The volcanic filling of the Central Valley, located in the western sector of the canton, corresponds to a flat undulating surface. It is possible that changes on the surface within the area are due to the presence of lava only a few meters below the surface. This constitutes a front. The sub-unit takes surface form from volcanic rocks, mainly lavas, tuffs and ignimbrites covered by ash in varying thickness. The sequence flow of lavas rests on sedimentary rock. The lavas are of the andesitic type. Geomorphologically, this unit is not a valley. Nevertheless, for political, socio-economic reasons and various other kinds of references, it is preferable to continue calling this region the Central Valley. The correct technical name is Tectonic Pit, due to the presence of a fault along the foot of the Central Volcanic Sierra, which is evidenced by the existence of thermo-mineral sources. Also there is an abrupt interruption in the foothills that align with the fault as they descend from the Central Volcanic Sierra mountain range towards the valley. Additionally, there is the presence of an unexplained "volcanism" (Pacacua Formation) that corresponds with or nears the position of the fault.