The geologic processes that evolve most abruptly are those that dominate and determine the appearance of a landscape. Factors such as climate, rock type, steepness of terrain, presence of water, and presence of wind are all contributing factors to a landscape's ultimate appearance. When landscapes are young and are being actively uplifted, the gradient is steep. When water, such as from a river, begins to erode under the force of gravity, it moves downslope with great erosive power. Young geologic features reflect these forces with their angular features.
In direct contrast, when a landscape has reached old age, the eroded features become level and rounded. This type of evidence—existing drainage channels and degree of erosion—tells geologists the history of the landscape in relation to mountain-building processes and the abundance of a relevant water source, both variables that also indicate what the past climate was like.
The same concept applies to the geologic evidence of weathering. In dry, arid climates, the weathering process is very slow; in humid climates, it is much faster. Therefore, the degree of weathering over a determined time period gives Earth scientists clues as to what the past climate was like. This is why arid landforms contain many angular features, such as pinnacles, outcrops, and scree (angular boulders). In warmer, more humid climates, such as tropical, chemical weathering processes dominate, leaving distinctive landforms. In extremely cold areas, still other features are formed, characteristic of the annual freeze/ thaw cycles, clueing the paleoclimatologist in to ancient conditions. The same holds for glacial landforms. They are very distinctive, and when seen in the field—even if it is in the Tropics today—they tell the experts that the area was once cold enough to provide the right environment for a glacier. Evidence of oceans and seas once existing in areas that are dry and mountainous today are indicative of a very different climate in the past. Landscapes around the world speak of climates that existed long ago, and when geologists can determine their age, it makes it possible for climate scientists to reconstruct past climate events.
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