- Parr IL. Szor. ii. §5.] EROSION OF ROCK-BASINS. 417 
unless, indeed, they chance to have been already filled up with glacial 
rubbish. 
It is now some years since Professor A. C. Ramsay drew 
attention to this peculiar power of land-ice, and affirmed that the 
abundance of exeavated rock-basins in Northern Europe and America 
was due to the fact that these regions had been extensively eroded 
by sheets of land-ice,! when the more northern parts of the two 
continents were in a condition like that of North Greenland 
at the present day. It is among the ice-fields of Greenland rather 
than among the valley-glaciers of isolated mountain groups that 
the operations which produced the widespread general glaciation of 
the period of the rock-basins find their nearest modern analogies. 

Fic. 154.—Srriatep SToNE FRoM BovuLDER CLAY. 
A single valley-glacier retires towards its parent snow-field as the 
climate ameliorates, leaving its roches moutonnées, moraine-mounds, 
and rock-basins, yet at times discharging its water-drainage in such 
a way as to sweep down the moraine-mounds, fill up the basins, bury 
the ice-worn hummocks of rock, and strew the valley with gravel, 
earth, sand, and big blocks of rock. Hence the actual floor of the 
glacier is apt to be obseured. But in the case of a vast sheet of 
Jand-ice covering continuously a wide region, there can. be but little 
superficial débris. When such a mass of ice retires it must leave 
behind it an ice-worn surface of country more or less strewn with 
the detritus which accumulated under the ice and was pushed along 
by it. This infra-glacial débris forms the Grundmordne (moraine 
profonde) or bottom moraine above referred to (p. 411). We know 
as yet very little regarding its formation in Greenland. Most of 
our knowledge regarding it is derived from a study of the till or 
_ boulder-elay in more southern latitudes, which is believed to re- 
present the bottom moraine of an ancient ice-sheet. In countries 
where true boulder-clay occurs, numerous rock-basins are commonly 
to be met with among the uncovered portions of the rocks. These 
and other features of glaciated Europe and America will be more 
fully described in the account of the Glacial Period (Book VI.).’ 
 1Q. J. Geol. Soc. xviii. (1862), p. 185. See also a paper by A. Helland (op. cit. 
xxxiii. p. 142), on the ice-fjords of North Greenland, and the formation of Fjords, Lakes, 
and Cirques. 

2 See the remarks already made (p. 338) on the possibility of the rotting out of 
25 
