Alps

The Alps emerged during the Alpine orogeny, an event that began about 70 million years ago as the Mesozoic era was drawing to a close. A broad outline helps to clarify the main episodes of a complicated process. At the end of the Paleozoic era, about 245 million years ago, eroded Hercynian mountains, similar to the present Massif Central in France and Bohemian Massif embracing parts of Germany, Austria, Poland, and the Czech Republic, stood where the Alps are now located. A large landmass, formed of crystalline rocks and known as Tyrrhenia, occupied what is today the western Mediterranean basin, whereas much of the rest of Europe was inundated by a vast sea. During the Mesozoic (245 to 66.4 million years ago) Tyrrhenia was slowly leveled by the forces of erosion. The eroded materials were carried southward by river action and deposited at the bottom of a vast ocean known as the Tethys Sea, where they were slowly transformed into horizontal layers of rock composed of limestone, clay, shale, and sandstone.

During the middle Tertiary period, about 44 million years ago, relentless and powerful pressures from the south first formed the Pyrenees and then the Alps, as the deep layers of rock that had settled into the Tethys Sea were folded around and against the crystalline bedrock and raised with the bedrock to heights approaching the present-day Himalayas. These tectonic movements lasted until nine million years ago. Tyrrhenia sank at the beginning of the Quaternary period, about 1.6 million years ago, but remnants of its mass, such as the rugged Estéral region west of Cannes, are still found in the western Mediterranean. Throughout the Quaternary period, erosive forces gnawed steadily at the enormous block of newly folded and upthrust mountains, forming the general outlines of the present-day landscape.

The landscape was further modeled during the Quaternary by Alpine glaciation and by expanding ice tongues, some reaching depths of nearly one mile (1.6 kilometres), that filled in the valleys and overflowed onto the plains. Amphitheatre-like cirques, arête ridges, and majestic peaks such as the Matterhorn and Grossglockner were shaped from the mountaintops; the valleys were widened and deepened into general U-shapes, and immense waterfalls, like the Staubbach and Trümmelbach falls in the Lauterbrunnen Valley of the Bernese Alps, poured forth from hanging valleys hundreds of feet above the main valley floors; elongated lakes of great depth such as Lake Annecy in France, Lake Constance, bordering Switzerland, Germany, and Austria, and the lakes of the Salzkammergut in Austria filled in many of the ice-scoured valleys; and enormous quantities of sands and gravels were deposited by the melting glaciers, and landslides—following the melting of much of the ice—filled in sections of the valley floors. The hills east of Sierre in the Rhône valley are an example of this last phenomenon, and they mark the French–German language divide in this area.

When the ice left the main valleys, there was renewed river downcutting, both in the lateral and transverse valleys. The river valleys have been eroded to relatively low elevations that are well below those of the surrounding mountains. Thus, Aosta, Italy, in the Pennine Alps, and Sierre, Switz., look up to peaks that tower a mile and a half above them. In the valley of the Arve River near Mont Blanc, the difference in relief is more than 13,100 feet.

Glaciation therefore modified what otherwise would have been a harsher physical environment: the climate was much milder in the valleys than on the surrounding heights, settlement could be established deeper into the mountains, communication was facilitated, and soils were inherently more fertile because of morainic deposits. Vigorous glacial erosion continues in modern times. Many hundreds of square miles of Alpine glaciers, such as those in the Ortles and Adamello ranges and such deep-valley glaciers as the Aletsch Glacier near Brig, Switz., are still found in the Alps. The summer runoff from these ice masses is instrumental in filling the deep reservoirs used to generate hydroelectricity.