River drainage patterns provide clues to the rock types and erosional
processes involved in landscape evolution. Here in the Gotel Mountains
along the border between Nigeria and Cameroon two distinct and highly
contrasting patterns are evident. The lowlands in the northern and
northwestern parts of the scene exhibit "dendritic" patterns, while the
highlands in the southern and southeastern parts of the scene show
"rectangular" and other linear drainage patterns.
Dendritic drainage patterns appear almost random. Moving upstream,
streams split into smaller and smaller channels, with finer and finer
spacing, and they show little preference for map orientation other than
to complete the pattern. In contrast, rectangular and other linear
drainage patterns show a distinct preference for certain orientations
in map view. In this scene, the two major preferred orientations are
rotated slightly clockwise of north-south and east-west.
Linear drainage patterns usually match the cracking patterns that
can occur in relatively hard rocks, including igneous rocks such as
granite and basalt. Stream erosion typically follows such lines of
weakness in these hard rocks. Meanwhile, the randomness of dendritic
patterns indicates that no such cracks nor any other geologic structure
controls the erosion where that pattern occurs.
Given the above, the topographic pattern in this scene appears to
tell us the following about the geology of this location. The lowlands
are composed of poorly consolidated (relatively soft) rocks or sediments
that are at least 100 meters (330 feet) thick and are "massive" (uniform,
with no prominent layering at the observed scale). The randomness of the
dendritic patterns further indicates that stream erosion is the only
significant dynamic process altering the lowland landscape. Forces such
as volcanism and tectonics are not altering these landforms.
Meanwhile, the neighboring highlands are composed of crystalline
rocks, such as granite, that are very hard (generally resistant to
erosion) and probably very old. Sometime through their history these
rocks cracked, perhaps when they cooled, perhaps under tectonic stress,
or perhaps when pressure upon them was relieved when they were unearthed
by erosion. Now at the surface, these cracks are zones of weakness as
these hard rocks otherwise resist stream erosion.
But while the topographic data gives clues, it does not always provide
definitive answers. Are the lowlands made up of soft sediments washed in
from elsewhere or are they simply rocks "softened" by weathering
(disintegration into sand) in place over time. If the latter, might
they in fact have been granite also? The two patterns interfinger
geographically, suggesting that the rugged highlands may be evolving
into the dendritic lowlands. Weathering products (loose sands) tend to
accumulate in place in low relief terrain because erosion there is slow
to remove them. Also, granites are typically "massive" and cracking
patterns vanish when the rocks disintegrate. The topographic data
indeed provide thought provoking evidence, but definitive answers will
require fieldwork or other additional evidence.
This image was created directly from an SRTM elevation model. A shade
image was derived by computing topographic slope in the north-south
direction. Northern slopes appear bright and southern slopes appear
dark. The shade image was then merged with a height-as-brightness image,
which helps clarify the continuity of the drainage networks.
Elevation data used in this image was acquired by the Shuttle Radar
Topography Mission (SRTM) aboard the Space Shuttle Endeavour, launched
on February 11, 2000. SRTM used the same radar instrument that comprised
the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar
(SIR-C/X-SAR) that flew twice on the Space Shuttle Endeavour in 1994.
SRTM was designed to collect three-dimensional measurements of the
Earth's surface. To collect the 3-D data, engineers added a 60-meter-long
(200-foot) mast, installed additional C-band and X-band antennas, and
improved tracking and navigation devices. The mission is a cooperative
project between the National Aeronautics and Space Administration (NASA),
the National Geospatial-Intelligence Agency (NGA) of the U.S. Department
of Defense (DoD), and the German and Italian space agencies. It is
managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., for NASA's
Earth Science Enterprise, Washington, D.C.
Size: 144.8 kilometers (89.8 miles) by 131.5 kilometers (81.5 miles)
Location: 7 degrees North latitude, 10 degrees East longitude
Orientation: North toward the top
Image Data: SRTM elevation shaded plus height-as-brightness
Date Acquired: February 2000