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yardang	A mushroom-like column with a resistant rock perched on an eroding column of softer rock; created by wind abrasion in deserts where a resistant rock overlies softer layers of rock.
yazoo stream	A small tributary that runs parallel to the main river in a floodplain because the tributary is blocked from entering the main river by levees.
Younger Dryas	An interval of cooler temperatures that took place 4,500 years ago during a general warming/glacier-retreat period.