Preston: Geology South of Grantham. 251 
angle now forming Grantham, and a short valley had been 
formed by the tributary stream. Such overflow springs rising 
on the east side of the valley may be seen at the present day, 
but in consequence of the N.W. to S.E. dip of the rocks, these 
springs only flow when the line of saturation of the limestone is 
reached, and have comparatively very little action in cutting 
tributary valleys. An examination of Geological Sheet 7o will 
show that the springs rising on the west of the Witham are 
much the more powerful and have formed tributary valleys at 
Stroxton, Wyville, and Skillington, whilst on the east side such 
springs as do occur have scarcely been at work long enough to 
be indicated on the map. 
After the Glacial period the whole of this limestone plateau 
at least was covered with Boulder Clay, and doubtless the 
which the rains and melting snow and ice would find a ready 
descent into the old valley, whilst the original spring would 
reassert itself and help to clear away the Boulder Clay which 
had blocked up its course. As time went on this spring would 
gradually cut back its head; rains and frosts would help to 
widen its banks; tributary streams from the west would 
augment the volume of its water; and thus the present valley, 
extending from Grantham to North Witham, would be formed. 
The colouring of the map shows that the stream has cut 
through the Boulder Clay, Lincolnshire Limestone, and 
Grantham to a point a little south of North Witham, but here 
denudation has not had time to work through the limestone, so 
that we may look upon this valley as still in process of growth. 
Whether the river will ever be much larger than at present 
depends upon what lies beneath the Boulder Clay to the west 
and south-west of the present valley. According to indications 
shown at the northern edge of this clay, the limestone does not — 
extend in these directions, for the outcropping of Northampton 
Sand and Upper Lias Clay along this line seem to show that 
these older rocks lie immediately beneath the drift and limit the 
present watershed. Also, it will be seen from the map that 
several areas of limestone to the west of the river are nearly 
surrounded by Upper Lias Clay, in fact the whole area’ north of 
Buckminster bounded by the river valley on the east and 
extending as far as Waltham-on-the-Wold on the west, an area — 
of fete Sr) like 30 is cate miles, is one great outlier of lime- 
