522 Heiieus — United States JVafional Museum. 



traced up Cwm P\v]l-y-wrach as high as the 800 foot contour. 

 Numbers of smaller boulders were found mixed with Old Red 

 Sandstone material in gravel deposits near Three Cocks Junction, 

 a little stream revealing good sections in mounds of rearranged and 

 roughly bedded drift deposits. No trace of these foreigners was 

 found in the valley of the Wye from Builth Wells to Three Cocks, 

 nor were any found in fine sections of Boulder-clay examined at 

 Llandrindod Wells. 



The author hopes to continue the investigation of these deposits, 

 but believes that sufiicient evidence has been collected to point to 

 a local glacier at first in each of the valleys of the Usk and the Wye. 

 The Usk glacier was fed from the Carmarthen and Brecon Fans, but 

 appears to have been overridden subsequently by a stream of foreign 

 ice from the direction of Llandovery, bringing the brecciated erratics 

 and pressing down the valley to Llangorse, Talgarth, and Three 

 Cocks. It is to the pressure of this foreign ice that he would 

 attribute the overflow of the Old Red Sandstone drift by the Cray 

 Valley, on to the Carboniferous rocks of Penwyllt, and up Dyffrya 

 Crawnon and through the faulted gap of Nant Trefil into the 

 Rhymney and Sirhowy valleys (as reported by the Geological 

 Survey). Among the erratics of the Wye Valley were tough green 

 grits, which were subsequently found quarried at Builth, but marked 

 on the geological map as ' Greenstone.' Several interesting stream 

 diversions, owing to accumulations of morainio material, were 

 observed. Amongst the more important were the diversion of the 

 Usk from a wide valley to a narrow gorge at Aberyscir by a moraine 

 at Cradog ; of the Honddu at ' The Forge,' Brecon, to the glacial 

 gorge which runs below the Priory Church ; and the reversal of the 

 drainage of the Afon Honddu and Olchon Brook at Llanvihangel and 

 Pandy by the morainio gravels which block the wide valley between 

 Bryn-ai'o and Skirrid-fawr, down which these streams no doubt 

 flowed in pre-Glacial times to join the Usk at Abergavenny, whereas 

 now they have been diverted into the Mounow, and so reach the 

 Wye at Monmouth. 



Only one case of a dry valley which had been a glacier-lake 

 overflow was noted, and that was the little gorge called Cwm 

 Coed-y-cerig, bj' which the drainage of Orvvyne Fawr appears to 

 have been carried off when the lower part of its present valley was 

 obstructed by a lobe of ice from Crickhowell, but it was not cut 

 deeply enough to continue to take the stream when the lobe was 

 withdrawn. 



IR E 'V" I E AAT S. 



I. — Smithsonian Institution : Unitbd St.^tes National Museum. 



I. The United States National Museum : an Account of the 

 Buildings occupied by the National Collections. By Riouard 

 Rathburn, Assistant Secretary of the Smithsonian Institution. 



