Geoscience Reference
In-Depth Information
West contains a wealth of evidence for past climates in some of the most
unexpected places. h e remarkable physiographic diversity of this region is
evident in a span of just a few hundred miles. We can illustrate much of this
diversity in an imaginary l ight from the Pacii c coast near San Francisco to
the east, across California and into Nevada and Utah. In this relatively short
distance, the landforms, plants, and animals change markedly, rel ecting
large dif erences in temperature, precipitation, elevation, and topography.
We begin outside the Golden Gate on the rocky coastline of the Pacii c
Ocean, including its magnii cent kelp forests and tide pools. Beneath these
dark blue and seaweed-green waters, sediments are slowly sinking to the sea
l oor, forming aggregates in the water column called “marine snow” that
eventually settles on the seabed. h e sediments, building up over the millen-
nia, are composed of silt washed in from the land, tiny shells of plankton, and
the remains of other marine life that live in the coastal waters. h e shells of
these tiny organisms hold information about past ambient ocean conditions,
like temperature and upwelling.
Our l ight moves inland over the San Francisco Bay, an estuary that receives
freshwater and sediment draining nearly half the state of California. h e organ-
isms living in the bay record information about the salinity and temperature
of these waters, which in turn rel ect the amount of river water that l ows into
the bay. h e shores of the bay are lined with lush tidal marshes, criss-crossed
by sloughs. Continuing east but still within the sprawling estuary, the water
becomes fresher and the wetland vegetation grows taller, such that a careless
boater could get lost in the winding sloughs that snake through the ten-foot-tall
tule bulrushes. h ese marshlands have been growing for thousands of years,
building up layers of organic peat as the level of the sea has slowly risen. h e
mix of marsh plants changes over time as rainfall and runof into the bay alter
salinity, and the marsh layers contain evidence of past droughts and l oods.
From the wetlands of the Sacramento Delta and its hundreds of islands,
we cross the great Central Valley, where rows of irrigated crops spread out
for miles on the l at valley l oor as giant rectangles of green that contrast
strikingly with the brown rolling hills surrounding the valley. Beneath the
ploughed i elds lie ancient accumulations of sediment that were eroded from
the Sierra Nevada range to the east. h
ese sediments contain layers deposited
during ancient l ood events.
Chaparral-covered foothills of the Sierra Nevada come into view next and,
continuing upslope, forests of pine, juniper, i r, Sequoia, and cedar. h e larg-
est of these trees have been growing for hundreds to thousands of years, the
