Simplified stratigraphical scale (the millions of years (MY) correspond to the period's beginning) :
4500 MY
540 MY
250 MY
65 MY
Eras
Precambrian
Paleozoic (primary era)
Mesozoic (secondary era)
Cenozoic (tertiary + quaternary eras)
tertiary era
quaternary era
65 MY
1,75 MY
0,01 MY
Epochs
Pleistocene
Holocene
La Loire and its tributaries dig their valleys and deposit alluviums...
From the beginning of the quaternary era, it is the alternation of glacial and interglacial periods that is going to stabilize the river system which we know today by the progressive digging of rivers and the formation of alluvial terraces.
To know more about these processes, click here.
Silts and wind sands settle on plateaus and colluviums cover hillsides and bottoms of small valleys...
The origin of silts in Touraine is very probably due to the wind: during glacial periods, the vegetation was very reduced and the erosion by the atmospheric agents on the bare grounds was important. Particularly, the gusts extracted fine particles and accumulated them further, in the form of silts or wind sands.
Colluviums are deposits of hillsides and bottoms of small valleys. Their origin is, as for silts of plateaus, linked to the erosion of grounds in cold period but the resemblance stops there because, on one hand, the driving force is not the wind but the gravity, on the other hand, the granulometry of colluviums can be much more important than the one of silts.
The composition of colluviums depends in fact on the nature of the grounds which were eroded and which slid towards the bottoms of slope.
The first men settle down in Touraine...
Touraine possesses a small number of natural cavities (not to confuse with the troglodytic dwellings which were dug in the historic period).
Among these some caves, those which were able to serve as rock shelter were occupied during Palaeolithic (la Roche-Cotard, near Langeais, les Roches and the shelter "Reignoux" in Abilly). Outdoor sites were also discovered in the valley of la Claise (la Picardie in Preuilly-sur-Claise, les Maîtreaux in Bossay-sur-Claise).
In Neolithic, after the last glaciation and the beginning of the agriculture, the human population intensified and left very visible traces: megaliths distributed on the whole Touraine (and built mostly from conglomerates of Paleogene) and flintknapping workshops (flint from upper Turonian) in the region of le Grand-Pressigny.