The rocks of Mulholland Hill

Over the last couple years, I’ve been more and more tempted by Mulholland Hill, the ridge shared by Moraga and Orinda that dominates its area and shelters the former village of Rheem. Tempted because I crave summits, but also tempted because its rocks, named the Mulholland Formation, are interpreted as the youngest in the region.

The Mulholland Formation is mapped in two shades of light tan on the geologic map, due east of Oakland. It extends from downtown Orinda past Moraga and into the watershed lands to the south; a finger of it (not shown) sticks beyond the rest across Cull Canyon and all the way to Crow Canyon.

Mulholland Hill sits in the northern part. Much of it is preserved as open space, and that’s where I went to see its rocks.

But first, what does it look like? This 2016 view east from the ridge above Wilder Valley shows Mulholland Hill’s level top just in front of Mount Diablo; the grassy ridge dominating the view is another hill that overlooks Lost Valley.

This February 2018 view north from Redwood Ridge shows Mulholland Hill against the horizon left of center, dotted with homes and trees.

And here are two closer views, the first looking southeast from 1204 Hill:

And the second looking northwest from Alta Mesa Drive in Moraga last week.

Here’s a closer look at the geologic map between downtown Orinda at top left and downtown Moraga at bottom right, showing the north half of the Mulholland Formation that underlies Mulholland Hill.

The formation is divided into upper and lower parts (Tmlu and Tmll respectively). Notice how the lower part flanks the upper part on both sides. That’s because the whole thing is folded like a taco, so the older rocks wrap around the younger rocks — a configuration called a syncline. The upper rocks have more sand and gravel in them and resist erosion better than the muddier lower rocks.

The paved trail is the middle part of Donald Road; you can get to the open space on Donald Road from north or south. I came up from the south and recommend that unless you’re in a hurry.

Along the way you may see cattle. Moraga originated as a cattle ranch in the 1840s, so these represent an old tradition. For all I know, Moragans still fill their household freezers with artisanal grass-fed Moraga beef.

Get off the pavement to see bedrock poking through the soil. It’s coarse sandstone with a fair share of pebbles.

Elsewhere it’s full-fledged conglomerate, mostly pebbles that represent a variety of different rock types.

These rocks are interpreted as freshwater deposits, laid down by a vigorous river draining hilly terrain. Nearby exposures of this unit contain horse bones and teeth and plant leaves that fix its age around early Pliocene time, some 5 million years ago.

These rocks are pretty tightly folded. This detail from the geologic map shows the direction and angle that the rock beds dip into the hill. You can see that over a short distance, their orientation changes by roughly 90 degrees. As surely as folding a taco, that would push the central belt of rocks upward. The red line with the arrows indicates the syncline’s axis and sense. (An opposite fold, with the arrows pointing away, would be an anticline.)

But by all means, look around from the top of Mulholland Hill. Depending on the weather and the direction, the vista can be stern, like this view of Round Top and the Oakland Hills,

or grand, like this view of Las Trampas and Rocky Ridges, with Bollinger Canyon between them,

or just splendid.

A fine place to visit. It’s also prime raptor habitat — but if you’re a birdwatcher you probably already know about it. I’m tempted to return.

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Landslide update from the Sports Center fire road

Ten years ago, I took my first walk on the fire road above the North Oakland Regional Sports Center (Caldecott Field), where I saw fit to document an incipient landslide there.

In June, standing on Skyline Boulevard, I noticed that the site was shrouded in black plastic, a surefire sign of a landslide. Passing by again last week, I noticed a change and made the time to visit. The change was that the center of the previous landslide had given way in a new landslide.

This new slide looks ugly, and of course the fire road will need fixing, but the slide material didn’t seem to go far downhill.

The picture seemed pretty simple to me, standing there, but then I came home and looked at the area, in Google Maps 3D, featuring imagery as of a couple years ago. The view is to the south.

Two things to see here. First, the fire road was resurfaced at the place where I shot the 2008 photo (just below the water tank). So the slide shrouded in plastic was likely the second one since 2008. Second, the hillside below the slide is stripped of trees — that is, it’s a landslide chute. Maybe the slide I shot in 2008 did that. Air photos from 1968 and 1939 show nothing distinctive at that location.

