In 1935, while climbing at Breakneck Ridge on the Hudson River, Wiessner spotted the gleaming white quartzite cliffs of the Shawangunks in the distance. The following weekend he set off in search of the tantalizing cliffs and immediately set about climbing the highest point in the area, a cliff now known as "Millbrook mountain". Along with John and Peggy Navas, he established a route now named Old Route 5.5, the first recorded technical rock climb in the Shawangunks, and in doing so helped to establish the area as a mecca for rock climbers.