Geology Walk
Cory
Golden with the Davis Enterprise attended the event and the
following article (Click
here to download as a PDF: Geology Hike Story) appeared
in the Sunday Sun-up section of the Enterprise on June 13, 2004.
If
you have suggestions for future hikes, would be interested in
leading a hike or helping with planning please contact Dawn
Lindstrom at (530) 757-2776 or coordinator@putahcreekcouncil.org
Hike
highlights changes fast --and ever-so-slow
By Cory Golden/Enterprise staff writer
If
Putah Creek is the unexplored back yard, then the geology of
the Central Valley is a mystery novel left unread by all but
a few. "What's
underneath our feet -- and why's it's so flat -- is actually
just as interesting (as the mountains)," Eldridge Moores,
UC Davis professor emeritus of geology, said recently.
Moores -- recognized nationally both as a booster of geography
education and as Pulitzer Prize-winner John McPhee's guide for
the book "Assembling California" -- likes to call
geology not a mystery novel, but a "historical archive
you can read if you learn the language."
That language is geology.
And on a recent, warmer-than-average Saturday in April, he and
Robert Matthews, also a UCD professor emeritus of geology, joined
about 45 people for a 3-mile hike along the south side of Putah
Creek, between Winters and Lake Berryessa in Solano County.
The Putah Creek Council sponsored the daylong event, which covered
a range of subjects, from everyday uses of the creek to ongoing
restoration to a long-view of the area in creeping geologic
time that can be hard to fathom.
Dressed
in khaki and using grass as a pointer, the bearded Moores explained
that the landscape before them was there for a reason -- geology
and geologic activity, that it had formed over millions of years
and that, right there in the sunlight, they were watching it
form and change at the rate it always had.
Putah Creek, he told the hikers, predated the uplift of the
Coastal Range, which began 3 million years ago; the uplift continues
today at a rate of one or two millimeters per year, as the Pacific
and North American plates rub as they slide past one another.
The Coastal Range is also moving slowly east.
Putah Creek has been able to continue cutting through the rock
fast enough to keep flowing.
Geologists believe the reason the valley is so flat is because
it's a big slab of oceanic crust and dense mantle sitting on
the edge of the continental plate, Moores explained. The valley
rests on a microplate, one that's "stuck" -- or relatively
stuck: moving northwestward with the Pacific plate, but even
more slowly.
Moores surveyed the green landscape behind him, seemingly soft,
rolling hills that in truth cover rock by a foot or two:
"If we look off to the slopes off to the north of us, what
we see here is Great Valley sediments and these are steeply
tilted. They're tilted at about 80 degrees or so ... These rocks
in here are about 80-90 million years old," he said. "As
you get into the valley, there are some rocks in there that
are about 50 million years old, so it's younger. And then up
on top (of the ridge) there's a black blob, that's actually
an outcrop or close to an outcrop, of some basalt, it's a volcanic
rock that's about 15 million years old.
"Above that you can see some lines up on the hillside that
are resistant layers which are tuffs, these are volcanic deposits
you can see on the road cuts down over and here and up Pleasants
Valley Road ... those are about 1.8 million years old. That's
the Putah Tuff, an air-fall deposit that was formed probably
by some eruption near the north end of Napa Valley, although
the eruptive center has never been found."
Moores told the group it was important to try to understand
the time such movement takes. He suggested the hikers think
of one year as one millimeter.
"All of recorded human history is within the last 10,000
years. Ten thousand years is 10,000 millimeters, that's 10 meters,
that's from here down to those logs...," he said. "If
you continue on eastward counting off your millimeters, get
on Interstate 80, the dinosaurs wipe out at about Sacramento.
"In order to get all the way to the origin of the earth...,
from here you've got to go to Boston."
The surface of the landscape, Matthews noted, can change all
too quickly.
He talked to the hikers about slides at the site of one of several
major ones in the area in January 1998, when Highway 128 was
closed in several places.
The coastal range, Matthew said, is home to in all shapes, sizes
and speeds of slides, from slow mud flows to boulders bearing
down the like Greyhound buses with the breaks out.
