The lower Conchos valley of northeastern Chihuahua is crossed by mountain ranges which are a part of the Sierra Madre Oriental of Mexico. The oldest exposed formation, the Plomosas shale, sandstone, limestone, and conglomerate, somewhat metamorphosed, is believed to be Paleozoic. Above follow the Upper Jurassic La Casita shale and a thick Cretaceous section, divided into the Las Vigas red and gray clastic rocks, the Cuchillo shale, gypsum, and limestone, the Aurora limestone, and the Chispa Summit flaggy limestone, shale, and sandstone. Early Tertiary volcanic rocks lie unconformably on the older formations, and the late Tertiary Conchos gravels fill the valleys to a depth of several hundred feet. The rocks are strongly folded along a north-northwest trend. In places the folds are overturned, and an overthrust fault with several miles horizontal throw occurs at the northeast base of the highest uplift. Normal faults parallel the trend of the folds. The main orogenic epoch was early Tertiary. After the ranges were deeply dissected, lavas covered the erosion surface, and folding recurred along pre-existing axes. Basins of interior drainage formed by the folding were filled with the Conchos gravels. As master streams connected the isolated basins, deep, narrow gorges were cut into the resistant rocks of the intervening ranges.