CHANNELS AND GLENS OF AYRSHIRE. 57 



south-east summit of the dyke there is an old fort, but without 

 ditches, the old fort-builder having evidently not been able to dig 

 into the trap. The old channel or glen probably lies somewhere 

 between the present stream and the Pockinan Burn. 



Polbaith Burn. — Opposite Alton Muirhouse the Polbaith leaves 

 the line of the old glen, and a short distance to the north-west of 

 it cuts through trap rock. Below this it comes into the line of 

 the old glen, and forms drift-scaurs, but again leaves it for short 

 distances in three places— at Castlehill forming a considerable 

 glen. 



Glen Water. — Where the glen issues from the wooded rocky 

 gorge of the Carlin's Craig, it has at one time turned to the left, 

 into the old valley, and has carved out an amphitheatre with 

 covenanters' seats, in the drift, where beds of sand and gravel are 

 to be seen on the slopes of the old bank. Above the mouth of 

 Mucks Water we find the stream pretty much in the line of the 

 old valley, and making scaurs 70 feet high in the shelly drift. 



Changue Burn. — Below where the Changue joins the Logan 

 Burn, it leaves its drift-scaured valley, and cuts a deep wooded 

 gorge for two furlongs through trap, emerging from it a short 

 distance above the great Score Tulloch scaur, where thick shelly 

 till is capped by about 90 feet of sand and gravel. 



River Irvine.— -The Irvine keeps better to the line of its old 

 valley than any other large Ayrshire stream, exhibiting but little 

 rock in its channel, and no cliffs of any magnitude. 



Cessnock Water. —The Cessnock, near Craighead and Craigmill, 

 cuts deeply into the calciferous series, forming splendid geological 

 sections, the drift-filled valley being probably a short distance to 

 the south. 



Avon Water.— The Avon at Main Castle (an old earth-work 

 fort) has left the pre-drift valley, and cuts through rock (metamor- 

 phosed old red sandstone of the Geological survey), the old channel 

 being on the west side of the fort and stream. Above Main 

 Castle the Dipple joins the Avon on the Lanarkshire side, coming 

 out of a rocky gulley. 



Above the moraine (see the Drift or Glacial Deposits of Ayr- 

 shire, page 67, by the author ; Transactions of the Geological Society 

 of Glasgow, 1898) the Avon again leaves the old valley, cutting a 



