During breaks from school over the next years, the family would vacation at various alpine resorts. One momentous destination for François was Bossons Glacier at Mont Blanc. Willem, during short visits, also taught the boys mountaineering and their favorite climb was Dent de Jaman. A gift of General Dufour series cloth-mounted military maps came with their father's personal training. They would orient with the maps during excursions into the mountains to collect insects.