This paper deals with the Jarama-Lozoya-Guadalix fluvial system evolution during the Pliocene and Pleistocene times. Duringt the Upper Pliocene and Lower Pleistocene time the Lozoya river was under incission and fluviokarst regimen; at the Middle Pleistocene beginning time (Cromer) after a local tectonic event, the jarama river shifted northwards and Lozoya river filled the "Huesos" cave with large amounts of mammal remains. Redueña alluvial fans were still active during the Upper Riss time, but because of local titling later became unactive