The long story of an old branch :
this fragment of branch, 90 million years old, is 21 cm long for a section of only 1 cm.
The preservation of such a vestige is a long story with multiple stages. This branch had at first to be buried in marine sediments which were going to become the tuffeau, then, after the formation of the tuffeau, the silicification had to concern the branch but also, on a radius of several centimeters and in a partial way, the gangue of tuffeau which surrounded it. Millions of years later, la Loire had to dig a more and more wide bed between two cliffs of tuffeau, by erosion on the base, then by successive collapses of both cliffs. It was then necessary that the blocks of tuffeau so released were affected by water of la Loire: Mechanical wear by the transported grains of sand and chemical wear by acid dissolving of the limestone. These phenomena of erosion have continued until the gangue, partially silicified and much more resistant, was reached. Finally, the gangue had to break in two parts under the effect of the frost/thaw and show in its middle this old branch found in the alluviums of la Loire.
