Other processes were at play during the Cenozoic era that shaped the present form of the Guadarammas. The erosion of the rocky massif provoked sedimentation which filled the mountain basins with sandstone. The action of glaciers during the Quaternary Period (1.8 mya up to the present) shaped several mountain profiles with small cirques, carved glacial lakes and left behind moraines. All three features can be found on Peñalara. Additionally, some traces of glacial passage are found in "El Nevero" and "La Maliciosa" in the form of sheepback-grooved rocks and small cirques. Finally, in the last million years, the action of glaciers caused consolidation of the network of rivers criss-crossing the mountain slopes, carved valleys and terraces resulting in the current appearance of the landscape.