400 Proceedings of the Royal Society 
elusive in the Spey to show that these terraces were not old sea 
beaches nor lake margins, but the fluviatile deposits of some former 
epoch when the floods rose to a greater height. The problem then 
came to be, In what way are we to explain the action of the river 
in throwing up deposits 60, 80 feet, or even more above its bed ? 
There are two ways, in one or other of which this may be accounted 
for, — either by supposing the river bed to have lain on its present 
level, and allowing rainfall sufficient to flood the channels up to the 
requisite height ; or by supposing the bed of the stream to have been 
formerly at a higher level, and that, after forming the terraces, the 
current had excavated its bed down to where it now is. It is the 
second of these views which has found most favour among geologists, 
and various suggestions have been offered as to how the bed of the 
stream was formerly elevated. 
One explanation is, that at the time of the highest terrace, the 
line of the valley, then comparatively shallow, was occupied by the 
original rock, still to a great extent in situ. In regard to our 
Scottish valleys this explanation is inadmissible. It was formerly 
shown, from the position of the boulder clay,* that the rocky struc- 
ture of these river-courses had been hollowed out nearly as deep as 
now previously to the formation of the terraces ; but apart from 
the Boulder clay the terraces themselves, as will be shown, prove 
the same thing, for example, the 70 feet terrace at Kingussie. 
Another explanation is, that during the last submergence of 
Scotland the valleys had been filled by marine gravels, &c., and 
that the river bed had been thus lifted to the requisite height. 
This view, however, must also be set aside, because after that sub- 
mergence, the valleys of Scotland were occupied by glaciers, which 
must to a great extent have cleared out these previous marine 
deposits.! Especially must this have taken place in Strathspey, 
lying so high above the sea, and connected with the central moun- 
tain-masses of the country. The glacier must have ploughed out 
the marine debris. It was after that the terraces were formed. 
There is a third suggestion, that the river had raised itself on 
its own alluvium, formed the terraces, and then re-excavated its 
* Trans. Roy. Soc. Ed., vol. xxvi., 171. 
t Sir C. Lyell’s Antiquity of Man, p. 206. Scenery of Scotland, by Mr 
Geikie, p. 847 
