172 STRIATED SURFACES 



the striations crossing them. Frequently the ends 

 directed one way are steeper, rougher, and not so 

 deeply striated as the slopes facing the opposite way. 

 These two sets of surfaces correspond exactly with the 

 "crag and tail" or "lee and stoss sides" of the roches 

 moutonne'es that are met with in every region where ice 

 has passed over hard rocks. 



At Jackal's Water in' Prieska the roches moutonnees 

 formed by the quartzites of the Kaaien beds are very 

 fine ; they are shown in Plates X. and XI. The lee sides 

 are on the south. At Vilet's Kuil in Hopetown the 

 hard lavas of the Beer Vlei (Ventersdorp) volcanic 

 group play a similar role to that of the quartzites of 

 Jackal's Water, the lee sides being again on the south. 

 In both of these cases the surface of the older rock re- 

 tains the roche moutonnee form for a distance of some 

 200 feet from the outcrop of the Dwyka boulder-bed. 

 Beyond this limit the rocks have lost their glaciated 

 surfaces owing to weathering since the removal of the 

 overlying tillite by denudation, but the characteristic 

 outlines due to glacial erosion nevertheless remain. 



Beautifully striated surfaces of Pniel lavas are found 

 on the Vaal Kiver near Pniel, Douglas and Kiverton, 

 to mention only a few of the important localities, and 

 throughout this wide area the direction of the glacial 

 stria3 is generally from north-east to south-west with 

 slight variations to the south or west. 



In Gordonia the tillite is frequently seen between the 

 Upington Commonage and Eietfontein, and it is the 

 only rock other than superficial deposits met with in 

 the few wells between the Hygap and the 22nd meridian ; 



