summer 07 / 20:2 - Grand Canyon River Guides

Evolving Landscapes, Stream Wars, and the
Darwinian Survival of the Fittest
In previous “Letters From the Grand Canyon,” I
have attempted to outline the various ideas on how
and when the Grand Canyon came to be, together
with the facts that have a bearing on the problem and
must be taken into account (but often are not) by any
theory that aspires to respectability. Whatever the theories,
the Grand Canyon is there, vast and mysterious. So,
it not only can exist, it does exist, no matter how much
we may argue about how this happened. This now gives
us leave to start having fun by looking at the happenings
within the Canyon once it had developed into something
much like what we see today—roughly the past
one to one-and-a-half million years. It is within this
interval that we can delineate with some degree of confidence
the events that have led to what is there now, and
the processes by which this has happened. And it is also
near the very end of this interval that we start seeing the
Canyon interact with that strange and self-important
bipedal primate, Homo, which is so pleased to call itself
sapiens.
We would be remiss, however, if we were to do these
things without first exploring several interesting lessons
that issue from the observations that have been made
and the intellectual turmoil that has occurred in
attempting to solve the great dilemma. This is the
subject of this “Letter.”
Evolving Landscapes
We humans know that the landscape
around us generally changes little, if
at all, within the time we are there
to observe it. It is natural, then, for
us to project this immutability to
intervals of time much longer than
our admittedly short life span: what
is there now must have been there
thousands, hundreds of thousands,
even millions of years ago. “The
eternal hills,” and so on. This may be
perfectly acceptable in the domain of
poetry, but it is not acceptable in the
field of science, where not taking
into account the rapid evolution of
landscape can lead to mistakes such
as using the present-day landscape to
develop theories on the origin of the
Grand Canyon several millions of
years ago. It is useful, then, to see
how this landscape has changed in
the past several million years during
which the Grand Canyon was born.
Today, the region surrounding the Grand Canyon is
underlain principally by the Permian Kaibab Limestone,
the cream-colored rimrock of the Canyon. The Kaibab is
tough and hard to erode in a dry climate, so erosion
slows down greatly when it has worked its way down to
the top of that unit. Meanwhile, rocks above the
Kaibab, which are much softer, get swept away. The end
result is that today’s topographic surface coincides with
the top of the Kaibab over wide areas, resulting in a
subdued landscape that slopes gently northeast because
that is the regional dip, or slope, of the Paleozoic strata
of which the Kaibab is part. This landscape is interrupted
only where some geologic feature such as a fold
disturbs the otherwise simple arrangement of the strata.
But even there the topographic surface mostly follows
the top of the Kaibab Limestone. For example, a topographic
feature—the Kaibab Plateau—follows exactly a
geologic one—the Kaibab Anticline (or dome), an uparching
of the strata.
Has this always been so Not at all. To begin with,
we know that even a short time ago, geologically, the
region where now the Kaibab Limestone forms the
surface rock was underlain instead by the Triassic
Moenkopi Formation, the brick-red sandstone that crops
out near the Lees Ferry boat ramp. We know this
because lavas that have been dated and are not very old
Figure 1. Schematic representation of a butte typical of many in the Grand Canyon
country. The basalt capping the butte flowed over the land surface as it existed at
the time. In most cases, the land surface was then underlain by the Triassic
Moenkopi Formation. Afterwards, the area under the tough lava was protected by
erosion, but surrounding areas were not. These were eroded down to the present
level, generally on top of the resistant Kaibab Formation of Permian age. Today, the
old land surface preserved under the lava caps is hundreds and, in some cases, more
than one thousand feet above the present-day topographic surface. This represents
the erosional lowering of the land since lava time. If one knows the age of the
lava, one can then calculate the rate at which the land has been lowered.
page 42
grand canyon river guides