Formation of "Natural Quarries." ]Q\ 



The " hack " and the floor of the quarry approximate in outline to 

 rectangles, which are frequently at right angles to each other. The 

 two sides of the quarry form triangles on vertical planes, or planes 

 ■which approximate to the vertical, and which tend to be at right 

 angles to the plane of the " back," The sides of the triangles are 

 therefore formed by the planes of the hillside slope, the "back" 

 and the floor. In the rounding off of corners there is a tendency 

 to destroy the sharp rectilinear outlines, btit such outlines are 

 clearly recognisable. 



A quarry may form on any hillside provided the rocks under- 

 lying the surface rocks are comparatively soft and easily removed. 

 Thus this type of quarry is found on hillsides of soft schistose or 

 stratified rocks, or of any decomposed rocks; and such hillsides may 

 include " dry " lake cliffs. It is not found on slopes with hard 

 granite boulders; but it does occur on slopes littered with small 

 but tough greenstone fragments, for often beneath the surface litter 

 the rock in situ may be much decomposed. Rain furrows and 

 small channels may lead to and from the quarry and across its 

 floor. Most quarries are of small dimensions, the average of 

 which for many would be about eight to ten feet high, six to eight 

 feet broad, and ten to twelve feet long. Some quarries greatly 

 exceed these measurements. One on the western shore of Lake 

 Goongarrie, at Comet Vale, is probably 30 to 40 feet high, 40 to 60 

 feet broad, and 60 to 70 feet long; but this is an exceptionally 

 large one so far as the writer's observations have gone. The 

 figures given are approximate only, as no actual measuring has 

 been done. 



The Triangular Qiifrrr//. — This type of quarry is more akin to 

 gulches produced by normal erosion at the heads of gullies. On the 

 plane of the slope which it dissects it is roughly triangular, and has 

 the base of the triangle on the upslope and the vertex pointing 

 downhill. It is usually V-shaped in a cross section parallel to the 

 base of the triangle, the sides having but moderate slopes. The 

 " back " however, is somewhat steeper, this feature being accen- 

 tuated when, as often occurs, a rock cap which tends to be under- 

 mined forms the coping to the back. The quarry is mostly con- 

 nected with a drainage line at the vertex of the triangle, the quarry 

 representing the present but somewhat abnormal head of that line, 

 as the steep slopes and gulches do in an area of normal erosion. 

 Similarly, as in such a normal erosion area, the quarry tends to be 

 at the top of the slope, with its " back " close to the crest line of 



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