Scottish Geographical Magazine a Bathymetrical Survey of the Chief
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This article was downloaded by: [University of Ulster Library] On: 12 January 2015, At: 02:09 Publisher: Routledge Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Scottish Geographical Magazine Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/rsgj19 A bathymetrical survey of the chief Perthshire lochs and their relation to the glaciation of that district James S. Grant Wilson a a H.M. Geological Survey Published online: 30 Jan 2008. To cite this article: James S. Grant Wilson (1888) A bathymetrical survey of the chief Perthshire lochs and their relation to the glaciation of that district, Scottish Geographical Magazine, 4:5, 251-258, DOI: 10.1080/14702548808554471 To link to this article: http://dx.doi.org/10.1080/14702548808554471 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. 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Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions EXPLORATION OF THE GULF OF GUINEA. 251 TABLE V.--ASCENSION TO CONAKR¥ (Serial Temperatures). 3 ° :'4N. 3 7 ° I"0N 3 ° 2'-3 N.I [4' *' 15 ° 0"9W.[15 ° 54"0W 15 ° 2"1 I Depth: )6~ 1892 2611 I" 2584 11 March. 112 March. 13 March. / 2 Jany, ~ahr. ° Fahr. o Fahr. ° Fahr. ° Fahr. ° Fahr. 0 1 ') 81 "6 81 "5 80 "8 82"3 82"9 10 0"2 81 '0 81'2 79"2 I 79"8 80 "0 82"0 15 ... 76 "1 81 '0 78"8 I ... 18 76'3 i 7g':v 20 0"2 7"3"7 8i"0 ... i 8b"o 8i':9 20 78 "4 23 7i'.'2 [ ... ,.. ..o 25 76 "2 6g:9 30 9 0 67".9 61.8 ' 6~':8 6~.o 64"8 30 ...... o,. 67"8 32 7io 40 50 •o 5~:9 5""37- ::: 5~':2 5~'.b 57 '8 5~:2 75 7 "9, 56"46 56 "2 ... 56 "7 56 "9 56 "2 100 2 "0 54 "92 55 -8 ... 55 '8 56'0 55 "8 5~:4 150 ... 52 '7 ... 50 '9 51 "2 51 '9 200 ... 5~':83 49.2 ... 47 "2 46 '9 48'0 55":0 300 ... 48 "64 42 '63 43"8( 44 '2 400 ... 42 "21 40 "95 ::: .... 40"6~ 50O 4~:9 6O0 ::: a;':41 89"7( 80O ... 39 '30 39'5~ 1000 38"3~ 8~:1 1250 37 '6~ 37-92 1500 ... o.. 36"9~ 36'98 2000 36 '5~ 2111 2397 ::: 3.~':62 2498 8g':57 ..° 2611 .. .... 2725 36'1~ A BATHYMETRICAL SURVEY OF THE CHIEF PERTHSHIRE Downloaded by [University of Ulster Library] at 02:09 12 January 2015 LOCHS AND THEIR RELATION TO THE GLACIATION OF THAT DISTRICT. :BY JAMES S. GRANT WILSON, H.M. GEOLOGICAL SURVEY. (Read before the Royal Society, Edinburgh, Feb. 20, 1888. Communicated by permission of the Director-General.) SITUATED within the Highlands of Perthshire are numerous fresh-water lochs, each surrounded with its own peculiar type of scenery. Not- withstanding this diversity, there are certain broad features which are common to them all, ,~nd suggest that the same agencies have been at work 252 BATHYMETRICAL SURVEY OF THE CHIEF PERTHSHIRE LOCI:IS in each case. During the progress of the Geological Survey in this part of the country four of these have been sounded, and the results obtained show the close connection that exists between the configuration of the various rock basins, and the glaciation of the district in which they are situated. The general trend of the chief Perthshire valley is from west to east, and these are in turn intersected by the transverse valley running south-east from the head of the Garry to Dunkeld. The river Tay belongs to both these systems; the upper portion flowing in an easterly direction from the head of Glen Dochart to Ballinluig, thence along the transverse valley in a south-east direction by Dunkeld to Perth. At Ballinluig the Tay receives the outflow from Loehs Rannoch and Tummel, while the surplus water of Loch Earn passes through Strath Earn, and joins the estuary a little to the west of Newburgh. In surveying these various loehs the system adopted was to take a series of soundings (50-80 yards apart) along parallel lines from one side of the lake to the other. These parallel lines were fixed at intervals of about a quarter of a mile, and the observations were checked by other soundings taken in lines extending diagonally across several of the former. A steam launch was employed on Loch Tay, while for the other lakes an ordinary rowing boat proved sufficient. LOCH RANNOCH.--This sheet of water, 9~ miles in length and having a mean breath of ~ of a mile, lies to the east of the Moor of Rannoch and at an elevation of 667"5 feet. This moor is a wild expanse of peat-moss and heather, and over its surface are scattered numerous small lakes and lochans which all drain into Loeh Rannoch. From the east shore of the loch the bottom descends very steeply, and at one point midway between Altruigh and Curie the greatest depth of 70 fathoms was obtained. Here the loch is a mile broad, and the depth of 70 fathoms is 16 of a mile from the east end--giving an average grade for the central plane of 1 in 22. From the deepest point to where the 10-fathom contour-line crosses the loch opposite Killiehonan, the central plane rises with tolerable re- gularity to the west with an average grade of 1 in 88. Between the 50 and 60 fathoms contour-lines the grade is 1 in 30, but this rise only continues for a short distance. At Carie, Aulieh, Killiehonan, and Crosseraig four large burns flow into the ]och, and the stony detritus borne down by these has in each case formed a large cone of alluvium that projects for a considerable distance into the lake. As might be Downloaded by [University of Ulster Library] at 02:09 12 January 2015 expected, the depth of the lake suddenly increases along the face of these cones, each resembling the tip end of a railway bank. Several measure- ments showed their angle of slope to be considerably in excess of 20 ° . ]n the deep water only two small banks have been discovered : one lies opposite the mouth of Dull burn, near to the north side and 29 fathoms from the surface, while the other is situated in the middle of the loch, to the north of Crosseraig, in 16½ fathoms of water. The general transverse section of this loeh is in the form of a V, the bottom sloping down from the one side till the maximum depth is reached, and then immediately rising towards the opposite shore. The only exception to this is the small area to the south-west of the Killichonan burn, where the bottom AND THEIR RELATION TO THE GLACIATION OF THAT DISTRICT. 253 is very flat--but this may in part be due to the silting up now in progress at this end. To the south of the Tower Island the water is very shallow, the main channel passing to the north of the island with an average depth of 8 fathoms. Between the mouth of the Erieht and Rannoeh Lodge there is a small depression to the extent of 13½ fathoms at its deepest point. From Kinloch-Rannoch to Aulich the north side of the basin shows a steep gradient towards the centre, becoming less steep to the west of Aulieh. The same steep gradient also prevails along the south shore as far as Dall, but between Altruigh and Carie a belt of shallow water, about 150 yards in breadth, lies between this sharp descent and the margin of the lake. In almost every case where the sounding-line brought up a sample of the bottom, it was found to consist of a very fine brown vegetable mud through which were scattered small particles of white mica. In the shallow water to the east and west of Killichonan this mud was often of a rusty brown colour, due to the presence of a small quantity of iron, and from the Tower Island to the mouth of the river Ericht the bottom was almost entirely composed of fine sand. The west end of this loch is slowly silting up by the deposition of sediment. But this is necessarily a very slow process. The two main feeders at this end, the Ericht and the Ghaoir, respectively drain Lochs Ericht and Laidon a few miles distant.