In most areas of the world, the landscape we see has been shaped by geomorphic agents over time spans of millions of years. Changes have occurred and continue to occur slowly and, for the most part, imperceptibly. A river bend that migrates 100 mm in a year, or even 500 mm, looks pretty much the same from one year to the next. A moraine that grows 50 mm higher each year will not appear noticeably different a year from now. Some geomorphic agents, however, produce changes on much shorter time scales, and these changes are readily detectable without precise measurements. Major volcanic eruptions, rapid landslides, and many human earth-moving activities are examples.

Four hundred thousand years ago Home erectus began moving rocks in a systematic way to support walls of dwellings. Ten thousand years ago, in the late Paleolithic, humans quarried flint. During the Mesolithic humans began congregating in villages, often constructed of earth materials, and began mining metals. Huge tombs were also built during this period, and the invention of the wheel led to construction of roads. The pace of human modification of the landscape accelerated further in the Iron Age, as iron tools facilitated earth moving while at the same time, in a positive feedback, requiring other earth materials, notably coal for smelting. Harnessing the power of steam during the Industrial Revolution accelerated this process still further, and today excavators based on the internal combustion engine can literally move mountains.

Thus, in the short space of 400,000 years, and particularly during the last two centuries, humans have left a mark on the landscape. While not yet approaching the effect of rivers and glaciers over millions of years, the present rate of modification of the landscape by this one new agent, humans, is unprecedented in geologic history.