8. Alluvial terraces in Arizona have been related to fluctuationsin climate.Here an alluvial filling of silt and fine sand in a valley was marked by formation of peat (black) in its upper levels, suggestinga relativelyhumid climate.Then a period of deep cutting followed (see men at lower right for scale), correlatedwith a changeto relativelydry climatewhen the slopeswere less well protectedby vegetation.Still later, minor fillingby white sand is recordedby the terraceon the left. Althoughthe cutting here occurredwithin historic time, much older intervalsof cutting and filling have been identifiedin the Southwestand correlatedwith earlypotteryculturesof this area. 7. Cross-section throughthe soil visiblein Figure6. The dark red soil protrudesdownward into calcareouswhite sand. The sand has been cementedby lime leachedfrom the soil to makea resistantrockwhichis now quarriedfor buildingstonein Beirut.The main soil is overlainby moresand in whichan incipientsoil was formed,and this in turn was buried. Artifactsdiscoveredin the buried soils might be correlated with climatic fluctuations. 23