Travel Reference
In-Depth Information
clues to ancient sedimentary environments. The sedimentary rocks forming
in a deep ocean look quite different from desert deposits, because they have
accumulated in much quieter, lower-energy environments and consequently
show different grain sizes and sedimentary structure than do desert deposits.
Any fossil types present are also important clues to reconstructing the envi-
ronments in which a particular sedimentary rock accumulates. Organisms
are restricted to very specific types of environments and, when preserved as
fossils, are among the best of all sources of information about what the
"place" was like where a given assemblage of sedimentary rocks accumulated.
When all of these types of evidence are considered together, they can yield a
detailed understanding of an ancient environment.
The sedimentary rocks on Sucia Island, for example, can be analyzed in
this way, by combining the evidence gleaned from grain size, sedimentary
structures, and the enclosed fossils. To make such an analysis, we need to
"walk the section," beginning amid the lowest (and hence oldest) of the is-
land's sedimentary rocks and finishing amid the highest, or youngest. If
Sucia's sediments retained their original horizontality, we would need a huge
stepladder to complete such a task. But Sucia's rocks have been contorted in
such a way as to make our job much easier.
The sedimentary rocks of Sucia Island have been subjected to enor-
mous compressional forces that have deformed the region such that the is-
land, as seen from the air, now describes a series of gigantic nested horseshoes
tens of miles across. We can see how this occurred by "performing" the fol-
lowing "thought experiment." Imagine that the ancient sedimentary beds of
Sucia Island are represented by a telephone book, each page representing
one layer of strata. The topic lying on a flat surface represents the strata of
Sucia prior to deformation, each page horizontal and parallel to every other.
However, the Sucia strata were not to be left in such peace. Imagine that the
two sides of the topic are squeezed, causing it to assume the shape of a half
tube, with the spine of the topic and the pages on the open side pointing up.
If we now take this deformed book and jam it into the surface it sits on at a
60-degree angle, we have roughly reduplicated the forces that the Sucia Is-
land strata have undergone. The resulting curvature is sure evidence that the
