As humans spread across the globe, we often encountered rivers that had cut canyons directly through a mountain ridge. Prior to our understanding of geologic processes and plate tectonics, these gorges were attributed to the power of the river which, apparently, had drilled its way directly through the mountain; in some cases, this assumption was conflicted by the fact that the stream could have easily flowed around the edge of the ridge.
Today, we know that these "water gaps" developed as the river (or its predecessor), entrenched in deposits above the ridge, sliced through the mountain as the river cut downward and/or the ridge was uplifted. This phenomenon is especially widespread across the Ridge & Valley Province of the eastern U.S.; prime examples include the Delaware Water Gap (on the border of Pennsylvania and New Jersey) and the Potomac Water Gap at Harper's Ferry, West Virginia. Out west, Split Mountain, in Dinosaur National Monument, was bisected by the Green River as it rose and the river cut downward; initially encased in Tertiary sediments, the Carboniferous rock is now a free-standing ridge, split by the stream.
Once again, our interpretation of the landscape, like our assumptions about the Universe, are based on our current knowledge and perspective. What was accepted as fact yesterday may prove to be folly tomorrow. Understanding water gaps required the closure of our own knowledge gap.
