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| Keywords: The Quarterly journal of the Geological Society of London (13204474205).jpg 50 <br> J W E DAVID ” GLACIAL ACTION IN SOUTH <br> the trend of the striae is 30° W of S ; at Cefn Cadlan they run 38° <br> W of 8 and at Gwern-cefn-y-garreg 42° W of S All these striae <br> are on mamillated surfaces of Millstone Grit At Gwern-cefn-y- <br> garreg their greatest height above the sea-level as taken by aneroid <br> is 1444 feet <br> The surface of the Millstone Grit here has been moulded into <br> elongated domes and hummocks fig 4 showing a bare bright surface <br> where they are not covered by peat or powdered rock Wherever <br> Fig 4 ” Bodies moutonnees Givern-cefn-y-garreg ; Millstone Grit <br> its surface has escaped being crushed it has been well striated <br> grooved and polished the striae pointing direct to the two tabular <br> tops of the Beacons distant 6 miles giving a fall of 244 feet per <br> mile A mile S W of Gwern-cefn-y-garreg on the right bank of <br> the M elite 500 yards below Ystrad-fellte church a recent slip of <br> Boulder-clay has exposed a well-glaciated surface of Carboniferous <br> Limestone Where it has not been completely shattered this rock <br> has taken a fine polish is firmly striated and slightly grooved <br> This surface forms a steep slope in a bend of the river which the <br> ice must have struck obliquely at an angle of 30° All its projections <br> have been striated in a determinate direction while the sheltered <br> ledges show striae running in various directions often at right angles <br> to the true lines of glaciation Tbe author thinks that all these <br> striae were formed contemporaneously The strike of the Carboni- <br> ferous-limestone beds here being nearly horizontal and the bedding <br> even and definite the stones which the ice was forcing obliquely <br> up the slope would be arrested at the points of junction between <br> the beds and would have a tendency to slip along these lines of <br> weakness and scoop out steps in the slope as has actually been the <br> case The direction of the grooves and coarse striae is less persistent <br> than that of the fine scratches the former often describing a curve <br> which increases with the steepness of the slope These striae run <br> down to the brink of the river Mellte and were evidently continued <br> below the present level of the valley- bottom Their height above <br> the sea-level here is 740 feet giving a fall from the top of the <br> Beacons 7 miles distant of 310 feet per mile <br> The furthest point west to which the author has traced these striae 36928627 113681 51125 Page 50 Text 39 http //www biodiversitylibrary org/page/36928627 1883 Geological Society of London Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 39 1883 Geology Periodicals Smithsonian Libraries bhl page 36928627 dc identifier http //biodiversitylibrary org/page/36928627 smithsonian libraries Information field Flickr posted date ISOdate 2014-03-16 Check categories 2015 August 26 CC-BY-2 0 BioDivLibrary https //flickr com/photos/61021753 N02/13204474205 2015-08-26 13 52 26 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1883 Photos uploaded from Flickr by Fæ using a script | ||||
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