The Lower Grindelwald glacier penetrated during his numerous advance phases in the post-glacial period (last 11,700 years) deeply into the coniferous forest zone. Numerous trees were pressed down and covered with moraine material, e.g. in the lateral moraines of the Stieregg and the Zäsenberg. During the still ongoing retreat phase came and are still coming to light residues of such trees (trunks, rhizomes and parts of trunks and roots). Partly the so-called fossil trees or woods are still at the original growing place and can be hundreds to thousands of years old.
Presumably such fossil trees have been discovered on the Stieregg as early as the mid-18th century. At the time the Lower Grindelwald glacier was somewhat smaller than during a high extension. Nearly two hundred years later, in 1947, when the Lower Grindelwald glacier was already strongly diminished, pastor Nil from Grindelwald discovered a trunk of a tree, which died around 700 years ago and indicates a late medieval glacier advance. In 1973, the French historian and climatologist Le Roy Ladurie found more trees in the area of the Stieregg, the Challi and the Zäsenberg, which date from the time of the early to the late Middle Ages. He calls this the "Fossil Forest in Grindelwald". The geographer and glaciologist Hanspeter Holzhauser recovered several trees between 1986 and 2011 in the lateral moraines of the Stieregg and the Zäsenberg and dated them by radiocarbon or tree ring analysis (dendrochronology). Often also the soil, on which the plants grew, was preserved. The oldest trees dated from the Bronze Age. The investigations of the tree species showed that earlier in the area of the Stieregg and the Zäsenberg at least three tree species occurred: The spruce, Swiss stone pine and sycamore. The trees were between 70 and around 380 years old.
The Stieregg and the Zäsenberg are today without significant forest cover, although with respect to altitude and climate the given conditions would be suitable. Most likely the men has already started to use the slopes of the Stieregg and the Zäsenberg for alpine farming in the late Middle Ages, probably as pastures for sheep and goats. Thus the trees disappeared. The fossil trees are therefore witnesses of a time in which the men did not yet or only very little intervene in the landscape in the area Stieregg - Zäsenberg.