Aletsch Glacier In Switzerland is the important rock formation. Aletsch Glacier covers more than 120 square kilometres (more than 45 square miles) in southern Switzerland and is the largest glacier in the Alps. It descends round the south of the mountain Jungfrau into the valley of the Upper Rhône and down its eastern extremity lies the glacier lake Marjelen. To the west rises the mountain Aletschhorn.