The formation of sawtooth moraine ridges in Bødalen, western Norway

Abstract

The forefield of the Bødalsbreen glacier, western Norway, exhibits a prominent set of end moraines. The outermost one, deposited at the Little Ice Age (LIA) maximum (A.D. ∼ 1755), is smooth, continuous and arcuate, thus reflecting an even ice front. In contrast, subsequently formed moraines (deposited A.D. ∼ 1767–1930) show a striking “sawtooth” morphology. In an attempt to elucidate the formation processes responsible for this complex landform assemblage, we have employed several methods of investigation including geomorphological mapping, sedimentological studies, and ground penetrating radar (GPR). In general, the detailed formation processes of all moraines include bulldozing and thrusting. Furthermore, the sediment composition and structure are similar for all the moraines containing recycled diamicton. The remaining major factor influencing the moraine morphology thus seems to be the shape of the glacier front. Due to the distinctive geometry of the Bødalen valley, the former glacier underwent strong changes of its transverse stress field. In the part where the valley was wider, splaying/radial crevasses developed. Further downvalley, where the valley again narrowed, these crevasses were closed. As previous studies have concluded, the change in valley morphology to a large extent explains the distinct morphology of the Bødalen valley moraines. This notion is further corroborated by studying sawtooth or sawtooth-like moraines elsewhere where valley geometry also seems to have been a major contributing factor for moraine morphology.