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XVII. On the Origin of the Parallel Roads of Lochaber and their Bearing 
on other Phenomena of the Glacial Period. 
By Joseph Prestwich, M.A., F.R.S., F.G.S., &c., Professor of Geology in the 
University of Oxford. 
Received March 27,—Read May 1, 1879. 
[Plate 46.] 
Contents. 
Page 
State of the Question . 663 
§ 1. The detrital and marine theories . 664 
§ 2. The glacial theory of Agassiz : Mr. Jamieson’s exposition . , .... 666 
§ 3. Objections to this exposition—1, the barriers; 2, the cols . 668 
§ 4. The terraces of Macculloch ; the deltas of Chambers and Jamieson. 671 
§ 5. The level of the snow-line. 674 
Views of the Author. 677 
§ 6. Depth of the ice-sheet in Lochaber: Extent of the glaciation. 678 
§ 7. Exceptional condition of the ice-sheet in Lochaber . 682 
§ 8. Distribution of the moraine detritus. 685 
§ 9. The level of the land at the incoming of the glacial period. 689 
§ 10. The effects of the subsequent submergence on the climate and on the ice-sheet 691 
§ 11. Structure of the “roads”: their inclination to the horizon. 696 
§ 12. Origin of the “roads”. 700 
§ 13. The “ minor barriers ” on the cols or passes at the head of the glens. 710 
§ 14. The “ main barriers ” at the entrance of the glens. 712 
§ 15. Irregularities in the levels of the “roads ” . 719 
§ 16. Further considerations in connexion with the great ice-sheet suggested by 
the local phenomena of Lochaber... 721 
§ 17. Conclusions. 724 
Explanation of plate . 725 
State of the Question. 
It is now forty years since the origin of the parallel roads of Lochaber was discussed 
before the Poyal Society by Darwin,'" in his well-known paper on the subject; but, 
although the problem had been for some time previously, and has since continued to 
be, the subject elsewhere of many communications, considerable difference of opinion 
MDCCCLXXIX. 
* Philosophical Transactions for 1839, p. 39. 
4 Q 
