advance, retreat, and then advance again. Their movement across the landscape ground down and shaped the mountains and the valleys we see today. Both the Mont Blanc massif, and the Aiguilles Rouges range to its north, now form part of the external zone of the Alps, effectively being the outermost layers of rock. The central granites make up Mont Blanc, the steep slopes of the Drus, the Grandes Jorasses and the Dent du Géant, and at the highest points are topped by schists, which are visible in places such as Grands Montets and near Mont Blanc's summit. The granite mountains around Chamonix have been eroded into steep needle-shaped peaks (known as ). Known locally as protogine, these rocks are lower in the mineral biotite mica and richer in quartz than the granites commonly found elsewhere. A large part of the massif is covered by ice fields and is cut by numerous glaciers, mostly flowing northwestwards on the French side; southeastwards on the Italian side and eastwards on the Swiss side. With much steeper slopes on the Italian side, many glaciers drop very sharply and some, such as the Miage Glacier and the Brenva Glacier, are very heavily covered in rock debris. The massif is itself defined by broad valleys which formed along fault lines and which have subsequently been shaped by ice during the last glacial period of the ice age. During the mid-19th century the granite of the Mont Blanc massif was an important source of stone for buildings; one hundred Italian stonemasons were brought to the Chamonix valley by Charles Albert of Sardinia for reconstructing the towns of Sallanches and Cluses, both of which had at that time just been destroyed by fires. Minerals The massif has been an important source of mineral specimens for crystal-hunters for over 250 years. The mountaineer and explorer, Edward Whymper, noted that the basin of the Glacier de Talèfre was "considered good hunting-ground for crystals", and that the slopes below les Courtes had yielded many large specimens. He recounted that in 1745 a guide had stated he had collected over of specimens there in just three hours. The first systematic account of the minerals of the Mont Blanc area was published in 1873 by Venance Payot. His list, entitled "Statistique minéralogique des environs du Mt-Blanc", catalogued 90 mineral types although it also included those present only as very small components of rocks.