490 L. Richardson — Glacial FeatureH near Biiilth Weils. 



by the thrust -plane. The observations at Langkofl confirm the 

 results I obtained for the district generally that the eai'liest folds 

 had an east-west strike, while the overthrusts took place during 

 a more advanced period of strong cross-compression. 



At Langkofl. the complex of thrust-slices has been segmented by 

 several north-south ' step ' faults of slight throw, the effect of which 

 has been to lower the segments on the east side towards the north- 

 south fault in Sella Pass with upthrow on the east. In former papers 

 I have shown that this is also the case in Sella Massive, and has 

 shattered the eastern aspect of that mountain as compared with the 

 western ; and that it also holds good for the Enneberg and Sett Sass 

 area where the step faults lower the Valparola exposures of Eaibl 

 and Dachstein Strata on the east of Sett Sass. The dolomitic rock 

 in these mountain-massives and in the Ampezzo and Gader Thai 

 district is part of the same thrust-mass as Sella and Langkofl. 



The Wengen Strata of the underlying or St. Christina and Seiser 

 Alpe thrust-mass may be traced continuously from the north of 

 Langkofl round the west of Plattkofl to the Duron Yalley slopes 

 on the south of the Dolomite Massive. There the thrust-mass of 

 Langkofl and Sella Massive is downthrown, and the underlying mass 

 upraised, on the north and south respectively of that leading east- 

 west fault which I called the Eodella-Buchenstein fault. 



The thrust-planes in the group of Lower and Middle Trias reappear 

 in the south or Duron Valley slopes, and have the same effect as at 

 St. Christina of duplicating or cutting out various horizons. The 

 chief thrust-plane of the Duron Yalley slopes occurs at 1700-1800 

 met. contour and continues as a well-marked plane of overthrust 

 throughout the Mt. Donna and Bufaure Mountains. Above it are 

 sheared slices of "Werfen, Muschelkalk, Buchenstein Strata, and the 

 Wengen lavas and tuffs; below it is a succession of Lower and 

 Middle Trias which exhibits the petrographical and paleeontological 

 characteristics of the so-called northern and southern, or ' Rand ' 

 facies of the Dolomites, i.e. with well-defined Upper Werfen 

 fossiliferous zones and the Middle Trias developed for the most part 

 as a homogeneous calcareous or calcareo-dolomitic mass. According 

 to my mapping of the thrust-planes in Fassa this calcareous facies 

 exposed in the Vallaccia, Costabella, Marmolata, and other Massives 

 represents the basal mass above which the thrust-slices with the 

 porphyritic, tufaceous, and dolomitic facies have been driven. 



Subsequent folds and faults have lowered the rocks of the over- 

 thrust slices in some places between portions of the basal mass, so 

 that in the Fassa and Contrin region the one facies suddenly gives 

 place to the other and the complication of detail is very great. 



V. — Some Glacl^l Features at Aberedw in' the Wte Valley, 



NEAR BuILTBC WelLS. 



By L. Richardson, F.R.S.E., F.G.S. 



(PLATE XXIX.) 



WHILE pioneering for an excursion of the Cotteswold Naturalists' 

 Field Club at Aberedw in the Wye Valley, between Builth 

 Wells and Hay, certain very interesting physiographic features, 



