The Middle Miocene sediments exposed in Somosaguas contain an excellent record of mammal fossil remains preserved in debris and mudflow deposits. In the sedimentary sequence of Somosaguas three facies associations are distinguished, representing alluvial fan and shallow lake margin environments. Alluvial fans are composed of mud and debris flow deposits. The whole of the lacustrine deposits are interbedded between debris and mud flow deposits. The shallow lake deposits are devoid of fossil remains, and they consist of well-sorted micaceous sandstone bodies with water flow structures, interbedded with thin mudstone beds. These sandstone bodies are interpreted as stream mouth lobes. Three integrated stratigraphic sections show that the lacustrine bedset overlain an erosion surface, probably scoured by flood waters. Palaeocurrent measurements indicate that the sediment filling the shallow lake came from the west and the north