i. eastern foothills of the sierra de perijá The Sierra de Perijá forms the western border of the Maracaibo Basin; it joins the Cordillera Oriental that rises athwart the basin in the southwest. Only the foothills of these great mountain ranges are shown on the map. North of Lat. 9.5°N., the structural picture is obscured by two conspicuous unconformities: (1) The Oligocene-Miocene sediments, a thick, nonmarine sequence of mottled claystones and siltstones, overlap westward across the Eocene and Cretaceous beds. (2) The sands and claystones of the Milagro formation, of Pleistocene (or possibly late Pliocene) age, transgresses westward in the same region, so that about 45 miles (75 kms.) west of Maracaibo the formation rests directly on the pre-Cretaceous igneous rocks of the Totumo-Inciarte uplift (center of B-10) 1 . Three structural features attract attention along the east front of the Sierra de Perijá: (1) the en echelon pattern of anticlinal prongs, which dominates the southern half of the east front of the Sierra de Perijá, and reappears in weak development and with reversed direction in the north; (2) the largely concealed anticlinal belts in the structurally high region west of Maracaibo; and (3) the abrupt northern termination of the Sierra de Perijá. The great Tarra anticline (Notestein et al . 1944; Staff, 1948) is the southernmost, the longest and highest of the en echelon fold prongs. It begins in the foothills of the Cordillera Oriented south of the Venezuelan border, in the Barco Concession, and extends north as a major unit for…

The three ranges of the Colombian Andes have distinct geologic characteristics. The Central Cordillera is an uplifted zone of crystalline rocks, mostly pre-Mesozoic in age. The Western and Eastern Cordilleras are composed, respectively, of eugeosynclinal and miogeosynclinal rocks, deposited during the Mesozoic, and deformed in Late Cretaceous and Tertiary time. Although the Andes as a topographic feature end at 8° N. lat, pre-Tertiary rocks are found to the northeast in the Santa Marta-Perijá region and the Guajira Peninsula. Recent work has shown that a three-fold structure, similar to that of the Andes, characterizes these regions as well, suggesting that the Andean mobile belt was more extensive than the present day topographic Andes.