Geology Reference
In-Depth Information
resent time and are deposited horizontally. This makes sense, since these rocks form by
settling through water under the influence of gravity, the way mud settles to the bottom of
a glass of water. The second is that rocks at the bottom of a pile are older than those above
them. This, too, seems obvious. And where one formation cuts off another, it is the young-
er that cuts across the older. Using these simple rules—determining what's above what,
and what cuts through or across what else—is how geologists decipher stories of time and
change written in stone. Of course, there is more to knowing how to read Earth's story,
such as how two other types of rocks—igneous and metamorphic—are made, but simple
interpretive rules apply to all rock types.
2
Everywhere on Earth is either eroding and losing material or receiving deposits of ma-
terial eroded off of somewhere else—one geologic realm sheds sediment, the other accu-
mulates it. But the places where each is happening change over time. The most obvious
change apparent in the walls of the Grand Canyon is that the marine rocks exposed in it
have switched from one domain (deposition) to the other (erosion). Eroded upland envir-
onments are not preserved in the rock record because there's nothing left to see—they've
vanished. The geologic signature of mountains is recorded by its absence, a gap in the re-
cord of time, while the story of our planet and life on it is archived in the sediments of
depositional lowlands and marine environments—the places where sediment piles up over
geologic time.
Deciphering earth history involves establishing the basic relationships between different
rock formations and the nature of the boundaries, or contacts, between them. Two layers
of sedimentary rock deposited one atop the other without any discontinuity are considered
conformable—they accumulated with minimal interruption. An eroded surface leaves a
discontinuity between two rock units, a gap representing missing time that geologists call
an unconformity. An unconformity represents how far down erosion wore into an ancient
landscape before additional sediment was deposited on top. A whole series of unconform-
ities exposed in the canyon walls tell of multiple rounds of deposition, deformation, and
erosion before the whole package of rocks rose from the sea to the level at which we find
them eroding today.
