du Chardonnet and the Aiguille du Tour, before dropping down to the Col de Balme. The Swiss – Italian border runs southwest from Mont Dolent, down to the twin passes of Col Ferret. The massif contains 11 main summits over in altitude, as well as numerous subsidiary points above this height. Crowning the massif is Mont Blanc (), the highest mountain in the Alps and in western Europe. From the summit of Mont Blanc to the Arve near Chamonix there is a drop in altitude over a distance of just . Because of its great elevation, much of the massif is snow- and ice-covered, and has been deeply dissected by glaciers. The Mer de Glace is the longest glacier in the range as well as the longest in France and the second longest in the Alps. The debris-covered Miage Glacier on the southern side of the massif is the longest in Italy. The summit of Mont Blanc is an ice cap whose thickness varies from year to year. The entire massif can be circumnavigated by the Tour du Mont Blanc, a walking route of approximately . It usually takes around 11 days to complete, but is also used for an annual mountain ultramarathon, with top competitors expected to complete the whole route in less than 21 hours. The main mountain passes, or cols, that connect different valleys and towns around the Mont Blanc massif are: Col du Bonhomme (path, links Contamines – Beaufort – Les Chapieux) Col de la Seigne (path, links Bourg Saint Maurice – Les Chapieux – Courmayeur) Col Ferret (path, links Courmayeur – Orsières) Col de Champex and La Forclaz, or Fenêtre d'Arpette (path and road, links Orsières - Trient Valley) Col de Balme (path, links Trient Valley – Chamonix) Col de la Forclaz (major road, links Chamonix – Martigny) Summits The Mont Blanc massif includes eleven independent and six subsidiary summits over in height. These (including Mont Blanc) are shared between Italy and France only, the highest in Switzerland being the Aiguille d'Argentière. The massif is amongst the three major subranges of the Alps having the highest concentration of four-thousanders, together with the Pennine Alps and the Bernese Alps. The first ascent of Mont Blanc in 1786 by Jacques Balmat and Michel Paccard initiated the sport of alpine mountaineering, and it was during the golden age of alpinism (185465) and the silver age of alpinism (186582)