FIGURE 2.1 The stratigraphic section in the Grand Canyon records a time span from about 1.8 billion to 250 million years ago—a substantial fraction of Earth’s history. SOURCE: National Park Service.

FIGURE 2.1 The stratigraphic section in the Grand Canyon records a time span from about 1.8 billion to 250 million years ago—a substantial fraction of Earth’s history. SOURCE: National Park Service.

in the future. Our growing ability to quantify processes specific to the near-surface environment also lets us reconstruct Earth’s past in increasing detail including aspects of its geochemistry, biotic processes, topography, and particle and solute fluxes. The information stored in landscapes and sediments provides a kind of “time telescope” that allows us to view what is in effect a temporal sequence of alternative Earths—recognizably our planet, yet often surprising and unfamiliar.

In addition, the history of landscapes strongly influences their present state and future evolution. In the Earth’s northerly regions, the current landscape pattern was largely created by glacial processes that peaked around 18,000 years ago, and present-day debates about agricultural best practices and practical water quality standards require a sophisticated understanding of how the landscape is evolving in response to the deglaciation. Likewise, coastal erosion is still influenced by uplift and subsidence in response to shifts in ice loading over Holocene time. The fate of global deltas on which hundreds of millions of people depend in turn depends on the delicate interplay of subsidence and sedimentation developed over geologic time. In upland regions, the general importance of tectonic history is obvious; more subtle are the possible effects of variations in uplift rate and climate in influencing the balance of sediment storage and release to downstream river systems. Soils are among the clearest examples of the influence of past time—the soils that support global agriculture represent the integrated effects of tens of thousands of years of biogeochemical processes.

The evolutionary road from the prebiotic Earth and an atmosphere devoid of oxygen to the human-dominated conditions of today has been well documented in the popular



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