CERTAIN" RIVER- VALLEYS INT LINCOLNSHIRE. 609 



valley to the Wash, the Idle ran via the present Trent valley north 

 of Gainsborough to the Huniber. The watershed between the 

 Trent and the Idle was then continuous, but was of course very low 

 where it crossed the Keuper marls by Tuxford, Marnham, South 

 Clifton, and Harby. 



The rills draining the northern slopes of this low watershed would 

 naturally be collected into a stream running northwards along the 

 valley subsequently usurped by the Trent, and falling into the Idle 

 somewhere north of Gainsborough. In these early Postglacial times, 

 the elevation of the land above the sea was probably higher b}>- 

 40 or 50 feet than it is now : the rivers consequently had a greater 

 fall, and erosion went on much more rapidly over the whole surface 

 of the country. The sources of this tributary of the Idle would be 

 carried back further and further southward, and the longitudinal 

 valley would be gradually extended and lowered until merely a low 

 col separated its head-waters from the slopes of the valley in which 

 the Trent was then running north-eastward to Lincoln. 



At this epoch in the history of the Trent, floods must have been 

 of frequent occurrence in this particular portion of its valley, owing 

 to the narrowness of the outlet at Lincoln ; and the large extent of 

 ground covered by its ancient gravels between Newark and Lincoln 

 testifies to the width of the area over which its flood-waters were 

 able to spread. Under such conditions, therefore, and on some 

 occasion when a large head of water was accumulated in the Trent 

 valley, it is not unlikely that an overflow would take place by 

 way of the low col leading into the aforesaid tributary of the Idle. 

 Every flood would then deepen the passage until the full stream of 

 the river found it easier to take this course than to maintain its 

 former channel through the Lincoln gap. 



The exact situation of this col is of course difficult to fix, though 

 some evidence may be obtained from the disposition of the gravels, 

 when these are completely mapped. 



There is also some reason to believe that there has been a slight 

 elevation of the high ground occupied by the outcrop of the Lower 

 Oolites east of the great escarpment in Postglacial times. The 

 valley in which Ancaster lies is a transverse cut through this high 

 ground, and is paved with deposits of sand and gravel, which are 

 continuous with those of the Witham valley north of Grantham ; and 

 it appears probable that the Witham passed through this valley at 

 the time when the Trent flowed through the Lincoln valley. 



Now, however, a low watershed crosses the Ancaster valley, just 

 west of the town, throwing off the Honington Beck to the west, and 

 the Sleaford Beck to the east ; and this watershed occurs in the 

 midst of a broad tract of stratified gravel, which must have been 

 deposited there before the watershed was formed. 



Eurther, the section in the railway -cutting west of Ancaster 

 station shows that the Lower Oolites (Lincolnshire Limestone and 

 Lower Estuarine series) are here bent into a low and broad anti- 

 clinal curve, the axis of which seems to strike nearly due north and 

 south. 



