It really warms my heart when I can read a good blog posting about geology. There are just a few geologists who blog regularly and it’s not so often that I stumble on an interesting and intelligent post about geology from outside this community. That’s why I really value Doc Searls’ occasional gems. I think Doc is really onto something great with his posts about tagging and annotating photos taken by air travellers.
This morning Doc asked about the geologic setting of a hill off Highway 101 just south of Gilroy, CA. According to the topo maps I consulted it’s called Lomerias Muertas (something died – my Spanish is rusty; Google’s translator is, too) and the southeast end of the uplift (near Hollister) is called the Flint Hills. I don’t personally have any detailed knowledge of the geology there (wrong side of the San Andreas in that region), but looking at the Santa Cruz 1:250,000 geologic map it appears to be constituted primarily of Pliocene marine rocks (probably not real strong stuff) at the northwest end and Pliocene and Pleistocene nonmarine rocks further south. These are all fairly young (geologically) and presumably are only weakly lithified – making them prime candidates for landsliding. It looks like someone teaching geology at Humboldt State likes Doc’s mountain, too – they’re using it as a teaching example in a class on using maps and airphotos to interpret geomorphology. Quite a few useful links there, including USGS orthophotos, DEMs and oblique airphotos (old B&W ones from CDMG Bulletin 158 on the Evolution of the California Landscape). I’ve also taken the initiative to overlay a portion of the geologic map (not real high resolution) in Google Earth – use the transparency slider bar to get the best effect.
I think word Doc was describing (“the kind of hill that gets squeezed upwards like a mudpie between the uneven walls of a moving fault”) is technically called a “pop-up” (John McPhee loves that sort of geological technical terminology), though I’m not sure whether it applies to this particular hill, strictly speaking.
Boy is it fun to have a conversation about California geology!
Later (Monday)… Doc asks:
Looking at that overlay map, I’m guessing (without the key) that Pml is Pliocene marine layer, or someting like that. I’m just wondering why the obvious slumps and slides here aren’t Qls, the customary label for Quarternary landslide.
Sorry for leaving off the key – the most relevant part is there among the Humboldt State links. My best guess is that the lack of mapped Qls has to do with the scale of the map. The 1:250,000 geologic map isn’t likely to show detail at the resolution of a single landslide (as seen in the aerial photos) – it’s blown up considerably when you view it in the overlay. A more detailed geologic map (if one exists – I don’t own it) would indeed likely show the landslides as Qls.
Upon further review… If you’re interested in more info on the landslides on this particular hill, Doc, you’ll want to stop at the Stanford University geology library and take a look at an unpublished Ph.D. thesis entitled “Slope stability of the Lomerias Muertas area, San Benito County, California” by Deane Oberste-Lehn (1977), 231 p. (My recollection is that Stanford doesn’t do interlibrary loans of its theses.)