part 3] GLACIATION OF NORTH-EASTERN IRELAND. 397 



has been signalized by numerous writers as characteristic of 

 County Down. 



So complicated is the surface with its rounded hills, kettle- 

 holes, and intervening strips of boggy land, that it is extremely 

 difficult to interpret the glacial phenomena. There are so many 

 streamless peat-filled valleys that it is, in most cases, impossible to 

 distinguish between true glacial overflow-channels and the boggy 

 strips between the drumlins. 



One or two facts stand out, and give a clue to the direction of 

 the ice-flow. 



The eastern portion of the area, comprising the districts of 

 Comber, Ardmillan, Saintfield, Bally nahinch, Crossgar, Killy- 

 leagh, Downpatrick, Killough, and Ardglass, was glaciated from 

 north to south by the Scottish ice. This is indicated by the con- 

 sistent direction of the striae, and by the presence of northern 

 erratics to the exclusion of Tyrone rocks. 



The western portion, lying between the Belfast & County Down 

 Railway from Comber to Crossgar and the Great Northern line 

 from Belfast to Bannbridge, is much less easy of interpreta- 

 tion. The striatums . vary much in direction, and in some cases 

 range through as much as 200° on the same surface of rock. 

 That this crossing of strise is due to varying direction of ice- 

 movement, and not merely to the irregularities of the surface, is 

 suggested by the general study of the drift-deposits. 



Thus, in the country immediately south of Newtownbreda there 

 are two distinct boulder-clays, a lower containing considerable 

 quantities of chalk and an upper characterized by the predomi- 

 nance of slate and grit. Immediately south of this boulder-cla}^ 

 area Ordovician slates crop out at the surface, and bear striations 

 varying in direction from north-west and south-east to north-east 

 and south-west. A flow of ice from the north-west would bring 

 chalk and basalt, while one from the north-east would pass over 

 Ordovician and Triassic rocks. 



Farther south the direction of striation becomes more regular, 

 and, although varying through some 30°, is, generally speaking, 

 from north-west to south-east. At the same time, a distinct change 

 takes place in the character of the drift, which in this southern 

 area contains numerous erratics from the Tyrone Axis. Thus at 

 Ballygowan, 4 miles south-west of Hillsborough, the drift con- 

 tains boulders of a quartz-porphyry which is intrusive locally in 

 the Ordovician, hornblende-gneiss, syenite, red granite from the 

 Tyrone Axis, Carboniferous sandstone (probably from the neigh- 

 bourhood of Dungannon), also red sandstone (Trias), Silurian grit,, 

 basalt, and flint. 



Between Dromore and Ballynahinch the country is covered with 

 drumlins of boulder-clay, with some gravels in the holloAvs and 

 occasionally on the tops of the hills. The erratics are mostly 

 basalt, dolerite, rocks from the Tyrone Axis, and flint ; but 

 Scottish rocks also occur sparingly, and a small pebble of Ailsa- 

 Craig eurite occasionally rewards a patient search. 



