Southern Britain tells of a similar sequence, which we may interpret as follows. Belonging either to the temperate climate of early Mousterian times, or to the period of the recession of the fourth glaciation, known in the Alps as the _Laufenschwankung_, are the Mousterian stations along the Lea and near the mouth of the Thames at Crayford (Worthington Smith,(14) Geikie(15)). These Palæolithic 'floors' of Mousterian times are buried beneath 4 to 5 feet of sand and loam and rest upon the surface of older river-gravels. Among the later river deposits several old land surfaces have been discovered; they consist of a few inches of angular gravel, crowded in places with unabraded implements and flakes which obviously occur just where they were left by Palæolithic workmen. At one point there is evidence that the flint maker squatted over his work, with his knees slightly apart, for the chips are thrown to the right and left in small piles. Here and there, mixed with these Mousterian implements, are more archaic forms which may have been drifted down from the older land surfaces above. [Illustration: FIG. 99. Excavations of the Mousterian culture layer under the cliff of Le Moustier. Photograph by N. C. Nelson.] One such floor has been traced by Worthington Smith(16) through Middlesex and on both sides of the Thames. Plant remains occur plentifully on this old land surface, including impressions of portions of leaves, stems of grass, rushes, and sedges. The birch, alder, pine, yew, elm, and hazel have been recognized. The common male fern is of frequent occurrence, while the royal fern (_Osmunda regalis_) is found in profusion. Upon the whole, this assemblage of plants indicates a temperate climate. The flints described and figured by Worthington Smith are either of the late Acheulean 'Levallois flake' type or else of early Mousterian age. This writer(17) notes the great number of instruments known as trimmed flakes, which are found on the Palæolithic 'floor'; these are flakes of large size, trimmed to an implement-like form on one side, while the other side is left perfectly plain; the examples are remarkably constant to one form. The type of implement here described resembles the flakes of Levallois or Combe-Capelle, or even the typical 'point' from Le Moustier. Such flakes, shaped into the Mousterian forms of racloir, or scraper, are very common in the gravels of the Lea and of the Thames.