Kind
of a gneiss sorys
By Jack Kinner
Saul Weisberg stood on a piece of land that was
once just a small bench of more or less level ground 300
feet above the Skagit River on the south side of Sourdough
Mountain, a steep piece of bedrock whose summit ridge, about
a mile to the north, is almost a mile straight up from where
he was standing. Hewas addressing a group of people in the
Environmental Learning Center’s (ELC) outdoor amphitheater.
A 910-acre reservoir lapped peacefully at a shoreline visible
through a screen of second-growth firs not far down the hill
from Weisberg, impounded behind the stylish Diablo Dam, erected
in 1929 by Seattle City Light as not just a dam but as one
of the few pieces of art deco bigger than the Chrysler Building.
Before the dam, the Skagit lay at the bottom of a narrow
gorge scoured out in the Pleistocene epoch by periodic surges
of water from overflowing lakes created and then released
by the comings and goings of ice age glaciers.
As the north-flowing
river was blocked, the resulting lake would back up and eventually
flow over the confining bedrock wall to the west, eventually
cutting it deeply enough to permanently reverse the flow
of the Skagit, in the process ending its days as a moderately
sized tributary of the Fraser, parallel to but smaller than
the Pasayten River. When joined by tributaries such as the
Sauk and the Baker, the Skagit became the second largest
river by volume in Washington, after the Columbia.
When it
originally broached the confining western bedrock ridge,
it cut and created the gorge that Highway 20 goes though
between Newhalem and the east end of Gorge Lake. Since its
more direct route to the sea was blocked by the Cordilleran
ice sheet laying in what is now the Puget Trough, a lake
formed between Rockport and Darrington that drained out the
south end to the Stillaguamish. As the Cordilleran sheet
subsided farther north, the Skagit reverted to its normal
course,fed by the Baker River. The Sauk then swung northward
after building a blocking alluvial fan where its outflow
slowed in the low gradient Stillaguamish, and it eventually
rejoined the Skagit, bringing the Suiattle River outflow
along with it.
A stop at Rockport’s Howard Miller Steelhead
Park shows you the confluence of two vigorous rivers, the
Sauk and the Skagit, both of which reversed their flow during
the ice ages. The Gorge, a narrow defile carved out between
two nearly vertical walls of solid metamorphic gneiss bedrock,
proved to be an ideal place for the first of the three hydro
dams Seattle City Light would build on the Skagit under the
leadership of J.D. Ross. In 1924, President Calvin Coolidge
flipped a switch in the White House and the generator son
the gorge dam began sending “juice” to Seattle.
The same metamorphic rocks made possible the dam at Diablo,
when built (1929) the tallest in the world at 384 feet, and
the Ross Dam,completed in 1949 to its current 540 feet. The
primary rock, gneiss,is a laminated and unusually picturesque
metamorphic rock similar to granite, and is one of the chief
constituents of what geologists call the crystalline core
of the North Cascades.
“Gneiss” comes from anold
German word for spark, after the way light reflects off small
embedded pieces of minerals such as mica, quartz or feldspar
(by far the most common). The Diablo Lake overlook, 1.5 miles
east on Highway 20 from the Colonial Creek campground, has
a spectacular exposed wall of gneiss that’s interlaced
with veins of white igneous rock that was still molten when
injected. What you won’t see, not very well at least,
are the 16 buildings that make up the campus, since they’re
hidden behind a natural screen of trees.
The rock that eventually
became the hard metamorphic gneiss arrived from the west
on its own tectonic plate that collided like a runaway barge
against the North American continent about 50 million years
ago, and was pushed 15 miles or more below the surface, down
into molten regions where even fossils can’t survive.
After being cooked like so much raku pottery the rock rose
at about the speed of fingernail growth, later to be polished
by successive glaciers a mile or more thick that stretched
unbroken from the Puget lowlands to Greenland during the
ice ages of the Pleistocene epoch, which in geological time
is like yesterday.
Sourdough Mountain, directly north of
the Environmental Learning Center’s (ELC) campus, is
creased by the Thunder Creek fault,one of the many in the
metamorphic or “crystalline core” that trends
north-south. The fault is revealed on the surface of the
south slope,where the ELC campus is located,by rubble – boulders
of broken bedrock ranging in size from beach ball to small
planet – that formed the bench Weisberg stood on, giving
the overlaying soil such good drainage that one finds small
groves of Ponderosa Pine, normally abundant only after passing
the Cascade crest 20 miles to the east. Much of this information
came from Geology of the North Cascades, by Tabor and Haugerud,
available at the visitor’s center in Sedro Woolley,
Washington. |
ARCHIVES
