166 PosTGLACIAL RAVINES IN THE CHALK WOLDS OF LINCOLNSHIRE. 
border for a breadth of about three miles, covering hill and dale alike 
in one continuous sheet. No traces of these clays have been found 
along the western ridges of the Wolds, though near the eastern 
border they run up to a height of 480 feet, a height which is only 
exceeded by the highest hills near Caistor. 
Whatever was the original extension of these deposits, it may safely 
be assumed that after the glacial conditions had passed away, and 
when more normal climatal conditions had returned, the eastern bor- 
der of the Wolds was completely invested with a mantle of Boulder- 
clay, covering the highest hills as well as filling the deepest valleys. 
From the fact that the streams have in so many instances reoccupied 
the pre-existent valleys, I infer that the surface of this mantle of 
Boulder-clay was by no means level, but that though naturally 
thicker in the hollows than on the hill-tops, it to a great extent 
draped the features of the former surface, and that the rain-streams 
were therefore naturally directed into the hollows of the pre-existent 
valleys. 
It was only, then, at points in these valleys where the drift 
happened to be heaped up in especially massive mounds that the 
stream was forced to take a different course, and to excavate an 
entirely new channel for itself outside the limits of the old valley. 
In every case the waters were probably ponded back, and a lake was 
formed (as at Croxby), the level of which rose till it overflowed the 
lip of the enclosing basin where that lip was lowest. | 
It is probable that in our climate the mechanical wear of rain 
removes the material of clay more rapidly than its chemical action 
removes the material of limestone. In Lincolnshire it certainly 
seems as if the Boulder-clays had wasted more rapidly under pluvial 
influences than the Chalk; consequently the tops of the barriers 
which have caused the diversion of the streams are now in most cases 
lower than the Chalk hills through which the ravines have been cut. 
In the olden days of geology, when cataclysmic ideas prevailed, 
*‘ some convulsion of nature” would have been the simple and easy 
method of accounting for such valleys; and perhaps even at the 
present day more than one observer might regard them as in some 
manner due to the pre-existence of faults and fissures. It may be 
well therefore to state that, though the Chalk is everywhere broken 
by small faults or slips of a few feet throw, such as might be 
caused by the upheaval of a mass resting upon a foundation of 
sand, yet no strong faults were detected anywhere in the neigh- 
bourhood of these valleys; certainly none exist which bring soft 
and hard rocks into contact, for the Chalk is all hard. Further, 
such lines of fracture are seldom or never so serpentine as is the 
course of these ravines. 
It is the simplicity of the general geology of the district which 
makes the alteration in the course of the streams so very striking, 
and enables the geologist to trace the connexion between cause and 
effect with greater ease than in other cases ; so that the ravines 
above described may be regarded as among the most remarkable and 
interesting instances of river-erosion to be met with in England. 
