Zeolites and dawsonite have formed rapidly during the past 20,000 years in the sides of Olduvai Gorge and over the adjacent Serengeti Plain of northeast Tanganyika. Nephelinite tuffs were altered to form a surface calcrete and the minerals phillipsite, natrolite, chabazite, analcime, and dawsonite; sodic trachyte tuffs were altered to form phillipsite, erionite, and chabazite. Zeolites and calcrete were also formed at the land surface in the Olduvai region during two dry episodes of the Pleistocene before the gorge was eroded. Zeolites and dawsonite were formed by reaction of volcanic glass and nepheline with solutions of sodium carbonate and bicarbonate that were concentrated by evaporation in the soil and surface layers of rock. Hydration and carbonation appear to be the principal chemical changes in zeolitic alteration.