Ice-dammed lakes of Würm age occupied pre-existing valleys in the Subalpine ranges of southeastern France. One of these, Lac du Triéves, accumulated over 200 m of predominantly clayey sediments in the form of varved and laminated deposits. The clay mineralogy of these sediments consists of the following suite: mica (65 to 85 percent), iron-rich chlorite (10 to 30 percent), and small amounts of poorly crystallized mixed-layer material. Bedrock lithologies in the vicinity of the lake vary from Jurassic and Cretaceous shale, silty shale, and carbonate rocks to Carboniferous meta-sedimentary rocks and Hercynian basement rocks. Jurassic shale, especially of the “terres noires” facies, consists primarily of an allevardite-type, regularly interstratified mica-montmorillonite and in some places directly underlies lacustrine sediments. The lack of incorporation of this material in lake clay indicates a relatively low surface erosion rate during Würm maximum glaciation and suggests that mechanical and possibly geochemical alteration of metamorphic mica derived from the cirque areas accounts for the bulk of the lake clay formed.