THE ICE AGE AND ITS WORK. 683 



Jura, and there deposited tlie erratic blocks whicli liad so puzzled 

 the diluvialists to explain. 



Other writers soon followed the clew thus given. In 1835 

 Charpentier, after a close study of the erratic blocks and of their 

 sources, adopted the views of Venetz. Agassiz followed, and by 

 his strenuous advocacy did much to spread correct views as to 

 the former extension of the Alpine glaciers, and their capability 

 of explaining the numerous superficial phenomena which in all 

 northern countries had been thought to afford proofs of enor- 

 mous floods and of the submergence of a large part of Europe un- 

 der a deep sea. He has, therefore, gained the reputation of being 

 the originator of the modern school of glacialists, which undoubt- 

 edly owes much to his energy, research, and powers of exposition, 

 though all the more important facts, as well as the logical con- 

 clusions to be drawn from them, had been pointed out by previ- 

 ous writers. 



Before proceeding further, it will be well to give a brief out- 

 line of the phenomena which lead to the conclusion that gla- 

 ciers have formerly existed in districts and countries where even 

 perpetual snow on the mountain tops is now unknown. These 

 may be briefly classed as 1. Moraines and drifts. 2. Rounded, 

 smoothed, or planed rocks. 3. Striae, grooves, and furrows on 

 rock surfaces. 4. Erratics and perched blocks. 



1. Moraines are those heaps or ridges of rock and other debris 

 which are deposited on the surface of a glacier from the preci- 

 pices or mountain slopes which border it, and which form what are 

 termed lateral and medial moraines while upon it, and terminal 

 moraines when, being gradually discharged at its end, either from 

 above or from beneath it, they form great heaps of rock and 

 gravel corresponding in outline and extent to that of the ter- 

 minal ice-cliff. Such moraines can be seen on and near all exist- 

 ing glaciers, and their mode of formation and characteristics are 

 perfectly well known. If the glacier is continuously retreating, 

 then the terminal moraine will form more or less irregular heaps 

 over the surface the glacier has formerly covered ; but when, as 

 is usually the case, the glacier remains stationary for a consider- 

 able period, then the terminal moraine will have a definite form, 

 . and will often stretch quite across the valley, but presenting one 

 or more openings through which the glacier stream has cut its 

 way. Such moraines form steep mounds, usually curved and 

 often very regular, seeming from a little distance to block up the 

 valley like an artificial earthwork. Among hundreds that might 

 be enumerated, good examples may be seen in Glen Isla (Forfar- 

 shire), in the Troutbeck Valley near Windermere, and in Cwm 

 Glas, on the north side of Snowdon, this latter being so regularly 

 curved, evenly sloped, and level-topped as to look from below 



