The terrain is composed of sedimentary rock, mostly layers of limestone, which present on the surface with medium or fine grain and sometimes as clay. It, like most of the Sierra Gorda, is on former sea bed from 150 million years ago. Recently, Arroyo Seco and other municipalities of the Sierra Gorda have reported numerous small earthquakes. According to seismologists, these are caused by the movement of water through sedimentary rock. Erosion causes gaps and then the gaps settle onto themselves, causing the small quakes. One thing scientists are checking for is to see if any areas stand over underground cavities which may give way.