160 A. J. JUKES-BROWNE ON SOME POSTGLACIAL RAVINES 
7. On some PostenactaL Ravines in the Cuatx Woxps of Lixcorn- 
sHirE. By A. J. Juxes-Browne, Esq., B.A., F.G.S. (Read 
December 5, 1883.) 
[Communicated by permission of the Director-General of the Geological 
Survey. | 
In a paper recently read before this Society I pointed out that the 
disposition of the Boulder-clays (Purple and Hessle series) along the 
eastern border of the Chalk Wolds enables us to determine the rela- 
tive age of the valleys which intersect the hills, and proves that some 
of them are older and some newer than the formation of those Clays. 
Most of the larger and wider valleys are occupied by deposits of 
glacial clay and gravel, which are continuous with the main mass or 
sheet of these beds lying to the eastward, and are, in fact, mere arms 
or prolongations of that sheet ; it is clear therefore thatallsuch valleys 
are of anterior date and must have existed prior to the formation of 
the East-Lincolnshire Boulder-clays. It would not be safe, however, 
to call them Preglacial, because these particular Boulder-clays are 
believed to be the latest glacial beds in England, and therefore the 
valleys may be really of Interglacial age. On the other hand such 
valleys or portions of valleys as can be proved to have come into 
existence after the formation of these Clays may safely be termed 
Postglacial in the most rigid acceptation of that term, and may 
consequently be regarded as some of the most recently formed valleys 
in the British Isles. 
There are several short valleys, from one to three miles in length, 
which open eastward on to the Boulder-clay plain, which are probably 
of Postglacial date ; for the boundary line of the Boulder-clay passes 
straight across their mouths ; no tongues of clay are thrust westward 
into these valleys, which are completely free from any trace of glacial 
deposits. Some of these valleys are narrow and ravine-like, but 
others are wider, and in the latter case there is nothing about them 
which would distinguish them from the older valleys occupied by 
glacial deposits. 
Much more interesting cases occur in the course of some of the 
older valleys where the original watercourse has been so blocked up 
with drift-deposits that, in the process of reexcavation, the stream 
has found it easier to cut anew channel for itself through the solid 
chalk than to remove the obstacle in front ofit. Itis some of these 
cases which I propose to describe in the following notes. 
1. Swaby Vale, near Alford.—The first instance I observed was in 
the course of a tributary to the Calceby Beck (see map and descrip- 
tion in Q. J. G. 8. xxxix. p. 600). The valley of the Calceby Beck 
is filled from end to end with glacial deposits, through which the 
modern stream finds its way without anywhere cutting down to its 
ancient bed in the Chalk. Near the village of Swaby, on the N.W. 
side of this valley, there are large mounds of glacial clay and gravel 
which completely block up the course of a tributary coming in from 
