452 
preserved when constructed of porous materials, or when built on 
porous strata. 
Some of these remarkable gravel hills reminded me of descriptions 
of Eskers, as well as of some I had been shown by Mr. T. V. Holmes 
on the gravelly plain west of Carlisle. Eskers, whose origin is a 
much-disputed question, are thought by Dr. James Geikie to have 
been heaped up principally by the action of sub-glacial waters 
during the melting of an ice-sheet. Having noticed the resem- 
blance of some of our tiny gravel hills to tumuli, I was much 
interested to read in a paper recently published by Mr. Holmes, 
that “ Esker mounds are often so extremely like artificial 
barrows as to be a very possible source of disappointment to 
the antiquary.” 
On impervious strata the sources of rivers may be almost entirely 
above ground ; and as the water-parting that separates streams 
running in opposite directions becomes reduced, we can conceive 
how two may rise near together, as is the case with the Little Ouse 
and the Waveney at Lopham Ford. Streams must often commence 
in this way underground. And tliis reminds me of the streams on 
either side of the high street at Chard in Somerset; one flows into 
the Bristol Channel as the Parrett, the other into the English 
Channel as the Axe. Even in the case of the Wensuni Valley, 
when Ave follow it up beyond West Eudham, towards Harpley, 
where the streamlets disappear underground, Ave may, Avhile 
gradually passing over the Avatershed, proceed in a kind of valley 
until Ave descend into that of a little stream that flows into the 
Wash near Castle Rising. 
Other cases of subterranean denudation occur in the sudden 
sinkings of ground Avhere “pip^s” have been formed in the Clialk 
(as previously mentioned), and the strata after a time “ cave in,” — 
I use the term in its original sense. Some such origin our meres 
may have had, and these I have had the advantage of seeing under 
the guidance of Mr. E. J. Bennett. They occur in a country Avhero 
Chalk is covered Avith sand and boulder clay. Granting tliat a 
huge pipe had been formed in the Chalk, then the Avashing in of 
boulder clay may liave “tamped” the holloAv, in much the same 
Avay as some of our ponds are rendered water-tight. 
* Trans. Cumberland Assoc, part vi, 1882. 