If only I’d been paying attention here over the last 10 years! But Oakland’s a big town with a lot to keep track of.

Landslides tend to be persistent: once one starts, others follow in the same place. This is especially true in the rock exposed here, which is mudstone of the Sobrante Formation. Here’s an exposure of it in a roadcut on Thorndale Drive. This stuff falls down real easy. Elsewhere in the hills I’ve called it “punk shale.”

A closeup of another roadcut shows wavy lines caused by shear within the rock.

The Sobrante was a big headache to the people excavating the Caldecott Tunnel bores. It caved in on the men digging the first bores, and the fourth bore required heroic engineering to keep it all shored up so the concrete could be poured. Likewise, houses built on this rock need strong foundations and designs that are sensitive to the site.

Tracing the old Thorn Road

Hiram Thorn took it upon himself in 1853 to build a road over the Coast Range hills from today’s Montclair to his redwood mill, which was either at the present site of Canyon or farther downstream where the former town of Pinehurst once sat. Thorn’s Road was a toll road for a long time, connecting Oakland to the Moraga Valley agricultural hinterland and beyond. “This was the main road into Contra Costa county in the early days,” wrote the Tribune in 1923, “and a daily stage ran over it to Walnut Creek, Danville and the top of Mount Diablo.”

This piece of the 1897 USGS topographic map shows the Thorn Road running from the lower left to the lower right corner.

There are a few things to point out. Kohler Creek is called Temescal Creek today, but back then Temescal Creek went straight uphill from the Lake Temescal reservoir. That streambed was obliterated by the later construction of upper Broadway, Route 24 and the Caldecott Tunnel bores. The dashed line from top to bottom is the county boundary, and the thick dot-dot-dash line running up the canyon along with the road is the boundary between Vicente and Antonio Peralta’s shares of the San Antonio rancho, the immense royal land grant made to their father in 1820. The Thorn Road was also the official line between the Oakland and Brooklyn Townships of Alameda County.

This 1878 map made by Malcolm King shows the landmarks at the time, including the location of the toll gate about where the Thornhill Coffee House stands today.

In the 1880s it was already being referred to as “the old Thorn road.” In 1889 the Tribune reported on the Alameda County Board of Supervisors meeting of 1 April: “The Committee of the Whole, to whom had been referred the petition asking for the placing in good repair of the Thorn road to Moraga valley presented a report saying, that upon examination of the ground, they were satisfied that it would be utterly impossible to ever make the same a good road on account of the steep grade.” They recommended surveying “a road to the summit over a new route and on an easy grade.” That new road was the Snake–Skyline–Pinehurst Road route. Between it, the existing Redwood Road, the Kennedy tunnel to the north and the Oakland Antioch & Eastern railway to the south, which went up Shepherd Canyon and cut through the hills to Eastport, the Thorn Road was no longer the best way over the hills.

Nevertheless, the 1936 street map showed Thorn Road still following its old route up to the Huckleberry saddle.

The 1947 topo map shows that the top segment of the Thorn Road, and all of it on the far side, had been abandoned. The newly named Thornhill Drive took a zigzag route incorporating what had been Idlewild Drive, and the part of the Thorn Road left behind was named Sobrante Road.

Here’s the modern Google map just to give an idea of the streets and terrain.

The Thorn Road took the gentlest way up Thornhill Canyon to the topographic saddle where the entrance to Huckleberry Preserve is today. It was still a very challenging grade near the top, about a 36% grade or 20 degrees, according to my phone compass. (This would rank among the steepest streets in notorious San Francisco.) It was even steeper on the Contra Costa County side.

For a while after the 1947 topo map was published, street maps connected Sobrante all the way to Skyline, but as of 1967 the upper end of Sobrante had been cut off. However, there’s still a right-of-way and a sewer line running down it.

That’s where I took a walk last week. This is looking back at the end of Sobrante and across Thornhill Canyon.

There are remnants of the old grade, but no path bigger than a game trail. I think a footpath should be built here, as an emergency route if nothing else.

