THE Biack WATER OF DEE. 187 
runs nearly parallel with the Solway. The rocks are very 
hard, and in some cases rise above the 1800 feet contour. 
In a former paper* it was shown that the consequent 
Ken originally traversed this ridge, thereby excavating the 
Doach Gap. 
Dr. Mort, in his paper on the Rivers of South-West Scot- 
land, suggested that the original Ken eroded this gap. Closer 
examination confirms his bold suggestion. Similar evidence 
is available to prove that the consequent Cooran Dee also 
traversed the granite barrier through what is now the Screel 
Gap. 
The 1000 Foot Plain. 
Let us visualise the geographical features of a 1000 foot 
plain. 
t. The great width of one mile in extremely hard rock that 
was cut in the floor of the Cooran Gap suggests a large 
stream—one that had travelled from a distance. 
2. The correspondence between the tooo feet peneplain of 
Arran and that of the Cooran valley floor would imply 
Arran as the only possible source of this master stream 
on its elevated path. 
43. The maximum height of Arran (2866 feet) against the 
Merrick of 2764 feet indicates an older slope or platform 
—hence the thousand feet stage would be a second stage, 
not a first. 
4. The predominant flatness along the top of the Screel range 
points to the former existence of a vast plain slightly 
over the 1000 feet contour. When it is considered in 
connection with the flat gap at Cooran and the plain 
remnants in Arran, it may be concluded that that pene- 
plain represented the base-level of a previous cycle of 
erosion. 
. The undulations of the plain as at Bengairn representing 
on 
a hillock of 100 feet, and the absence of marine deposits, 
infer that the plain was due to sub-aerial erosion—in 
other words, part of a huge river system. 
* «¢ The Galloway Dee,’’ Transactions, 1918-19., pp. 78-88. 
