The Inventario Minero National (IMN), a four-year cooperative geologic mapping and mineral resources appraisal project, was accomplished under an agreement between the Republic of Colombia and the U. S. Agency for International Development from 1964 through 1969. Subzone IIB, consisting essentially of the east half of Zone comprises nearly 20,000 km2 principally in the Department of Antioquia but including also small parts of the Departments of Caldas and Tolima. The rocks in IIB range from Precambrian to Holocene. Precambrian feldspar-quartz gneiss occupies a mosaic of fault-bounded blocks intruded by igneous rocks between the Oto fault and the Rio Magdalena. Paleozoic rocks are extensive, and include lightly metamorphosed graptolite-bearing Ordovician shale at Cristalina, and a major suite of graphitic quartz-mica schist, feldspathic and aluminous gneiss, quartzite, marble, amphibolite, and other rocks. Syntectonic intrusive gneiss included many of the older rocks during a late Paleozoic(?) orogeny, which was accompanied by Abukuma-type metamorphosing from lowermost greenschist to upper amphibolite facies. A Jurassic diorite pluton bounded by faults cuts volcanic rocks of unknown age east of the Otu fault. Cretaceous rocks are major units. Middle Cretaceous carbonaceous shale, sandstone, graywacke, conglomerate, and volcanic rocks are locally prominent. The Antioquian batholith (quartz diorite) of Late Cretaceous age cuts the middle Cretaceous and older rocks. A belt of Tertiary nonmarine clastic sedimentary rocks crops out along the Magdalena Valley. Patches of Tertiary alluvium are locally preserved in the mountains. Quaternary alluvium, much of it auriferous, is widespread in modern stream valleys. Structurally IIB constitutes part of a vast complex synclinorium intruded concordantly by syntectonic catazonal or mesozonal felsic plutons, and by the later epizonal post-tectonic Antioquian batholith. Previously unrecognized major wrench faults are outstanding structural features of IIB. Some are traceable for several hundred kilometers and probably have displacements measurable in kilometers, although only the Palestina fault, with right-lateral displacement of 27.7 km, is accurately documented. Correlations of rocks mapped in IIB with those of outlying areas including neighboring IIA are discussed.