The Middle Triassic Edderfugledal Member of East Greenland is interpreted as lacustrine because of: (A) nonmarine fossils such as conchostracans and small bivalves, (B) non-marine trace fossils such as Isopodichnus , Cruziana , Pelecypodichnus and Fuersichnus , (C) stromatolites with algal moulds, (D) sedimentary structures, which indicate shallow water, weak to moderately strong wave action and frequent subaerial exposure, (E) absence of tidal current generated structures and (F) great lateral continuity of single beds, which indicate a low topographic gradient in the basin. Six major facies are distinguished within this ancient lake content: (1) green mudstone (open lacustrine), (2) yellow dolostone (carbonate mudflat), (3) flat pebble conglomerate (beach lag or stromatolite breccia), (4) stromatolitic limestone (nearshore lacustrine), (5) greyish wave rippled sandstone (shoreline sandflat) and (6) reddish sandstone and mudstone (alluvial mudflat). The stromatolitic columns, commonly between 5 and 15 cm high, are subcylindrical to hemispheroidal, discrete and laterally linked. They consist of mm-thin couplets of organic-rich micrite laminae with both coccoid and filamentous algal moulds and dolomicrite laminae. The member displays well developed cyclic sedimentation both in the basal open lacustrine unit (the Sporfjeld Beds) and in the overlying shallow lacustrine unit (the Pingel Dal Beds). In the Sporfjeld Beds open lacustrine facies dominate and marginal lacustrine facies are restricted to carbonate mudflats. In the Pingel Dal Beds open lacustrine facies become relatively rare and shoreline sandflats and alluvial mudflats are added to the marginal lake environment. This change in sedimentation is explained mainly in terms of intermittent tectonic uplift. These tectonic events were probably accompanied by a gradual change towards more humid conditions during the Middle Triassic.