on a hill of melange, a marble-cake rock unit that mixes chert, basalt, sandstone and serpentine in a mudstone matrix.

Some of it can be downright psychedelic. Stuff like this was quarried in Piedmont before any buildings were here at all.

Even the sidewalks, the oldest ones anyway, incorporated aggregate from Piedmont’s original quarries.

The town has grown up since then. It puts on a good front — very nice homes, lovely grounds, a fine place for walking and taking in the views (especially when the leaves are down in winter). But it began as a rugged, dusty mining district with horses and dynamite.

Tour of the Fan: Lobe 3

22 January 2018

This post takes a look at lobe 3 of the Fan. To orient you as we start, here’s where lobe 3 fits in the bigger picture on the geologic map.

This is the best time of year to walk around the Fan, in the Oakland midlands. When the trees are bare you can see farther and better — the landscape’s bones stand out, and the homes do too. So get yourself on out!

This lobe is more difficult to explore than the others. One problem is that the trees are larger and the landscaping thicker, which makes it hard to get a good view around you. Another is that the streets are laid out in a distributary fashion — that is, you aren’t expected to drive through it except in a couple of places. You’re supposed to drive into it, to your single-family home or small apartment building, and out of it, to work or shop. So getting around in any other direction is not straightforward.

Maybe this will be clearer on the geologic map. For convenience, I’ve divided the lobe into four sublobes.

I tend to analyze landforms from their ridgetops and divides and saddles, features on top of the land. But lobe 3 may be easier to comprehend as a set of stream valleys dissecting a highland, as shown in this watershed map from the county flood control district.

As with the other parts of the Fan, the edges of lobe 3 are pretty clear-cut. This is typical of eroded landscapes in arid country, which is interesting because Oakland isn’t especially arid these days. It was during glacial times, though. Here’s the north edge at the south end of Ramona Avenue, just west of Mountain View Cemetery in the valley of Glen Echo Creek.

This view across the Glen Echo Creek valley up Monte Vista Avenue gives a better idea of the height of this lobe — about 80 feet here, but it seems like much more.

At its westernmost edge, the lobe starts right where the Whole Foods store stands. Everything in front of the far hills is part of the Fan.

This shot from the toe of the Calmar sublobe shows why the Fan has always been land to make homebuilders salivate: fantastic views of the city, the Bay and the Golden Gate. Adams Point fills the middle ground.

And here’s a final view across the edge of the lobe, this time as seen from the freeway overpass over the homes in Trestle Glen. Longridge Road along the top gives its name to this sublobe.

The Adams Point sublobe is larger, but a little lower than the others, reaching about 160 feet elevation. Its highest ridge, along Fairmount, Kingston and Rose Streets, offers good views north toward lobe 2.

Points on the other side, like Jean Street and the knob of Nace Avenue, overlook nice vistas south and east. Or Cambridge Avenue.

The Warfield sublobe is a highlight of geology ramble 4. Whereas the Adams Point sublobe has saddles where three roads cross it (Santa Clara, Linda and Grand Avenues), you have to hump Warfield Ridge over Mandana Boulevard, as here:

or go up the Bushy Dell Creek valley on Wildwood Avenue and over to Winsor Avenue. In between it’s a bit intimidating, as seen from the foot of Jean Street.

The Calmar and Longridge sublobes are the highest, reaching well over 200 feet elevation at their upper ends. Longridge is particularly hard on landscape photographers because there are so many mature trees and big homes. (If only it were 100 years ago, when this whole tract was bare hills.) But I like this shot from Long Ridge. On the right edge it shows the red-brick Grandview Apartments building (Warfield sublobe), the Spanish Revival apartment complex on Crescent Street (Adams Point sublobe), and behind them the big palm tree at the top of MacArthur by the Chetwood overcrossing. A little bit of Elwood Street is visible to their left.

But if finding big views is frustrating, it’s easy to find cute little pocket valleys all over lobe 3. The only one I’ll point to here is the one with the Morcom Rose Garden in it.

Here’s where all twelve photos were taken.