14 September 2015

Benchmarking Time: California Collection

Posted by Jessica Ball

I’ve been neglecting this series, but I didn’t stop “collecting” benchmarks when I moved to California. In fact, working at the USGS makes it really easy to find markers, because there are at least three on campus.

Number one...

Number one…

...two...

…two…

...and three.

…and three.

I won’t give away the locations (you’ll just have to look for them if you come to campus!), but the dates tell a bit of a story. The Menlo Park campus was originally part of a WWII army hospital that was decommissioned in 1947, and over the years various buildings have been added. The 1956 benchmark is the oldest I could find (it’s near the building I work in, which looks like it was probably built around then). As you can see by the elevation on the oldest of the benchmarks, we’re only 53 feet above sea level – unsurprising, since we’re three miles from the Bay.

If you’re just dying to know where to find these kinds of benchmarks, the National Geodetic Survey has an interactive map you can use to look them up. (I didn’t realize that it’s usually the NGS that puts benchmarks in place, not the USGS – but geologists sure make good use of them!)

Here are a couple from King’s Canyon in the Sierras (unfortunately, like much of the state, the park is currently on fire and not the best place to visit at the moment):

King's Canyon near caves

King’s Canyon near caves

This first one is near Boyden Cavern, in an area where narrow bands of marble sit in the granite of the Sierra Nevada batholith. They’re called roof pendants, and they’re made from the metamorphosed remains of Paleozoic and Mesozoic rocks (including limestones) that came in contact with the magma chamber that eventually formed the batholith. This particular marble is called (surprise) the Boyden Cave Roof Pendant. The marble really stands out when you drive down the canyon between Sequoia and King’s Canyon National Parks:

Marble "roof pendant" in King's Canyon

Marble “roof pendant” in King’s Canyon (the bluish-grey rocks in the background). The marble weathers differently than the granite (to the right) and tends to form sharper features.

Another great place to visit in King’s Canyon is Mist Falls, which is firmly in the granitic part of the bedrock. There’s at least one benchmark:

King's canyon near waterfall

King’s canyon near waterfall

And, of course, a waterfall (or two):

Mist falls in King's Canyon

Mist falls in King’s Canyon

Earlier in the summer, before everything started to catch on fire, I joined the William & Mary field geology course to talk volcanoes in Long Valley Caldera. One of my favorite stops was at Lookout Mountain, where we talked with Stuart Wilkinson, part of the USGS’s California Volcano Observatory, about the seismic network in the caldera. In between checking out landmarks I snapped a quick photo of a much used-and-abused survey marker:

Long Valley Stop with Stuart

Lookout Mountain in the Long Valley caldera

But mostly I just enjoyed the view:

Stuart talking seismology with the W&M crowd

Stuart talking seismology with the W&M crowd

I think this month I’ll have to catch up with my summer trips – I still haven’t blogged about King’s Canyon, Lassen or Yosemite. (I’ll have to break up all that granite with California’s most recent volcanic eruption and a few hydrothermal features!)