II.

Geologic Structure and Formation

The Alps are a complex fold-mountain system. Sedimentary deposits (see Sedimentary Rock) of vast thickness, mainly limestone and dolomite, were laid down in the ancestral Tethys Sea during the Triassic and Jurassic periods. Subsequently, enormous pressure generated by a collision between the African and Eurasian tectonic plates thrust these rock strata upwards and northwards to form recumbent folds (nappes), which in the process of movement were detached from their roots.

The four glaciations of the Quaternary Period (beginning about 2.5 million years ago) were of great importance in the sculpturing of the Alps. Vast ice masses moved through the valleys, transforming them into deep troughs with steep walls; the overflow of ice across the mountain divides shaped the passes. Glacial deposits in the form of moraines dammed the streams and rivers and produced the region's many lakes, the two largest of which are lakes Geneva and Constance (or Bodensee).