156 Professor J. W. Gregory — 



sides would have been short and blunt. Their pre-Glacial walls 

 probably stood quite close to the existing walls. One of the best- 

 known ice-worn slopes in Arran is the southern face of Creag Rosa. 

 But a section across this sIojdc based on the Ordnance maps (Fig. 3), 

 even with the vertical scale magnified twice, does not give the 

 inijoression of any great overdeepening. If the floor of the valley 

 had been at its present depth in pre-Glacial times, the dotted line on 

 the figure would indicate the approximate position of the pre- 

 Glacial slope, The greatest thickness of rock removed by the ice 

 would only have been about 100 feet, and this layer would have been 

 weakened by pre-Glacial disintegration. The thickness of hard 

 fresh rock removed would have been insignificant. 



5. Garbh Allt axd the Glacial Deepening op Glen Rosa. 



Dr. Mort, however, holds that Glen Rosa was glacially deepened 

 to the extent of between 500 and 800 feet, a conclusion based mainly 

 on the hanging valley of Garbh Allt. This stream rises on the 

 south-eastern slopes of Beinn Tarsuinn, and at first flows south- 

 south-east parallel to Glen Rosa ; at 800 feet it bends suddenly 

 eastward, and is joined by a tributary from the west ; it follows the 

 direction of this stream and discharges to the east-north-east by 

 a series of cascades to Glen Rosa. It comes from a hanging valley 

 which ends 500 feet above its confluence with the Rosa Water ; 

 and Dr. Mort holds that the difference in level between the hanging 

 valley and the Rosa Water is due to glacial deepening of at least 

 500 feet and perhaps of 800 feet. 



The limited corrosion by the lower Garbh Allt is capable of another 

 explanation. Dr. A. Scott suggested to me the possibility that the 

 Garbh Allt was originally the head of the Basbuig Burn (Bishop's 

 Glen), a tributary of the Machrie Water ; and a visit to the locality 

 showed the high probability of the suggestion. Figs. 4 and 5 show 

 five sections across the hills to the west of Glen Ro^a. Section 4a 

 crosses the upper part of the Garbh Allt, which there flows through 

 a valley clearly separated from the parallel valley of Glen Rosa. 

 Section 4b crosses the middle part of the Garbh Allt, where the 

 eastern side of the valley is inconspicuous. Section 4c is along the 

 lower course of the Garbh Allt below the right-angled bend at 

 800 feet ; the section is continued westward along the tributary 

 Avhich continues the course of the cascades of the Garbh Allt. 

 Section 5d is along the low flat rise which separates Garbh Allt from 

 Glen Easbuig to the south-west, and from the tributaries of Glen 

 Shurig to the south-east. Section 5e crosses Glen Easbuig from 

 north-west to south-east at the same distance from D as that is 

 from Section C and as C is from B. These sections show a continuous 

 valley which was probably the original course of the Garbh Allt 

 when, instead of turning to the east at C, it descended from 1,114 feet 

 in B, over the depression at a little over 1,000 feet in Section D to 

 Glen Easbuig. The Garbh Allt has now been diverted eastward tO' 



