Archive for the ‘oakland geology views’ Category

Caballo Hills rocks and views

20 July 2015

The Caballo Hills is the neigborhood flanking Merritt College on the Bay side; it consists of Campus Drive and the Ridgemont development. It’s geologically monotonous, underlain entirely by Leona “rhyolite.” And while all the roads have sidewalks, it’s a steep and exposed hike. Nevertheless the area is highly scenic, and I walked it in the course of a few separate visits this spring.

Let’s start with the map. It shows that essentially all the homes here were built on graded lots, removing whatever soil was there.


I guess the original vegetation was scrub like that on 1175 Hill, the high knob in the Leona Canyon Preserve. Now it’s mostly bare rock. Here’s a typical example, a roadcut on the Merritt College campus.


And here’s another farther down Campus Drive.


So seeing interesting things underfoot is not easy as you visit this area. The “rhyolite” is not a stratified rock—you won’t see bedding, or fossils, or sedimentary features. However, its rusty butterscotch color is very even and pervasive. This is caused by the presence of iron oxides, which are steadily being released from the rock, probably from pyrite and other metal sulfides being oxidized in the presence of air and water.

That’s why this bedrock exposure caught my eye at one spot. It features little pockets of intact pyrite that were exposed by the developers’ excavators.


The Leona Heights sulfur mine exploited this same rock unit, and you’ll see similar pyrite pockets in the rocks exposed there, at the end of McDonell Avenue. Outside the mine area, the rest of the “rhyolite” probably weathers slowly enough that sulfuric acid from the pyrite oxidation is neutralized with no problem.

But lift your eyes off the ground and the best part of this neighborhood reveals itself: the views. I surveyed the Ridgemont end on a cloudy day in April. Here’s a look at the northern end of Ridgemont Drive from the parking lot on the western edge of the college.


And this is looking back up from there. It’s beautiful land. Would be nice if Leona Heights Park could take up some of it.


The south end of Ridgemont overlooks the former Leona Quarry and the uppermost catchment of Chimes Creek.


I surveyed Campus Drive a couple weeks later on a sunny day, which made the hike harder but offered better views. This view is to the southeast (it’s 1000 pixels, click and enjoy). The high ridge in the back appears to be The Knife, an eminence overlooking San Ramon about 8 miles away that reaches over 1800 feet and consists of the Briones Formation.


And this view (also 1000 px) looks right over the old Oak Knoll grounds and Knowland Park hill to the 800-foot peak of Fairmont Ridge.


Down at the southern foot of Campus Drive, you get eye-soothing views of Leona Canyon.


This city is just gorgeous.

Lobe 8 of the Fan: Evergreen Cemetery hill

15 June 2015

Cemeteries occupy some of Oakland’s nicest hillsides. Evergreen Cemetery, unlike the others, has a whole hill to itself. The hill consists entirely of sand, silt and gravel laid down in a large alluvial fan (the Fan) back some time in the Pleistocene. Later, stream erosion cut the Fan into pieces, which I have numbered west to east from 1 to 8. This one’s the smallest and, all things considered, the sweetest.


This is the view from 64th Avenue, the hill’s gently sloping side. The NL bus runs past here, too.

Below is the terrain view in google Maps, where I’ve marked it along with lobes 6 and 7.


This is the view from across MacArthur Boulevard at 68th Avenue/Church Street. The hill is pretty secluded, with trees screening it on most sides. When you’re there, your eyes are lifted to distant views, which is fitting. The homes on the far side of 68th probably excavated into the edge of the hill when they were built.


When the weather’s right, you can get a good view of it from Outlook hill, for instance from Delmont Avenue:


or from the overlook at the end of Simson Street. These days the cemetery is doing its part in the drought. Most homeowners are, too.


The geologic map (as seen in this earlier post) shows a little more Fan alluvium to the north and east of the hill, but it’s topographically subdued.

Landslides of Outlook hill

5 March 2015

I’ve been surveying the low hill between Mills College and Holy Redeemer College, home of the Millsmont and Eastmont Hills neighborhoods. Its western face has no bedrock, either on the geologic map or in my experience. Here’s the relevant portion of the geologic map.


Its crest is supposedly Jurassic basalt, which would be part of the Franciscan assemblage. But the Hayward fault runs right along its length, and I lean toward calling it a pressure ridge. Long story short, it is squeezed up, shattered, and oversteepened, and these make it prone to landslides. Here are some, starting with the notable example at the top of 64th Avenue. This is its toe . . .


. . . and this is the view from its head, at Delmont Avenue.


Another is above Outlook Avenue, south of 76th Avenue. As you walk along its base, you’ll see bits of concrete from the homes that once stood here.


Above it, on Hillmont Drive, there is a gap in the houses that offers a nice view. I have no business saying whether a landslide is responsible.


Between these two obvious slides are some fine hillsides. This one, below Simson Street, makes a lovely backdrop to the Eastmont mall and, it seems, a nice informal park for the residents.


It isn’t really vacant—all of the lots that subdivide it are extremely long for some reason. I think that spaces like this, shared without fuss by the landowners around it, are very precious.


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