Because of its geological variety and particularities Checa could be considered a “geological theme park”. In a relatively small space, Nature has concentrated many substrata and layers which can be read as an Earth´s history book.
The oldest geological level is paleozoic, mostly consisting in slates and quarzites of 490 to 250 million years of age.
After this there is a mesozoic level, with three goups of rocks. Most ancients first:
- Triassic rocks consist mainly in conglomerates and sandstones, eroded into river canyons and peculiar shapes. In this group we must include clays and gypsum that lay at the bottom of several valleys and are the reason of the existence of saline springs of 250-205 million years of age.
- Jurassic rocks: limestones and dolostones, some of them containing a great amount of fossiles.
- Cretacean: mostly caoliniferous earths, limestones and dolostones. These are found on the topmost layer of canyons, and are 145 to 65 million years old.
- Cenozoic: two types of rock; from the Paleogene and Neogene, mostly sands and conglomerates 65 to 2 million years old; and Quaternary rocks, mostly travertines and calcareous tufa, heavily related to processes that have been going on along the hill and mountainsides, and in rivers and creeks, for the last two million years.
Pizarras de Checa (Checa´s Slates) is the scientific name of a specific tratigraphic unit. This unit is made up of pyritiferous black slates with quartz intersections towards the bottom, where oscillation ripples can be seen. Some sedimentary structures may be observed in the slates, such as horizontal lamination and millimiter-sized laminae of sand and silt. However, their main particularity is the presence of a penetrative schistosity that gives Pizarras de Checa a leaf-like appearance. This lower section is considered to be about 100 metres thick, however, it being a remarked decollement level, it is hard to actually determine its real thickness. A transition towards an alternation of about 90 metres thick of black slates, sandstones, and grey quartzites can be observed at the top of the unit. An abundance of graptolites and even orthoceras are found in the whole of this unit, allowing us to date Pizarras de Checa as Llandoverian-Wenlockian.