Montgomery Ridge

St. Mary’s Cemetery, north of Mountain View Cemetery, is on a ridge that runs toward the bay and peters out at the Kaiser hospital on Macarthur and Broadway. The ridge is on bedrock at the high end and changes to old alluvial fan sediment just past Pleasant Valley Boulevard. I call it Montgomery Ridge because Montgomery Street runs approximately up its crest. My yard lies on the edge of this ridge down near its end. I find these Franciscan chert cobbles scattered thinly in the dirt, and I’ve been putting them aside. They are rough, but not jagged, so I take them to be natural, in-situ alluvium rather than fill or crushed rock. That’s where things stood until the other week, when I found a cutbank on upper Howe Street dug into the ridge, and the same chert was tumbling out of the hillside from a layer just beneath the topsoil. Walking down Montgomery, I saw more chert chunks in the soil by the road at the corner of John Street. My favorite pieces are the greenish ones, like this one by the side of upper Howe Street.

This chert comes from the Piedmont block, but the geography is different today. Today, streams have incised the old fan and they’re too feeble to carry this kind of material. I picture much drier conditions, and flash floods strewing the chert across the surface of the ancient fan. The next thing is to see where else it occurs. Let me know if you find it in your neighborhood.

Sausal Creek in flood

With the heaviest rains I’ve seen in years, I checked out Dimond Canyon today to assess the power of the stream in it, Sausal Creek. The water was brown and impressive. It looked about waist-deep at most. I don’t know how this stream cut the canyon, which is a gorge more than 50 meters deep with stone walls. But I have a theory involving stream capture and movement on the Hayward fault, just upstream from the gorge. At various times, the fault has pulled the canyon past different watersheds. Perhaps lakes lay upstream, or landslides formed dams, that collected enough water to give the canyon a good downcutting once in a while. I hypothesize that the stream’s watershed was once quite a bit larger, perhaps even the valley now occupied by Chabot Reservoir. But the timing has to work.

There are at least two other gorges in the Oakland foothills that appear oversized to me: the upper reaches of Cemetery Creek, along Moraga Road, and the canyon of Peralta Creek in Redwood Heights, best seen from Rettig Avenue north of 35th Avenue.

It is recorded that the early loggers who stripped the redwoods out of the Oakland hills used to float their logs down Sausal Creek to the bay. All I can say is, there must have been a lot more water in the hills back in the 1850s, because even today’s deluge couldn’t have done that.

BTW see the Friends of Sausal Creek site.