Oakland Geology

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.

This entry was posted on 6 August 2018 at 8:02 am and is filed under Oakland chert, Other topics. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.