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This is the second of two pages designed to start geology students thinking like geologists. The
first one
simply laid out the ways geologists see the world today, whereas this one lays out what geologists see from the past and into the future.
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As the first of two examples, let's look at two mesas in Monument Valley, in the Navajo Nation or Dinè Bikeyah (The Land of The People) in northern Arizona and southern Utah. Here's a little of what any visitor sees:
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A geologist sees the same, two towers of sandstone, but immediately envisions the geology from which they formed. In this case, the similarity of layering in the two mesas clearly evokes continuous layers of sandstone, of which our mesas are only two last remnants:
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In fact, extending this concept across all of Monument Valley and beyond leads to the conclusion that there were layers of sandstone that existed across thousands of square miles. Sandstone is made of of sand now solidified into rock, and that's a lot of sand.
"A lot of sand" leads to the next thing a geologist thinks about here: where did all that sand come from? Study of modern erosion and sedimentation shows that vast quantities of sand inevitably come from large mountain ranges that are being eroded. In our example above, the sands of Monument Valley seem to have come from the Appalachians that were being eroded in the Triassic and Jurassic, the times in which the great sandstone formations of the U.S. Southwest were deposited. If that seems far-fetched, bear in mind that the sands in the Mississippi delta come, at least in part, from the mountains of Montana and Wyoming.
The final question that most geologists would ask is "How and when did all this sand get deposited?" The answer here has mostly to do with sand dunes in the Jurassic, when dinosaurs might have strayed into the shifting sands. The evidence for those answers would come from structures and fossils in the sandstone, both of which are beyond our scope today. The point is that these towers of rock bespeak far more than they are today - they tell of vast layers of sand eroded from ancient mountains, and sand deposited in shifting dunes long ago.
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As the second of our two examples, consider Stone Mountain east of Atlanta, Georgia. Any visitor sees a small mountain of granite that stands above the Piedmont's gneisses. Here's one view:
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A geologist sees the same but envisions much more. Granite is an igneous rock that forms at depth in Earth's crust; its coarse crystals preclude an origin near Earth's surface. Thus a geologist has to envision the thousands of feet of overlying rock that must have been present when the Stone Mountain granite formed, but that have since been eroded away.
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Of course, that's looking back into the past. Once you appreciate that the present is just one instant in time, you can also think about where geological processes will lead in the future:
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Again, a geologist envisions much more than the tourists sees. In this case, it's a mountain range into the depths of which the granite was intruded, and an overlying mass of rock that has since been eroded away while dinosaurs, early mammals, and mastodons strode across it. As in our first example from Monument Valley, it's not just what you see, but what you can envision once you understand a little about the geologic processes that generated these landscapes.
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