It was also during this geologic era that an ocean shift took place, causing the present-day location of the mountains to be part of the ocean for a time. It is possible that the then-peak formations were only small islands, barely rising above the level of the ocean. This accounts for the presence of limestone (a sedimentary rock formed predominately from calcite derived from marine organisms) found in the rims of Guadaramma mountain peaks and in some of their interior caves. Limestone formations are evident at a number of the peaks, notably El Vellón, La Pinilla and Patones.