CEETAEN" RIVER-VALLEYS IN LINCOLNSHIRE. 603 



broad plain between two of the steep hill-ridges into which the 

 chalk escarpment is here cut up. This plain now forms the water- 

 shed between the Calceby and Steeping valleys ; its centre is occupied 

 by a broad strip of peat and alluvium, the western end of which 

 drains into the Tetford Beck and the eastern end into the Ormsby 

 Beck. This plain only exists because the ancient valley is choked 

 up with glacial sand and gravel ;' and it is perfectly clear that if 

 these accumulations were removed, the Tetford brook would continue 

 its easterly course into the Ormsby brook, and so into the Calceby 

 valley. Indeed, the strip of alluvium indicates that this was the 

 course of the brook up to a very recent period (geologically speaking), 

 and thac it was only deserted when an easier exit was found by 

 the present channel. 



The cause of this diversion now remains to be considered, but is 

 not far to seek. The broad ridge of Neocomian sandstone which 

 lies to the south of Tetford is traversed by a deep and narrow trench, 

 and through this the brook now escapes from the upper plain, and 

 descends to the lower level of the Steeping valley. It could not 

 have done this until the spring-heads of the Steeping had receded 

 to their present position. 



It is most probable that this trench was originally formed by a 

 small tributary of the Tetford brook., draining the district to the 

 southward and running northward to join it below Tetford. The 

 drainage-basin of this tributary was gradually invaded acid sapped 

 by the recession of the spring-heads on its southern border, until 

 it ceased to convey any water into the Tetford valley and its northern 

 portion would remain as a dry trench. As the combined action of 

 rain and springs carried the head of the Steeping valley further and 

 further back, they worked down to a lower base-line than that of 

 the Tetford valley, and the Kimmeridge Clay was gradually bared 

 along the course of this dry trench. The strong springs which now 

 issue from the base of the sandstone in this trench near Somersby, 

 show how the work was done. Eventually when the country had 

 assumed its present configuration, and probably when the Tetford 

 brook happened to be in flood, the waters overflowed from the Tet- 

 ford valley along this trench into that of the Steeping; and when 

 this communication was once established it would be maintained, 

 because, being cut down to a lower base-line, the fall along the new 

 channel is much greater than that along the old one. 



If this was the manner in which the Tetford brook became part 

 of the Steeping-river system, it is very likely that other streams 

 may originally have drained into the Calceby valley, and have been 

 diverted in the same manner. 



The breadth of the valley in which Brinkhill stands, its occupa- 

 tion by sand and gravel which is continuous with the drift of the 

 Calceby valley, and its abrupt termination, are all facts suggestive 

 of its once having had a longer extension to the south-west. In all 

 probability the beck which rises near Warden Hill, and now flows 

 southward through Harrington Carr, was originally a tributary of 

 a stream flowing N. E. down the Brinkhill valley into the Calceby 



