Internationale des Associations d'Alpinisme (UIAA) has defined a list of 82 "official" Alpine summits that reach at least . The list includes not only mountains, but also subpeaks with little prominence that are considered important mountaineering objectives. Below are listed the 29 "four-thousanders" with at least of prominence. While Mont Blanc was first climbed in 1786 and the Jungfrau in 1811, most of the Alpine four-thousanders were climbed during the second half of the 19th century, notably Piz Bernina (1850), the Dom (1858), the Grand Combin (1859), the Weisshorn (1861) and the Barre des Écrins (1864); the ascent of the Matterhorn in 1865 marked the end of the golden age of alpinism. Karl Blodig (1859–1956) was among the first to successfully climb all the major 4,000 m peaks. He completed his series of ascents in 1911. Many of the big Alpine three-thousanders were climbed in the early 19th century, notably the Grossglockner (1800) and the Ortler (1804), although some of them were climbed only much later, such at Mont Pelvoux (1848), Monte Viso (1861) and La Meije (1877). The first British Mont Blanc ascent by a man was in 1788; the first ascent by a woman was in 1808. By the mid-1850s Swiss mountaineers had ascended most of the peaks and were eagerly sought as mountain guides. Edward Whymper reached the top of the Matterhorn in 1865 (after seven attempts), and in 1938 the last of the six great north faces of the Alps was climbed with the first ascent of the Eiger Nordwand (north face of the Eiger). Geology and orogeny Important geological concepts were established as naturalists began studying the rock formations of the Alps in the 18th century. In the mid-19th century, the now-defunct theory of geosynclines was used to explain the presence of "folded" mountain chains but by the mid-20th century the theory of plate tectonics became widely accepted. The formation of the Alps (the Alpine orogeny) was an episodic process that began about 300 million years ago. In the Paleozoic Era the Pangaean supercontinent consisted of a single tectonic plate; it broke into separate plates during the Mesozoic Era and the Tethys sea developed between Laurasia and Gondwana during the Jurassic Period. The Tethys was later squeezed between colliding plates causing the formation of mountain ranges called the Alpide belt, from Gibraltar through the Himalayas to Indonesia—a process that began at the end of the