Complex sentence: The Walensee is one of the larger lakes in Switzerland, for about 2/3 of its surface in the Canton of St. Gallen and for 1/3 in the Canton of Glarus.
Split into simpler sentences: The Walensee is one of the larger lakes in Switzerland. About 2/3 of its surface in the Canton of St. Gallen and about 1/3 in the Canton of Glarus.