During the last ice age, the Bürgenstock was completely covered by an ice stream flowing from the Gotthard into the foothills of the Alps. Abrasion marks left by the ice on the limestone are found including in the highest altitudes. Large granite boulders which were transported by the ice from the Gotthard, are distributed over the whole mountain, as for example an 18 m3 large, round specimen on a steep slope on the Allwägli land reserve, which in 1949 was placed under protection. After the regression of the ice, the Bürgenstock was at first an island in Lake Lucerne. In the course of a few thousand years, however, the Engelberger Aa River filled up the area between the Engelberger valley entrance and the Bürgenstock with sediments, creating today's flat valley between the townships Ennetbürgen, Buochs, Stans and Stansstad.