60 Mr Marr, The Meres of Breckland. 
Fowlmere. Again mechanical erosion of the watercourse above 
the mere would widen the bed there, while that of the deserted 
stream below the mere would be no longer lowered, and a larger 
area to be filled with water would be thus produced. In other 
cases, direct collapse of the roofs of underground caves might — 
initiate meres. 
In hollows of other districts bearing some resemblances to 
those of Breckland, the upward pressure of artesian waters due 
to erosion of overlying i impervious deposits has been regarded as 
the cause of formation of the hollows, ‘The conditions in Breckland 
do not seem suitable for such occurrence. 
The existence of deserted mere-hollows down-stream near 
Roudham Junction is of importance. At the time of their 
formation, according to the above views, the junction of glacial 
clays and chalk would be situated near these sites, it having 
been gradually driven up-stream to its present position by 
erosion. 
When the glacial clay junction was near these hollows, their 
underground drainage would be partly blocked by deposit of clay 
on their floors. As the junction between clay and chalk was 
shifted up valley, the supply of clay would be stopped in the 
lower meres, being now deposited in newly formed upper ones. 
Solution would now proceed unchecked until a free underground 
passage for all water draining into the hollows would be established, 
and the meres would then be permanently dry. 
In connexion with this question, however, is that of the mode 
of infilling of the meres, and here we find conflicting views; one, 
that they are filled by the rise of the saturation-level of under- 
ground water, the other that they are fed by surface streams. 
(See W. G. Clarke, loc. cit.) If the latter, the supply of water 
would cease when the glacial clay was denuded. 
As to the age of the meres, they are clearly post-glacial in 
the sense that they were formed after the accumulation of the 
boulder-clay of the district. They may have been formed at 
various subsequent periods, and are probably still in process of 
formation. One of the meres at Wretham was drained and its 
floor dug out, and the results were described by Mr C. J. F. 
(afterwards Sir Charles) Bunbury (Quart. Journ. Geol. Soc. XI. 
1856, p. 355). Over twenty feet of black peaty mud was found 
resting on a light grey sandy marl. In the mud were a number 
of horns of the red deer, which had been sawn off just above the 
brow-antlers. In connexion with this discovery Mr Skertchley 
(Geol. Survey Mem. Geology of the Fenland, p. 248), refers to 
the horns used for picks in the flint-mines at Grimes Graves. 
These mines have been hitherto referred to the Neolithic Period, 
but Mr Reginald Smith has recently advocated their assignment 
