§78 /. Croll. — Boulder-clay of Caithness. 



depth of more than 1,600 feet. The hill is situated about four or five 

 miles to the south of Edinburgh, and forms, as has already been 

 stated, the northern termination of the Pentland range. Immediately 

 to the north lies the broad valley of the Firth of Forth, more than 

 tvs^elve miles across, offering a most free and unobstructed outlet for 

 the great mass of ice coming along the Midland valley from the 

 west. Now, when we reflect how easily ice can accommodate itself 

 to the inequalities of the channel along which it moves, how it can 

 turn to the right hand or to the left, so as to find for itself the path 

 of least resistance, it becomes obvious that the ice never would have 

 gone over Allermuir had not only the Midland valley at this point, 

 but also the whole surrounding country been covered with one con- 

 tinuous mass of ice to a depth of more than 1,600 feet. But it 

 must not be supposed that the height of Allermuir represents the 

 thickness of the ice ; for on ascending Scald Law, a hill four miles 

 to the south-west of Allermuir, and the highest of the Pentland 

 range, we found, in the debris covering its summit, hundreds of 

 transport-ed stones of all sizes, from one to eighteen inches in 

 diameter. One of them, a Greenstone boulder about eighteen inches 

 in diameter, which we dug up, was finely polished and striated. 

 The height of the hill is 1,898 feet, consequently the thickness of 

 the mass of ice covering the surrounding country, must have been 

 at least 1,900 feet. How much thicker it was than this I have 

 not as yet had any opportunity of ascertaining. But this is not 

 all. Directly to the north of the Pentlands, in a line nearly parallel 

 with tbe east coast, and at right angles to the path of ice from the 

 interior, there is not, with the exception of the solitary peak of East 

 Lomond, and a low hill or two of the Sidlaw range, an eminence 

 worthy of the name of a hill nearer than the Grampians to the north 

 of Forfarshire, distant upwards of 60 miles. This broad plain, ex- 

 tending from almost the Southern Highlands to the Northern, was 

 the great channel through which the ice of the interior of Scotland 

 found an outlet into the North Sea. If the depth of the ice in the 

 Firth of Forth, which forms the southern side of this broad hollow, 

 was at least 1,900 feet, it is not at all probable that its depth in the 

 northern side, formed by the Valley of Strathmore and the Firth of 

 Tay, and wLich lay more directly in the path of the ice from the North 

 Highlands, could have been less. Here we have one vast glacier, 

 more than sixty miles broad and 1,900 feet thick, coming from the 

 interior of a country not much over sixty miles distant. Such a 

 breadth and thickness of a glacier, proceeding from so small an area 

 of drainage, is inconceivable upon any other hypothesis than that 

 it was dammed back by land-ice from Scandinavia occupying the 

 North Sea. 



[Note. — The figures indicating the ^5° of latitude, on the right side 

 of Plate X. are misplaced ; they should have been attached to the line 

 beneath. The line against which they are placed is intended to 

 indicate the course of the ice across the Faroe Islands. Also the 

 curved lines, indicating the path of the ice from Scandinavia, should 

 have extended down to at least latitude 55'^. — J. C] 



