Geology Reference
In-Depth Information
7
Catastrophic Revelations
B EFORE THE EARLY NINETEENTH CENTURY , natural philosophers paid little attention to deposits
of loose gravel, sand, and boulders lying above solid rock. But northern Europe's geological
blanket of unconsolidated material became far more interesting once it was thought that the
part of earth history that overlapped with human history was preserved in surficial sediments
rather than in the solid rock below. It helped that geology arose as a science in countries that
had been glaciated, where a regional cover of glacial deposits—gravel, sand, boulders, and
mud—resembled what you might expect a big flood to leave behind. These surface depos-
its and topography, the form of the land itself, became the link between the modern world
people knew and the former worlds preserved in the rocky depths of geological time.
I came to appreciate the potential for catastrophic rearrangement of surficial deposits in
the Philippines. At the time, I was doing fieldwork in the Pasig-Potrero River, where one of
my graduate students was studying changes after the catastrophic 1991 eruption blew the top
off Mount Pinatubo and buried the surrounding countryside under hot pumice and ash. The
whole landscape around the volcano changed, as river valleys filled in with sediment only to
have great canyons cut back down hundreds of feet into the loose debris in just a couple of
years. We saw the Passig-Potrero River as an ideal place to study how rivers behaved when
supplied with as much sediment as they could carry.
On a beautiful tropical morning, we started out from Delta 5, an abandoned military
checkpoint perched on a rock outcrop sticking up from the riverbank. We headed upstream,
leaving the coastal plains to enter the volcanic upland. Walking up the riverbed, we surveyed
it in three-hundred-foot sections. One person would stay behind, sighting through a tripod-
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