26 



THE FARMER'S MAGAZINE. 



floating with five-sixths of their bulk submerged, 

 must frcqueutly touch the grouud, aud, grinding over 

 its rocky bed, must produce effects very similar to 

 those produced by the descent of a glacier down its 

 valley. Or it may be produced by masses of shore 

 ice, like those of the arctic regions, advancing over 

 slowly-sinking laud. Glaciers, it will be remembered, 

 bear on their surface large and small fragments of 

 stone, which the frost has detached from the bound- 

 ing rocks. In the slow descent of the glacier, some 

 of these are deposited on its sides, where they form 

 what arc called lateral moraines ; but the more im- 

 portant accumulations of this kind ai'e the terminal 

 moraines, which are formed where the glacier reaches 

 a point having an elevation of not more than 3,000 

 or 4,000 feet above the sea, where the temperature 

 is such that it can advance no further, but melts and 

 deposits a confused heap of boulders, gravel, and 

 mud. The mud is formed by the grinding of the gla- 

 cier over the rocks; the water which escapes from be- 

 neath a glacier is charged with it. Hence the rivers 

 which descend from valleys occupied by glaciers are 

 rendered turbid during the summer by the melting 

 of the ice. It is shown by the position of most of 

 these terminal moraines in the Alps that the glaciers 

 had formerly a greater prolongation into the valleys. 

 Their fluctuating limits arc well known, as it is on 

 record that in the seventeenth and eighteenth cen- 

 turies they began to reach points to which they had 

 not extended in the eleventh and fifteenth centuries. 



The extension of a glacier down a valley not unfre- 

 quently dams up the river of a transverse valley, and 

 forms what is called a glacier-lake. When the icy 

 barrier Inirsts, the descending torrents disperse the 

 terminal moraines in the main valley, and re-arrange 

 their materials in a rudely stratified mass, not un- 

 like some portions of the erratic tertiaries. The 

 grooving, polishing, and scratching of the rocks 

 beneath them, is another feature common to the ter- 

 restrial glaciers and the erratic tertiaries. The 

 surface of the rocks beneath a glacier is grooved, 

 polished, and scratched in the same manner as the 

 surface of the rocks beneath the boulder clay of the 

 erratic tertiaries. The furrows on the rocks be- 

 neath the glaciers are produced by large blocks 

 frozen into the under edge of the ice ; the sand be- 

 tween the glacier and the rock polishes the surface 

 of the latter, and the small pebbles set in the ice 

 produce scratches and striae. 



The blocks which the deposits of glaciers and the 

 glacio- marine erratic tcrt'aries contain are also 

 polished and scratched in a similar manner by the 

 same processes. This appearance is frequently 

 manifested only on one side, which has ground over 

 the rock in the descent of the glacier ; while the 

 other, which has been imbedded m the ice, is sharp 

 and uuabraded. A remarkable difference, however, 

 must be noticed in these two very similar glacial de- 

 posits. The boulder clay of the erratic tertiaries 

 contains marine shells, and exhibits a much greater 

 amount of stratification. It is a common mistake to 

 represent the boulder clay as unstratifled. This 

 arises from a crust of earth washed down by the rain, 

 which generally covers the face of a cliff of this clay, 

 and conceals the stratification. When, however, 

 a clean section is obtained, distinct marks of strati- 

 fication may be very generally observed. There are 



also uustratified heaps of transported materials in 

 the boulder clay. In Norfolk, these generally con- 

 sist of chalk and Kimmeridge clay, which appear 

 to have been shot down in separate heaps, as we 

 might suppose them to have been, from rafts 

 of melting ice. On the flanks, however, of these 

 masses, we find the same materials finely comminuted 

 and re-arranged in a stratified form. This may be 

 attributed to the action of the sea washing on these 

 heaps of ice -transported matter, by which their outer 

 portions have been re-arranged. Another distinction 

 between the materials of moraines, re-arranged by 

 the bursting of glacier lakes, and the boulder clay, 

 it is usual to represent the latter as destitute of 

 shells, except in Norfolk, where they are supposed 

 to have been washed out of the crag. In that dis- 

 trict there may be some ambiguity arising from this 

 cause ; but we have seen the same kinds of broken 

 shells in the boulder clay in so many parts of Wales, 

 liolderness, and Ireland, that we must regard that 

 deposit as generally but irregularly fossiliferous. 

 The general rarity of shells, and the paucity of spe- 

 cies, is another fact in accordance with the arctic 

 character of these erratic deposits. 



Passing, however, from this digression on the 

 points of distinction between giacio-mariue deposits 

 of the erratic tertiaries and the detrital deposits of 

 glaciers.on the land, let us proceed to the conside- 

 ration of the transport of erratic blocks outwards 

 from the central region of the Alps. 



The great valley of Switzerland, which separates 

 the Alps from the Jura, is fifty miles broad, and the 

 average height of the Jura is about one-third that of 

 the Alps ; but everywhere on that chain, as well as 

 on the higher regions of the Alps, where there are 

 no glaciers at present, all the indications of former 

 glacial action are observable, such as moraines and 

 rocks with polished and rounded, grooved, and stri- 

 ated, surfaces. These appearances are likewise ob- 

 servable far below the present limits of the glaciers. 

 Immense blocks of granite, gneiss, &c., which can 

 have come from nowhere but the Alps, are lodged on 

 the flanks of the Jura, to which they must have tra- 

 velled a distance of fifty miles, across one of the 

 deepest valleys in the world. These travelled blocks 

 are of immense size, some of them containing 50,000, 

 G0,000, and one as much as 1 61,000 cubic feet. 

 From the mineral composition of these transferred 

 blocks, it is easy to determine the parts of the Alps 

 from which they have been derived ; and it appears 

 that they have crossed the valley in a direction 

 nearly at right angles to its length. These facts 

 are strongly opposed to the supposition of their 

 having been transported by currents of water. Pre- 

 ceding naturalists had supposed the ghciers of the 

 separate valleys of the Alps to have had an extension 

 to the Jura, when these blocks were transported. 

 Agassiz, on the contrary, supposed the whole valley 

 to have teen filled with ice, which extended in a con-: 

 tinuous mass from the Alps to the Jura. Others 

 suppose the blocks to have been transported on float- 

 ing ice, when the greater part of the chain aud the 

 whole of the Jura were under water. The absence 

 from the valley of Switzerland of marine deposits of 

 tJie period when these blocks must have been trans- 

 ported apfiears a formidable objection to this expla- 

 nation. 



