structural:expansion, linguistic:paraphrasing, quality:corruption, stylistic:style_transfer
Geology

The geological formations in the Loire river basin can be grouped into two sets of formations, namely, the basement domain and the domain of sedimentary formations. The basement domain primarily consists of metamorphic and siliceous fragmented rocks with groundwater occurring in fissures. The sedimentary domain consists of limestone and carbonaceous rocks, that, where saturated, form productive aquifers. Rock outcrops of granite or basalt also are exposed in the river bed in several stretches.

The middle stretches of the river have many limestone caves which were inhabited by humans in the prehistoric era; the caves are several types of limestone formations, namely tuffeau (a porous type of chalk, not to be confused with tufa) and Falun (formed 12 million years ago). The coastal zone shows hard dark stones, granite, schist and thick soil mantle.