88 PARALLEL ROADS OF GLEN ROY. [Cn. YH 



accounting more easily for the temporary existence and entire disappear- 

 ance of lofty transverse barriers, although the height required for the im- 

 aginary dams of ice may be startling. 



Before the idea last alluded to had been entertained, Mr. Darwin examined 

 Glen Eoy, and came to the opinion that the shelves were formed when the 

 glens were still arms of the sea, and consequently, that there never were 

 any seaward barriers. According to him, the land emerged during a slow 

 and uniform upward movement, like that now experienced throughout a 

 large part of Sw^eden and Finland ; but there were certain pauses in the 

 upheaving process, at which times the waters of the sea remained station- 

 ary for so many centuries as to allow of the accumulation of an extraor- 

 dinary quantity of detrital matter, and the excavation, at many points im- 

 mediately above, of deep notches and bare chfFs in the hard and sohd rock. 



The phenomena which are most difficult to reconcile with this theory are, 

 first, the abrupt cessation of the roads at certain points in the different 

 glens ; secondly, their unequal number in different valleys connecting with 

 each other, there being three, for example, in Glen Roy and only one in 

 Glen Spean ; thirdly, the precise horizontality of level maintained by the 

 same shelf over a space many leagues in length requinDg us to assume, 

 that during a rise of 1250 feet no one portion of the land was i.iised even 

 a few yards above another ; fourthly, the coincidence of level already al- 

 luded to of each shelf with a col, or the point forming the head of two 

 glens, from which the rain-w^aters flow in opposite directions. This last- 

 mentioned feature in the physical geography of Lochaber seems to have 

 been explained in a satisfactory manner by Mr. Darwin. He calls these 

 cols " landstraits," and regards them as having been anciently sounds or 

 channels between islands. He points out that there is a tendency in such 

 sounds to be silted up, and always the more so in proportion to their nar- 

 rowness. In a chart of the Falkland Islands by Capt. Sullivan, R. N., it 

 appears that there are several examples there of straits where the sound- 

 ings diminish regularly towards the narrowest part. One is so nearly dry 

 that it can be walked over at low water, and another, no longer covered 

 by the sea, is supposed to have recently dried up in consequence of a 

 small alteration in the relative level of sea and land. " Similar straits," 

 observes Mr. Chambers, " hovering, in character, between sea and land, 

 and which may be called fords, are met with in the Hebrides. Such, for 

 example, is the passage dividing the islands of Lewis and Hams, and that 

 between North Uist and Benbecula, both of which would undoubtedly 

 appear as cols, coinciding Avith a terrace or raised beach, all round the 

 islands, if the sea were to subside."* 



The first of the difficulties above alluded to, namely, the non-extension 

 of the shelves over certain parts of the glens, may be explained, as Mr. 

 Darwin suggests, by supposing in certain places a quick growth of green 

 tuif on a good soil, which prevented the rain from washing away any loose 

 materials lying on the surface. But wherever the soil was barren, and where 

 green sward took long to form, there may have been time for the removal of 



* " Ancient Sea Margins," p. 114, by R. Chambers, 



