A HISTORY OF TOURAINE THROUGH ITS ROCKS
2 - TOURAINE'S DEPTHS

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)
Periods Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous
540 MY 500 MY 435 MY 410 MY 355 MY 295 MY 250 MY 203 MY 135 MY
Epochs Lias Dogger Malm

The most ancient strata rising to the surface in Touraine are 150 million years ago and correspond to upper Jurassic (= Malm), in the middle of the secondary era. All the previous history is known only by oil drillings which reached the primary basement, constituted mainly by shales, sandstones, granites and coal deposits.

Before the beginning of the primary era, the sea...
This sea, or rather this ocean, deposited mainly clayey sediments.

Then a mountain range at the beginning of the primary era...
This ocean gradually closed by the bringing together of two continental plates. The collision wrinkled and lifted the ancient clays which were transformed into shales. Then the erosion gradually destroyed this mountain range (cadomian range).

Then again the sea...
The sea came back from Ordovician to Devonian. It deposited clays, sandstones, limestones...

Then again the mountain in Carboniferous Period...
This new mountain range is the Hercynian range which rises to the surface in the Armorican Massif and the Central Massif. It began to form at the end of Devonian by wrinkling strongly the ancient marine deposits which were mainly transformed into shales. At the same time, these strengths created on the mountain's base an important rise of temperature which melted the deep rocks in the form of magma. By turning colder very slowly, this magma crystallized in the form of granite.

Coalfields...
During Carboniferous and Permian periods, the luxuriant vegetation accumulated locally in series of mountain lakes, further to landslides. These accumulations of vegetables in these collapse's ditches formed, after burying, the coal. We know at least two coal belts in the Touraine's depths.

The erosion gradually destroys the Hercynian range and the sea returns in Triassic...
The beginning of Triassic is marked by the erosion of the Hercynian range which is gradually transformed into peneplane. The first deposits have a continental origin, then, towards the end of Triassic, a little deep sea, lagoonal, settles on the remains of the Hercynian range. This laguno-marine gulf signs the birth of the Paris Basin. The open sea was then eastwards and oceanic sediments deposited in what will become the Alps much later.

The sea predominates during lower and medium Jurassic...
After a short ebbing of the sea between the end of Triassic and the beginning of Liassic (= lower Jurassic), the sea settles during Liassic and deposits mainly marls (mixture of limestone and clay). In Dogger (= medium Jurassic), the marine deposits are rather constituted by limestones.

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