Underfoot is Claremont chert, not a surprise because this is right next to, and a hundred feet downhill from, the endangered chert roadcuts of Elverton Drive.

The habitat has possibilities. The ground was wet during my visit, thanks to fog drip. But crews have dumped a bunch of eucalyptus slash, which not only obstructs passage but also presents a fire hazard.

Also prominent in the human litter is a bunch of slash consisting of For Sale signs. There are still lots available up here.

On the far side of the ridge, in Contra Costa County, a stub of the Thorn Road got the name Winding Way. It was known as a shortcut for motorcylists when CHP Captain George Kallemeyn, chasing a group of hotrodders down the road, went over the edge and died in July 1959. Winding Way was still shown as open as of 1967, though it went only a short distance down the canyon.

Some time after that a landslide took out the highest segment of the road, and today the Huckleberry Path edges around the scar. About a hundred yards down the trail, a bench marks the spot where the old roadbed, heavily eroded and overrun in roadcut rubble, picks up again. It’s passable on foot all the way down to the hairpin turn of Pinehurst Road.

I recommend visiting this end of the old road starting down at Pinehurst. You can park beside Pinehurst a little bit downhill from the hairpin, where the old rail tunnel came out. (The cut is still there, filled with rubble and leaking a steady stream of groundwater.) But just as convenient, and more tempting, is the new Wilcox Station staging area, an access point to Sibley Volcanic Preserve’s eastern annex where the Eastport station once stood.

The road starts out along San Leandro Creek, then soon starts to climb.

It’s a steady grade, but the road was never more than one lane wide. As you walk it, imagine the work it took to trailblaze by pickaxe and oxteam. Imagine driving the daily stage to Danville over it. Between raveling ground on the uphill side and landslides on the downhill side, this road, once a vital link in the commerce of the redwood era, is reduced to a precarious trail today.

At any time an earthquake or rainy winter could cut it off, either until repairs can be made or once and for all.

An Elverton update

After a visit five years ago, I had high praise for Elverton Drive: “From end to end, it offers the best exposures anywhere of the Claremont chert.”

This stuff, as seen a few weeks ago during a return visit.

Those of you who’ve followed along know the amazing striped chert of the Claremont Shale, which crops out in a belt from Claremont Canyon along a couple miles of Skyline Ridge to Huckleberry Botanical Preserve and beyond in the hinterland. The fat pale stripes are layers of microcrystalline silica — chert — and the thin dark ones are layers of claystone — shale.

During this visit I walked from the south end of Elverton past the newest set of houses, near Huckleberry, and had a good stop in the old borrow pit. The wall has crumbled a bit since five years ago, opening this fine exposure.

I was hoping to find pieces of dolomite rock, which are present as an uncommon third ingredient, so I gave the rubble a good look. None of that there, but I was interested to see some extra-thick pieces of the chert and shale.

The chert, in fact, was very light. It was barely changed from its original state as diatom ooze on the seafloor, almost the balsa-wood lightness of the Pinole diatomite. I did not expect that.

At the other end of the pit is the same big-ol’ boulder that was lying there in 2013. This is not a decorative rock placed there to look good; no, it fell here from the beetling cliff above and stopped rolling just short of the roadway. I recalled writing in 2013, “if you feel an earthquake while you’re there, step the hell back.”

Every time I visit the high hills, the pleasure of geologizing gives way, sooner or later, to a sense of dread at the state of the roadcuts. The eucalyptus roots in this scene were exposed as the hillside crumbled away, and behind them is a concrete cast meant to slow down a landslide.

But thinking ahead I looked forward to admiring this again after five years away. Google Street View still shows it.

Instead, it’s being shored up and fitted with a shotcrete shroud.

And another splendid exposure farther along is being smothered too, with no finesse.

In fact, not long afterward I started to despair of Elverton Drive. Is this the point of occupying such a spectacular setting? To cover it with property? To look outward and not downward?

The Claremont chert isn’t as solid as it might seem. Given the tendency of these young rocks to crumble, there’s no guarantee a new house in the high hills will survive its first mortgage. Or that the road will last that long.

Look out. Don’t look down. Elverton Drive is falling apart while it’s still filling up.

I already miss the place.