The Girón Formation (, Jg) is an extensive geological formation stretching across from the north in Teorama, Norte de Santander, across the Mesa de Los Santos and Chicamocha Canyon towards west of Nobsa, Boyacá in the northern part of the Altiplano Cundiboyacense in the south. The formation extends across the northern and central part of the Eastern Ranges of the Colombian Andes. The formation forms the basement in the Middle Magdalena Valley and is forming the Serranía de Los Cobardes. The Girón Formation consists of thickly bedded red sandstones, shales, siltstones and conglomerates deposited in a terrestrial alluvial and fluvial to fluvio-lacustrine environment in an extensional basin setting. The formation dates to the Late Jurassic period and has a maximum thickness of at its type locality in the valley of the Lebrija River. Definition The formation was first defined by Hettner in 1892 and named after Girón, close to Bucaramanga, Santander. The type locality was defined by Langenheim in 1954 in the valley of the Lebrija River. The formation was studied by various authors in the 1940s, Oppenheim in 1940, Dickey in 1941 and Trumpy in 1943, who described the formation as a lateral equivalent of the La Quinta Formation of northern Colombia and western Venezuela. Julivert (1958) assigned a thickness of , while Navas in 1963 observed a thickness of . Cediel in the same year defined the thickness at its type locality of and divided the formation in seven units. Research by Osorio and Velandia, published in 2015, concluded a maximum thickness of at the type locality. On the Cáchira Páramo in Norte de Santander, the thickness is . Description The Girón Formation extends over a distance of about from Teorama in Norte de Santander in the north to Nobsa in Boyacá in the south, stretching across three departments; Norte de Santander, Santander and Boyacá. Lithologies The Girón Formation is characterized by a thick sequence of red feldspathic and micaceaous sandstones and thinly bedded reddish siltstones, conglomerates with quartz and lithic clasts, shales with white spots and purple to dark red claystones. In the south, the formation includes fragments of schists, quartzites, and red and green limonite. The middle part of the sequence contains alternating greyish green sandstones and red to purple siltstones in beds up to . In the Middle Magdalena Valley (VMM), the Girón Formation has been drilled and proved to comprise rhyolitic, rhyodacitic flows and