He talked about how cattle grazing can exacerbate the problem,
and how little expensive engineering can do to stop slides.
In 1906, Matthew said, Cache Creek was dammed by a mudslide
before breaking through. To date, about 100 slides have been
mapped along that waterway, along with two active fault lines.
Said Moores, "The moral of the story is, don't put a dam
there."
The land where the geology hike took place is part of three
parcels totaling 526 acres along Highway 128. It has been partially
owned since 1989 by Tom Cahill, a UCD professor emeritus of
physics and atmospheric science and research professor in engineering,
and his wife, Ginny, a deputy attorney general who also teaches
environmental and water law at UCD.
The hike began just off the Cahill property at Four Winds Nursery,
where John and Mary Helen Seeger grow 50 varieties of dwarf
citrus trees and ship them off to grow on patios across the
country.
John Seeger walked the hikers through the process, detailing
how weed seeds are filtered out from creek water and the water
treated with ozone to effectively sterilize it, eliminating
the need to use fungicides, and how a vintage drip system helps
the growers minimize the amount of water they use.
An enthusiastic host, Cahill told the story of how thanks to
taking out another mortgage on their Davis home and the bumbling
of a marijuana farmer who lit a blaze that ended up damaging
part of the land, reducing the asking price, Cahill, his wife
and a friend could afford the land.
"It looked terrible," he said.
Their goal since has been to restore it, as best they can, to
its natural state. By sparing the ground the cattle grazing
that had ravaged it, already the land is surging back to its
natural state.
Cahill stood on a grassy spot, dotted by lacy blue larkspur
and dozens of varieties of other, smaller flowers, and talked
about the "surprising and delightful" return of purple
needle grass and other native species to a spot burned in a
fire last year.
He pointed to an area where two weeks earlier he founded matted
grass and blood; there, he felt certain, a mountain lion had
leapt down upon its dinner.
The land, which climbs steeply about 1,000 feet in places, is
also home to more than 35 species each of birds and butterfly,
from hawks to western bluebirds to turkeys, fox and some healthy-looking
coyotes, he said. Blue oak trees and manzanita give way to gray
pine, valley oaks, black walnuts. Red buds have come back, reaching
up through poison oak.
Cahill lamented poachers and trespassers with off-road vehicles,
then chatted about how the creek reached 8,700 cubic feet per
second over the winter, compared the 1,500 cubic feet per second
of the American River in summer, wiping out an island of willows
and scrubbing rocks clean of vegetation.
"The creek constantly reworks itself like that," he
said.
The Cahills have opened the land up to researchers from UCD,
ranging from botanists to biologists, and small groups of campers
and hikers.
On the day of the Putah Creek Council's hike, he relaxed with
his visitors as they chewed their lunches at creekside.
"That's what it makes it so fun for us," Tom Cahill
said. "I just love sharing (the land). I'm sorry it was
under-appreciated before, but I like taking people to the land
that appreciate it now."
Cahill said he planned to see to it the land is never developed
and will always remain open to the public in some way. He imagines
property owners someday stitching together hiking trails stretching
east all the way to the Vic Fazio Yolo Wildlife Area.
"It's an amazing treasure we have here -- and it's just
sort of a hidden one," he said.
At one point along the trail, Cahill stopped at what, at first
glance, appeared to be ordinary puddles along the trail.
In fact, they were salt springs, vaguely smelly spots of brackish
water that can be found up and down the coastal ranges. The
puddles are often an indicator of fault lines, though Moores
said no thorough geologic examination of that reach of Putah
Creek has been done.
"They're actually remnant marine waters that were incorporated
in the sediment at the time of the formation..," Moores
said. "This water stayed in the sediments and it's being
squeezed out right now, in part because of the tectonic activity
taking place."
Salt water, oozing to the surface -- a sign of the intense pressure
below our feet, another sign that the plates of the earth are
still moving on, as slowly as fingernails grow.
"You don't not want to step in this mud," Cahill warned
the hikers, "your shoes will never be the same."
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