I have no doubt that from remotest times the whole valley was filled with water. Everywhere the shape of the promontories and their steep slopes reveals the ancient shore of this alpine lake. We find vast tracts of land, formerly flooded, now cultivated with banana, sugar cane and cotton. Wherever a hut is built on the lake shore you can see how year by year the water recedes. As the water decreases, you can see how islands begin to join the land while others form promontories or become hills. We visited two islands still completely surrounded by water and found, under the scrub, on small flats between 4 and 8 toises above the water-level, fine sand mixed with helicites deposited by waves. On all these islands you will discover clear traces of the gradual lowering of the water.