Landscape Evolution

In arid climates, prehistoric landscapes easily reveal theselves due to a lack of obstructing vegetation and slower landscape-altering erosional processes.  Geologists love arid climates because of their unobstructed views.

Geomorphology, a sub-discipline of geology, examines landscapes and their evolution through time. A car-window view while driving across the Colorado Plateau of northern Arizona reveals many prehistoric landscape remnants. This image from House Rock Valley between Lees Ferry and Jacob’s Lake, Arizona, illustrates this perfectly:

(click on the photo for a larger image, where you can actually read the annotation)

A geologic structure called a “monocline” forms the long ridge along the horizon in the photograph. Tectonic forces folded the Earth’s upper crust, creating a “step” of tilted rock layers with one side being higher than the other. You can trace the tilted layers of the monocline in this photograph, with the fold highlighted by the blue dashed lines. Notice that the rock layers start to flatten again near the top of the ridge. In this case, the visible rock layers are Permian Kaibab limestone, around 275 million years old. This limestone is resistant to weathering and erosion, as evidenced by its prominent position as the rim of the Grand Canyon.  A diagram of a monocline is included below.

Both the resistant nature of the limestone and the structure of the monocline lend major influence to the modern landscape by creating topographic relief. This relief influences drainage patterns, creates both an area of erosion and a place for sediment accumulation. Precipitation falling on the elevated upper limb of the monocline flows towards the lower limb, creating canyons and carrying sediment. As the sediment-laden water emerges from the confines of the canyons, it spreads out and deposits sediment in a wedge near the base of the ridge. Remnants of this sediment exist today as gravel atop the terraces seen in this image.

The terraces tell another story. A terrace is simply a relict floodplain. As a stream erodes deeper into its valley, it leaves behind remnants of former floodplains perched above the modern, active floodplain. Thus, the terraces are evidience of valley erosion, revealing a valley floor that was once higher than the modern valley floor.

If a stream erodes downward at a constant rate, we would expect the resulting landscape to be relatively smooth and uniform.  The photo above reveals three very distint terrace levels, or three distinct former floodplains.  What was the stream doing during the time at which its floodplain was at a level intermediate between the terraces?  Does no evidence exist from this intermediate stage?  The broad, uniform terraces represent times when the stream was not actively eroding downward.  During these times, the stream had time to smooth the landscape as it migrated and meandered laterally, depositing a veneer of sediment in the process.  One could say the erosion and deposion processes of the stream were stable, or in a state of equilibrium.

Erosion to the next lower terrace level meant something changed, and disrupted that equilibrium.  Changes that could cause such downward erosion may be local or global.  Changes in precipitation patterns and amounts, tectonic forces, changes in vegetation, wildfires, or simply Changes in the dowstream steam and river network can all cause disruptions in the equilibrium, leaving behind terraces.  Usually, the complete answer is complicated.

Next time you take a trip, pay attention to the view from the car or airplane window, and think about the processes and events required to create the modern landsape.

A USGS geologic map of the area can be found HERE.

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