Planet Earth


Different rocks weather at differnt rates. In warm, humid environments chemical weathering is fast so granites and limestones tend to weather rapidly. In drier, colder environments, freeze-thaw is a major process of weathering and so layered rocks, expecially shales tend to weather rapidly. The cliff in this photo is a layer of sandstone underlain by shale. The more easily weathered shale is broken and moved down slope by gravity and rain. The quickly weathered material cannot support a steep slope and so it does not form cliffs. The sandstone of the cliff, is "strong" and weathers slowly. It is, however, constantly undercut by the down slope movement of the weaker shale. Loosing support from underneath, blocks of the sandstone break off the cliff face and roll down the slope.

The pattern seen in the photo is typical with a cliff of resistant rock capping a more gentle slope of easilty weathered material. Take the time to apply this information to the two photos that follow. Identify the resistant and not resistant layers in each.

GRAND CANYON NATIONAL PARK

BADLANDS NATIONAL PARK


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