Underneath the even sheet of the alluvial deposits the surface of the sub-Pampean platform is very irregular. Its shape has been discovered by deep borings in search of arterial waters. It has been warped and cut up by faults, some of these deformations being probably synchronous with the formation of the Pampean deposits which have concealed them as they have been produced. A subterranean rocky ridge continues the Sierra de Cordoba southward and joins it with the sierras of the Colorado. The granite emerges at Chamaico, on the western railway, and on both sides the borings have passed through great depths of clay and sand.[79] This ridge isolates the eastern Pampa from the sub-Andean chains, and marks the limit of the area with sheets of underground water. In the north of the Pampean region, between the Sierra de Cordoba and the Parana, the loose continental formations are more than 2,000 feet thick at Bellville, and more than 3,500 feet north-west of Santa Fe (fodder farms of San Cristobal and El Tostado). At Buenos Aires the granite has been found 985 feet below the surface. [79] At Rancul, in the east, 660 feet of loess overlying red sandstone: at Telen, in the west, 2,800 feet of sand, marl, sandstone and gravel. [Illustration: THE PAMPEAN PLAIN. TRES ARROYES (BUENOS AIRES PRAIRIE BETWEEN THE SIERRA DE TANDIL AND THE SIERRA DE LA VENTANA). _Zone of wheat and oats on large scale. The Pampa is a tableland here (400 feet above sea-level), with clay overlying the limestone of the Tosca. The valleys are well marked._ Photograph by the Author.] [Illustration: THE PAMPEAN PLAIN. TOAY, ON THE CENTRAL PAMPA (590 FEET). _The tableland, with a strong framework of limestone Tosca, is cut across by well-marked dry valleys which sink lower toward the east. At the edges of the valleys the sand is the prey of the winds. Here we are near the limit of the wheat belt._ Photograph by the Author. PLATE XV. To face p. 166.]