548 SECTIONAL TRANSACTIONS.— 0. 



Friday, September 7. 



Mr. M. Macgregor. — The Pre-glacial Valley of the Clyde and its Tributaries. 



The present drainage system of the Clyde is superimposed upon a much older 

 system filled in and concealed by glacial and post-glacial deposits. These buried 

 valleys are deeper, and as a rule more open and less gorge-like, than the present 

 ravines. The most remarkable feature in connection with them is the relation of the 

 great Kelvin or Bearsden channel to that of the Clyde. Rockhead was found in the 

 former at a depth of 240 feet below O.D. (boring near Drumry), while the greatest 

 depth of superficial material recorded below O.D. in the case of the pre-glacial Clyde 

 is only about 100 feet (near Glasgow Bridge) and in the case of the old valley of the 

 Black Cart 115 feet (between Paisley and Renfrew). This superdeepening of the 

 Kelvin has been ascribed to glacial erosion, but it may be suggested that in the pre- 

 glacial Clyde-Kelvin channels we are dealing with two entirely different drainage 

 systems. The pre-glacial topography of the Glasgow district must have sculptured at 

 a time when the land stood at least 300 feet higher than it does at present, and the 

 drainage system seems to have developed as follows : — 



(1) The Kelvin-Lower Clyde an obsequent stream ; the Kelvin hollow eroded 

 towards a base-level at least 300 feet higher than present-day base-level. 



(2) Submergence of the land by about 100 feet accompanied by silting-up. 



(3) Renewed erosion towards a new base-level. To this period may be referred 

 the development of the Upper Clyde as the important obsequent stream. The earlier 

 Kelvin-Lower Clyde may have had many of its head-waters diverted, while the 

 Upper Clyde was cutting backwards and extending its drainage area. The Clyde and 

 its tributaries (Black Cart, &c.) were eroding rock-gorges, while the Kelvin was merely 

 a sluggish stream ' winding over alluvial flats and cutting here and there into the 

 valley sides ' (Glasgow District Memoir, p. 219). 



The alluvial materials with which the pre-glacial Clyde valleys are mainly filled 

 seem to be of late glacial age since they rest frequently on boulder-claj'. But in the 

 case of the deep Kelvin channel the lowest beds may be taken to represent the relatively 

 undisturbed detrital material of pre-glacial times. 



Dr. W. F. P. McLiNTOCK and Dr. J. Phemister. — On a Gravitational 

 Survey by means of the EotviJs Torsion Balance in the Neighbourhood 

 of Drumry, Glasgow. 



The drift-filled preglacial valleys of the River Clyde and its tributaries do not 

 coincide entirely with their present courses. Borings north and west of Glasgow 

 have shown that the preglacial Kelvin valley can be traced from Kirkintilloch to 

 Drumry and that at Drumry the depth of the buried valley is at least 300 feet below 

 the present surface or 240 to 2.50 feet below present Ordnance datum. The contour 

 map of the rock surface round Glasgow which is contained in the Geological Survey 

 Memoir on the Geology of the Glasgow District shows that at Drumry the buried 

 valley of the Kelvin appears to fork against a rock mass which has been shown by 

 boring to rise to within 74 feet of the surface. It was suggested that the outlining of this 

 rock mass would be a suitable problem for solution by means of the torsion balance. 



The survey was carried out in December 1927, and in January and February of 

 this year. Sixty-eight stations were investigated. During the survey an area of 

 one-quarter of a square mile was contoured at five-foot intervals to allow topographical 

 corrections to be calculated. Since the problem is the investigation of a buried 

 topography it is evident that no symmetrical results are to be expected and that, 

 therefore, an areal survey is necessary. Stations were put down with a view to 

 constructing an isogam map of the area, that is, a contoured map on which the 

 contours connect all the positions at which the force of gravity has the same numerical 

 value when normal earth and surface effects are eliminated. Owing to an in- 

 sufficiency of stations in the west of the area it was possible only to construct the 

 isogams for the eastern part. The isogam map indicates the existence of an embayed 

 valley flank stretching north and south and falling to the east. Detailed analysis 

 shows that in the locality where the gravitational evidence is most complete, the 

 slope of the valley side is one in six. A bore in this neighbourhood shows 74 feet of 

 drift, while the depth of drift au the mapped position of this bore is, from the 

 gravitational results, about 80 feet. One of the embayments of the valley side is 



