MORPHOSTRUCTURAL REGIONS OF SOUTH AMERICA The present structure of South America is mainly the result of the Andean movements which began in inter-Senonian time, continued intermittently throughout the Tertiary, reached a climax at the close of the Pliocene, and persist even at present. Though these movements represent primarily the closing tecto-orogenic phase in the evolution of the Mesozoic Andean geosynclines, they were not circumscribed to the western tectogenes but were felt with varying intensity throughout the whole continent, from the Goajira peninsula to Cape Horn and from Recife to Talara. In the western belt they resulted in the successive uplift of the different mountain ranges which now form the so-called Andes. In the extra-geosynclinal areas they resulted in extensive subcontinental fracturing and in the differential uplift and down-faulting of protracted regions. Both in the Andean and extra-Andean areas the location of the movements was controlled, to a considerable degree, by the pre-Mesozoic structure and history of the regions concerned. The Frontispiece is an attempt to plot the main morphostructural regions or provinces of South America which can be distinguished according to their internal constitution and external relief. Almost all the units shown can be subdivided into subprovinces on both morphological and structural grounds. The Eastern Cordilleras of Peru and Bolivia, plotted as a single unit, can be subdivided into at least four subprovinces, and the Eastern Cordilleras of Colombia into not less than seven or eight. These subdivisions, however, though convenient and useful in the detailed study of particular areas,