590 PKOCEEDINGrS OF THE WASHINGTON MEETING 



an ancient hike, a vale well known to travelers who have visited the great gorge of 

 the Aar above Meiringen. It is not the object of this paper to describe this locality, 

 except to observe that we have here apparently a case of erosion similar to those 

 seen about the Griinsel. This basin has been described as a product of glaciation, 

 though some Swiss observers, notably Heim, hold otherwise. 



In general, the basins appear to mark points of effective ice-work, and seem, in 

 several cases at least, to lie at the foot of what must have been a sharp descent of 

 the ice, and therefore a place of strong abrasion. The Griinsel lakes occupy similar 

 basins, and seem to have escaped the filling to which the others have been sub- 

 jected by being left out of the track of the debris-laden torrent. 



Below the lower Aar glacier, on the south side, a stream descends over the steep 

 cliff face, carrying the waters of the upper Aar glacier. The lateral valley enters 

 its principal some hundreds of feet above the floor of the latter, and thus is a 

 typical case of the hanging valley, interpreted by Davis and others as due to more 

 effective glacial erosion in the trunk valley, in contrast with the law of ordinary 

 stream and valley development, by which the chief and lateral valleys are graded 

 to the same level at the point of union. Similar hanging valleys enter from east 

 and west at Innertkirchen. Another enters the Rhone valley from the east, imme- 

 diately adjoining the fall of the Rhone glacier, and other cases may be seen along 

 the same valley, between Visp and Martigny. 



In discussion of Professor Brigham's paper, W. M. Davis showed some 

 lantern views illustrating the topic, and made the following remarks : 



The discordance between the depth of a main valley and that of its tributaries, 

 as shown by Professor Brigham in the case of the Aar, is very striking in the 

 glaciated areas of the Alps. No locality exhibits the discordance better than the 

 valley of the Ticino, followed by the Saint Gotthard railway southward from 

 the great tunnel into Italy. The main valley is not a gorge, but a broad trough, 

 gravel-floored, and with steep, cliff-like walls for several hundred feet of height, 

 above which the mountain slopes flare out, as if they were the upper parts of a 

 more ordinary V-shaped valley. The side valleys are V-shaped in cross-section, 

 and the streams from them cascade into the main valley, thus repeating the features 

 shown by Professor Brigham at several points in the valley of the Aar. Although 

 most writers on glacial erosion have given little importance to the discordance of 

 main and side valleys in glaciated regions, and although the discordance is ex- 

 plained by so experienced an observer as Heim as a result of normal river erosion 

 after a period of uplift (the river of the main valley outstripping the streams of the 

 side valleys in the work of valley deepening and widening), the greater depth and 

 breadth of the main valley than of the side valleys is by several Alpine observers 

 not only held to be beyond explanation by other than glacial action, but to be in 

 itself an excellent index and measure of such action. It seems at first as if the 

 discordant floors of glaciated valleys were abnormal. They do not appear to fall 

 under the general relation— the accordant levels of main and tributary valley 

 floors — that was employed at the beginning of the century by Playfair as one of 

 the best proofs that valleys are eroded by the rivers that occupy them. But a 

 closer examination shows that glaciated valleys are not abnormal in this respect. 

 Glaciers are slow moving streams ; the cross-section of their bed is a considerable 



