THE COURSE of the River Bain, the longest stream in the Lincolnshire Wolds, is a curious one. In general, this river flows southward for twenty-two miles from its source in the central part of the Chalk Wolds to join the River Witham near Tattershall, and the most striking feature is the way in which it transects obliquely the various Lower Cretaceous rocks that outcrop along the western part of the Wolds. Thus from its source on the Lower Chalk west of Ludford, it successively crosses the Red Chalk, Carstone, Upper Tealby Clay, Tealby Limestone, Lower Tealby Clay, Claxby Ironstone and Spilsby Sandstone to pass on to the Kimmeridge Clay at Donington-on-Bain. From this village to Hemingby, it flows over the Kimmeridge Clay, but the outcrop of the latter is never more than one mile wide. The divisions of the Cretaceous rocks indicated in Figure I are more generalized than those described by the Geological Survey,1 but the outcrops of these rocks and the areas of superficial deposits were mapped by the author in the field on relevant sheets of the Ordnance Survey I:25,ooo map between 1955 and 1959. The differentiation of the glacial drifts is a further result of this fieldwork, though it was clearly anticipated in the descriptions of the deposits contained in the various Sheet Memoirs of the Geological Survey, and in the writings of A. J. Jukes-Browne.2 Influenced by the rocks and drifts over which the river flows, the Bain valley shows great variety of form (Fig. 2). The uppermost part of the Bain occupies a broad shallow valley, directed generally eastward, flanked by remnants of the highest planation surface to be identified in the Wolds, the High Street-Bluestone Surface.3 Below Wykeham, the river turns south across the upper members of the Lower Cretaceous until the Lower Tealby Clay is exposed in the valley floor near Girsby. Here the eastern valley side is developed in glacial drift, and is steeper and less broken than the western side, where two tributary streams enter the Bain. At Biscathorpe, the river turns sharply east to collect Grimblethorpe Beck, which occupies a fairly deep valley trending north-east to south-west and cut into the Chalk, Carstone and Tealby Beds. This tributary in turn receives, from the south-east, Gayton Beck which is etched deeply into the High Street-Bluestone Surface along the strike of the Chalk. South of Biscathorpe, the middle section of the Bain passes into a broad vale, the east side of which is formed by both Lower and Upper Cretaceous rocks rising in a bold but fretted escarpment. The western side, however, is part of a ridge that is largely composed of glacial till overlying gravel, resting on the lower members of the Lower Cretaceous. The floor of the vale south-south-east of Donington is composed mainly of gently inclined plateaux of Spilsby Sandstone, to the west of which the Bain has excavated a relatively deep asymmetrical trench into the Kimmeridge Clay. Towards Goulceby, this vale is obstructed by Colley Hill, an outlier of Tealby Limestone capped by highly weathered gravelly drift, west of which the