Underlying landscape similarities might not be evident under casual observation by the untrained eye flying over southwestern Pennsylvania or over northwestern Arizona. Pennsylvania’s landscape is shrouded by trees thriving on its humid climate. Arizona’s landscape is rather bare under semiarid conditions. Both regions are plateaus—uplifted areas—composed of sedimentary rock layers that overlie metamorphic and igneous rocks. On both landscapes streams and valleys form branchlike, or dendritic, patterns.
In northwestern Arizona, the Colorado River and its current and former tributary streambeds are not unlike those of the Monongahela River in western Pennsylvania. Both streams flow on and cut into table-top sedimentary rocks whose elevations and climate control valley shape and depth. The higher plateau in Arizona is largely unprotected by lush vegetation that covers western Pennsylvania. Without plants to retain soil and rock steeper-sided valleys formed. And on the higher plateau the contrasting relief, or difference in elevation, is greater than on the lower Appalachian Plateau, even though the stream patterns are essentially the same. In both regions the water falls to an ocean basin, the Monongahela, through the Ohio and Mississippi rivers, to the Gulf of Mexico and the Colorado to the Gulf of California. The flow of the latter plunges a greater vertical distance than the flow of the former.
There’s an analog in human behavior. All of us undergo erosion of some kind and depth, the eventual loss of muscle and sharpness of reflex, the loss of some mental facility, or the loss of “beauty.” And much of the erosion stems from the elevation to which we rose in earlier times. Learn more, forget more. Lift more, lift, in absolute numbers, less. The loss of 10 percent is a greater absolute number even though the percentage is the same for both.
Whether or not the plateaus of Arizona and Pennsylvania are still rising is a matter of debate. They might have reached their maximum elevations and their futures lie in erosive destruction. But Earth is a planet that renews as it destroys, though not all renewal is proportional or locked in place. Some revitalized tectonic activity might renew the rise of either or both plateaus.
What seems true of both plateaus is that a lower elevation and a secure protection of trees results in a gentler landscape that is a bit less spectacular than what the higher elevation and naked exposure to the attack of streams engenders. More people flock to see the Grand Canyon’s mile-deep and steep-sided canyons than to see the Monongahela Valley’s hundreds-of-feet deep and gentler-slope cuts.
Landscapes that have undergone the same kinds of dendritic erosion can differ dramatically because one has reached a higher elevation than the other. Yes, more erosion occurs where greater uplift and unshielded exposure occurs compared to where lower uplift and efficient protection prevails. Which one, I ask, do you prefer to see? And which one, I also ask, would you prefer to be?
In northwestern Arizona, the Colorado River and its current and former tributary streambeds are not unlike those of the Monongahela River in western Pennsylvania. Both streams flow on and cut into table-top sedimentary rocks whose elevations and climate control valley shape and depth. The higher plateau in Arizona is largely unprotected by lush vegetation that covers western Pennsylvania. Without plants to retain soil and rock steeper-sided valleys formed. And on the higher plateau the contrasting relief, or difference in elevation, is greater than on the lower Appalachian Plateau, even though the stream patterns are essentially the same. In both regions the water falls to an ocean basin, the Monongahela, through the Ohio and Mississippi rivers, to the Gulf of Mexico and the Colorado to the Gulf of California. The flow of the latter plunges a greater vertical distance than the flow of the former.
There’s an analog in human behavior. All of us undergo erosion of some kind and depth, the eventual loss of muscle and sharpness of reflex, the loss of some mental facility, or the loss of “beauty.” And much of the erosion stems from the elevation to which we rose in earlier times. Learn more, forget more. Lift more, lift, in absolute numbers, less. The loss of 10 percent is a greater absolute number even though the percentage is the same for both.
Whether or not the plateaus of Arizona and Pennsylvania are still rising is a matter of debate. They might have reached their maximum elevations and their futures lie in erosive destruction. But Earth is a planet that renews as it destroys, though not all renewal is proportional or locked in place. Some revitalized tectonic activity might renew the rise of either or both plateaus.
What seems true of both plateaus is that a lower elevation and a secure protection of trees results in a gentler landscape that is a bit less spectacular than what the higher elevation and naked exposure to the attack of streams engenders. More people flock to see the Grand Canyon’s mile-deep and steep-sided canyons than to see the Monongahela Valley’s hundreds-of-feet deep and gentler-slope cuts.
Landscapes that have undergone the same kinds of dendritic erosion can differ dramatically because one has reached a higher elevation than the other. Yes, more erosion occurs where greater uplift and unshielded exposure occurs compared to where lower uplift and efficient protection prevails. Which one, I ask, do you prefer to see? And which one, I also ask, would you prefer to be?
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