J. Q. Goodchild — On the Origin of Coums. 493 



present open to us. As, in the present state of science, we have no 

 means of discovering what is actually taking place in and beneath 

 the ice of a great continental mass like that of the ice-sheet of 

 Greenland, we are compelled to rely upon the data supplied by the 

 glaciated rock surfaces left by the ice of the European Ice-sheet 

 Period, and to supplement these by the data, less satisfactory in 

 some respects, that are afforded by the puny descendants of the Ice- 

 sheet of Mid and Northern Europe. Even in the Arctic regions we 

 should hardly expect to meet with ice in a state precisely similar to 

 what obtained during the Ice-sheet Period in England. Wherever 

 perennial ice has lingered from the Glacial Period to the present 

 day, the erosion of the old weathered surface by the ice must be 

 greater in proportion to the length of time that has elapsed since 

 the last traces of the Ice-sheet left these parts. When the Ice-sheet 

 left the North of England, it is highly probable that there was much 

 weathered rock left to furnish the materials for more drift ; but in 

 the glacier regions of the present day the ice has accomplished much 

 more, and there is probably little else than perfectly sound rock left 

 for the ice to erode. 



When we examine any large extent of nearly flat glaciated 

 surface lying near the bottom of a deep valley, we usually find 

 that most of the striae run for considerable distances without any 

 great deviation and without interruption, and that the larger 

 grooves, even those of an inch or more in width, are ploughed out 

 in lines paralled to those of the finer scratches around. As a rule, 

 these larger grooves bear no necessaiy relation to any structural 

 planes in the rock, and they bear every appearance of having been 

 produced at one operation by the steady grinding of the rock by 

 the slow onward movement of the sole of the Ice-sheet armed with 

 bigger stones than those that produced the adjacent finer scratches. 

 As scratches of this character occur along the whole length of the 

 bottoms of valleys, right up to the source, we need no other evidence 

 to convince us that the very lowest strata of the old glaciers were 

 impelled forward as well as the higher strata were ; and that, even 

 at a considerable depth from the surface, grooves of large size could 

 be made at one operation, although the absolute rate of flow of the 

 sole of the Ice-sheet may have been so slow as to be almost imper- 

 ceptible if it could have been tried by any of our most carefully 

 constructed modern instruments. When it is remembered that 

 a sheet of ice " 1000 feet in thickness has a pressure on its rocky 

 bed equal to about 25 tons on the square foot," 1 and that the actual 

 thickness of the ice in the Yorkshire Dale District equalled, and 

 in places exceeded 1500 feet, the amount of erosion accomplished 

 during the whole of the Glacial Period may have been something con- 

 siderable. It is a very noteworthy point that where a large branch 

 valley joins the main valley, the main valley striae are more or less 

 deflected from their general parallelism with the larger contours 

 of the part where they occur ; and, what is of still greater impor- 



1 J. Croll, On Geological Time and the Probable Date of the Glacial and the 

 Upper Miocene Period, Phil. Mag. Nov. 1868. 



DECADE II.— YOL. II. NO X. 32 



