The best glacial remains of Sierra de Guadarrama
The vestiges of Quaternary glacier and the current periglacial processes can be observed in Peñalara Massif. Much of the current geomorphology is explained by these processes, occurred thousands of years ago, but they have currently resulted in a great diversity of species and habitats and in a high landscape quality.
The preglacial period
Before the cold episode, this picture shows the appearance the South-Eastern slope between the Dos Hermanas and Peñalara peaks would have probably had. Instead of the circuses and deep depressions existing at the bottom of the peaks, there would have been torrential water catchment areas of diverging sizes. These catchments would hold some soft eroded margins, unlike the abrupt slopes between the peaks areas and the circuses cliffs we can currently observe.
The maximum glacial growth episode
Under the enabling weather conditions, great accumulation zones formed. At the time their size expanded, the snow compacted, turned to ice and started flowing down the slope. All the torrent top area, from the Dos Hermanas peak to the vicinity of the Peñalara peak, was taken up by a huge mass of ice, which reached 1.870 m height and filled a 2,5 km long surface.
The wave ogives development episode
Then a retreat of unknown breadth occurred, and a new expansion to a height about 1.950 m followed. This episode is likely the time in which the circuses, though being under the ice, were modelled to their current configuration. In the Large Lagoon (Laguna Grande) circus area, the glacier could reach about two hundred metres width.
The wave ogives retreat episode
A progressive change in weather conditions boosted a significant retreat of the Peñalara glacier. As the ice volume reduced, the glacier split in two much smaller glaciers: the Dos Hermanas glacier, having a small ice pack, and the one of the Large Lagoon. The latter came to stretch far beyond the circus area and its terminus experienced up to three small stabilization episodes, giving rise to just as much decreasing radius wave ogives.
The rock glaciers episode
This episode represents the last chapter in the glacier history, which main feature was the glacier circuses had no terminus. The one from Dos Hermanas could be defined as a rock glacier. This particular glacier stands a great scree accumulation “messing” all the ice pack, even though holding some flow. The Large Lagoon area was taken up by a greater typical glacial circus, which held in its Southern section an associated embryonic rock glacier.
The post-glacial (current) stage
When glaciers withdrew definitively, the entire area was cleared and periglacial and torrential activity started, forming the current landscape. Thus, in La Hoya de Peñalara -the area which, in previous episodes, used to be covered by the circus terminus- the subsequent peatlands, torrents broke entirely moraine bars and they expanded in the massif foothills direction. In circuses, great rocky grounds formed soon as result of gelifraction and in the Large Lagoon circus. The lake formed inside the overburden excavation basin the ice had made.