Geoscience Reference
In-Depth Information
thickness of their growth rings rel ecting the temperature and precipitation
of these past periods. Small lakes dot the landscape, accumulating sediments
eroded from the surrounding watershed and windblown pollen that provides
snapshots of changing vegetation over time.
Soon the majestic granite peaks of the Sierra Nevada come into view,
carved and polished by glaciers during the previous ice ages into stark peaks
and valleys, glistening in the sunlight. h en we make an abrupt descent down
the steep eastern slope of the Sierra Nevada, where moraines—elongated
accumulations of sediment eroded and transported by glaciers—were let
behind as the ice age glaciers melted. h ese moraines mark the entrance into
the drier Great Basin, with its patchwork of brown shades of earth, sparsely
dotted with drought-tolerant shrubs. Here, we can see lakes encircled by
what appear to be bathtub rings but are in fact ancient shorelines let in the
landscape that attest to earlier, wetter periods. Finally, our transect takes us
across a series of block-like mountains separated by deserts, together forming
the Basin and Range Province that stretches across Nevada and into Utah.
Lakes, and the remnants of ancient lakes, dot the desert landscape. h e lakes
have grown and shrunk over the millennia in response to dramatic climate
l uctuations.
h is l ight over the western landscape from the coast of California to
Utah has shown us many clues about the history of the region's climate. h e
environmental archives tell us that the prehistoric climate was not always as
benign as what we have experienced in the past century and a half. h ey paint
a picture of past droughts much larger and longer than any we have recorded
in recent history, followed by enormous l oods and extended wetter periods.
We will return to these archives of western climate—how they were dis-
covered and interpreted, and what they tell us—in the next few chapters of
this topic. First, however, we need to sharpen our forensic skills in i nding
and understanding the evidence.
the climate backdrop
For nearly two million years, the earth's climate has swung like a pendulum
between two primary climate states: glacial (ice age) and interglacial. h is
period is known as the Pleistocene, and it includes the previous thirty glacial
cycles. h e pendulum has not kept a steady rhythm, however. h e glacial
periods have lasted much longer than the interglacials over the past million
