Geology Reference
In-Depth Information
packed cliffs flank the western shore of Chesapeake Bay. Walking along the narrow strip
of sand between land and water, one can find an abundance of extinct clams and spiraled
snails, corals, and sand dollars. Once in a while on a very lucky day, a six-inch serrated
shark tooth or six-foot streamlined whale skull turns up. These prized relics tell of a time
fifteen million years ago, when the region was warmer and more tropical, like today's
Maui, and majestic whales came to calve and monster sharks sixty feet long feasted on the
weak. The fossils populate three hundred vertical feet of sediments, laid down over three
million years of Earth history. The layers of sand and marl dip ever so gently to the south,
so walking along the beach is like a stroll through time. Each stride to the north exposes
slightly older layers.
To get a sense of the scale of Earth history, imagine walking back in time, a hundred
years per step—every pace equal to more than three human generations. A mile takes you
175,000 years into the past. The twenty miles of Chesapeake cliffs, a hard day's walk to
be sure, correspond to more than 3 million years. But to make even a small dent in Earth
history,youwouldhavetokeepwalkingatthatrateformanyweeks.Twentydaysofeffort
attwentymilesadayandahundredyearsperstepwouldtakeyouback70millionyears,to
just before the mass death of the dinosaurs. Five months of twenty-mile walks would cor-
respond to more than 530 million years, the time of the Cambrian “explosion”—the near-
simultaneous emergence of myriad hard-shelled animals. But at a hundred years per foot-
step, you'd have to walk for almost
three years
to reach the dawn of life, and almost
four
years
to arrive at Earth's beginnings.
How can we be sure? Earth scientists have developed numerous lines of evidence that
point to an incredibly old Earth—to the reality of deep time. The simplest evidence lies in
geological phenomena that produce annual layerings of material; count the layers, count
the years. The most dramatic such geological calendars are varve deposits, thin alternating
light and dark layers that represent coarser-grained spring sediments and finer winter sedi-
ments, respectively. One meticulously documented sequence from glacial lakes in Sweden
records 13,527 years of layering, with a new light-dark layer deposited every year. The
finely laminated Green River Shale, which is exposed in scenic steep-walled canyons in
Wyoming, features continuous vertical sections with more than a million annual layers.
Similarly,icedrillcoresthousandsoffeetdeepfromAntarcticaandGreenlandrevealmore
than eight hundred thousand years of accumulation, year by year, snow layer by snow lay-
er. All of these layerings rest atop vastly older rocks.
Measurements ofslowergeological processesstretchthetimescale ofEarthhistoryeven
further. The massive Hawaiian Islands required slow and steady volcanic activity, as suc-
cessive lava blankets piled up—at least tens of millions of years, based on modern rates
of eruption. The Appalachians and other ancient rounded mountain ranges were formed by
hundreds of millions of years of gradual erosion, and the barely detectable motions of tec-
