The western Altos Cuchumatanes is a deeply dissected, northwest-trending, fault-bounded uplift in northwestern Guatemala. About 7,500 m of sedimentary rocks, which is perhaps the thickest exposed section of sedimentary rocks in Central America, lie upon basement rocks consisting of slate, schist, quartzite, gneiss, amphibolite, and intrusive and volcanic rocks of unknown age and thickness. The lowermost overlying strata, consisting predominantly of clastic rocks, are included within the Santa Rosa Group which is composed of the Chicol (800 to 1,200 m), Tactic (800 m), Esperanza (300 to 500 m), and Chóchal (300 to 1,000 m) Formations. The Tactic and Esperanza are probably of Late Pennsylvanian(?) to Permian age, whereas the age of the Chicol, which is probably lithostratigraphically equivalent to the Sacapulas Formation to the east, can only be said to lie within the Ordovician-Permian interval. The Chóchal Formation, composed of fossiliferous limestone and dolomite, locally includes an upper sequence of fine-grained siliciclastic beds intercalated with abundantly fossiliferous carbonate layers, informally known as the Tuilán member. The age of the Chóchal ranges from Leonardian to early Guadalupian. The unconformably overlying Todos Santos Formation is comprised of conglomerate, sandstone, siltstone, shale, and locally contains limestone lenses near the top. Its thickness ranges from a few meters to about 1,200 m, and its age is considered to be Late Jurassic–Early Cretaceous. The Ixcoy Formation, of Cretaceous age, consists of about 2,500 m of carbonate rocks with a few interbeds of fine siliceous clastic material. The Sepur Formation is composed of 245 m of fine-grained red beds, marl, and calcarenite of Campanian to Maestrichtian age. The Colotenango beds (an informal name applied to a lithologic unit confined to the Selegua and Cuilco Valleys) consist of about 500 m of conglomerate, sandstone, mudstone, and volcanic rocks. These beds are probably of late Tertiary and Quaternary age. The youngest sediments in the western Altos Cuchumatanes are Quaternary deposits of till, volcanic ash, and alluvium. Plutonic rocks range in age from early Paleozoic (or older) to Late Cretaceous-early Tertiary.
In western Guatemala, and within the western Altos Cuchumatanes, the Chixoy-Polochic fault zone separates pre-Permian crystalline rocks to the south from younger sedimentary rocks to the north. The regional tectonic grain, which is parallel to the Chixoy-Polochic and Motagua fault zones to the east, swings northwestward near Huehuetenango and continues into Mexico where it coincides with regional trends in Chiapas, Mexico. Several probably strike-slip faults splay to the west-northwest, away from the Chixoy-Polochic fault zone. This divergent regional grain is also characterized by broad folds and high-angle normal and reverse faults. Structural relief on the Todos Santos-Ixcoy contact is more than 1,500 m. The age of the latest folding is early Eocene or younger. Two, and possibly more, older episodes of tectonism are evident in the western Altos Cuchumatanes. Intensely folded rocks belonging to the Santa Rosa Group, intruded by late Paleozoic plutons, represent a late Paleozoic mountain structure that lies buried beneath Mesozoic rocks. This buried erosioral high, the Poxlac uplift, was apparently completely covered by the Todos Santos Formation.