 In 1915, Marshall climbed his first Adirondack peak, the Ampersand Mountain, alongside his brother George and family friend Herb Clark, a Saranac Lake guide.The two brothers learned the arts of woodcraft and boating through Clark, who accompanied them on most of their longer trips during adolescence and early adulthood.By 1921, they became the first climbers to scale all 42 Adirondack Mountains believed to exceed , some of which had never been climbed.