James Croll — Mechanics of Glaciers. 363 



however, incorrect. The ice filling the hasin and the glacier overlying it are united 

 in one solid mass, so that the latter cannot move over the former without shearing ; 

 and although the resistance to motion offered by the sloping sides of the basin may 

 be much greater than the resistance to shear, still the ice will be slowly dragged out 

 of the basin. However, in order to obviate this objection to which I refer, the 

 advocates of the glacial origin of lake-basins point out that the length of those 

 basins in proportion to their depth is so great that the slope up which the ice has to 

 pass is in reality but small. This no doubt is true of lake-basins in general, but it 

 does not hold universally true. But the theory does not demand that an ice-formed 

 lake-basin cannot have steep sides. We have incontestable evidence that ice will 

 pass up a steep slope ; and, if ice can pass up a steep slope, it can excavate a basin 

 with a steep slope. That ice will pass up a steep slope is proved by the fact that 

 comparatively deep and narrow river- valleys are often found striated across, while 

 hills which stood directly in the path of the ice of the glacial epoch are sometimes 

 found striated upwards from their base to their summit. Some striking examples of 

 strise running up-hill are given by Professor Geikie in his ' Glacial Drift of Scotland.' 

 I have myself seen a slope striated upwards so steep that one could not climb it. 



" A very good example of a river-valley striated across came under my observation 

 during the past summer. The Tay, between Cargill and Stanley (in the centre of 

 the broad plain of Strathmore), has excavated, through the Old Eed Sandstone, a 

 channel between 200 and 300 feet in depth. The channel here runs at right angles 

 to the path taken during the glacial epoch by the great mass of ice coming from the 

 North- West Highlands. At a short distance below Cargill, the trap rising out of 

 the bed of the river is beautifully ice-gr©oved and striated, at right angles to the 

 stream. A trap-dyke, several miles in length, crosses the river about a mile above 

 Stanley, forming a rapid, known as the Linn of Campsie. This dyke is moutonnee 

 and striated from near the Linn up the sloping bank to the level of the surrounding 

 country, showing that the ice must have ascended a gradient of one in seven to a 

 height of 300 feet. 



" From what has been already stated in reference to the resolidifying of the mole- 

 cules in the insterstices of the ice, the application of the molecular theory to the 

 explanation of the effects under consideration will no doubt be apparent. Take the 

 case of the passage of the ice-sheet across a river-valley. As the upper surface of 

 the ice-sheet is constantly receiving heat from the sun and the air in contact with it, 

 there is consequently a transference of heat from above downwards to the bottom of 

 tbe sheet. This transference of heat from molecule to molecule is accompanied 

 by the melting and resolidifying of the successive molecules in the manner already 

 detailed. As the fluid molecules tend to flow into adjoining interstices before 

 solidifying and assuming the crystalline form, the interstices of the ice at the bottom 

 of the valley are constantly being filled by fluid molecules from above. These 

 molecules no sooner enter the interstices than they pass into the crystalline form, 

 and become, of course, separated from their neighbours by fresh interstices, which 

 new interstices become filled by fluid molecules, which, in turn, crystallize, forming 

 fresh interstices, and so on. The ice at the bottom of the valley, so long as this 

 process continues, is constantly receiving fresh additions from above. The ice must 

 therefore expand laterally to make room for these additions, which it must do unless 

 the resistance to lateral expansion be greater than the force exerted by the molecules 

 in crystallizing. But a resistance sufficient to do this must be enormous. The ice 

 at the bottom of the valley cannot expand laterally without passing up the sloping 

 sides. In expanding it will take the path of least resistance, but the path of least 

 resistance will always be on the side of the valley towards which the general mass 

 of the ice above is flowing. 



" It has been shown (Chapter xxvii.) that the ice passing over Strathmore must 

 have been over 2000 feet in thickness. An ice-sheet 2000 feet in thickness exerts 

 on its bed a pressure of upwards of 51 tons per square foot. When we reflect that 

 ice under so enormous a pressure, with grinding materials lying underneath, was 

 forced by irresistible molecular energy up an incline of one in seven, it is not at all 

 surprising that the hard trap should be ground down and striated. 



" We can also understand how the softer portions of the rocky surface over which 

 the ice moved should have been excavated into hollow basins. We have also an 

 explanation of the transport of boulders from a lower to a higher level, for if ice 

 can move from a lower to a higher level, it of course can carry boulders along with 

 it." — Climate and Time, pp. 523 — 527. 



