6 llOKN EXPEDITION — PHYSICAL GEOGRAPHY. 



feet above sea level. The mean elevation of this ridge is about 2.500 feet above 

 sea level, that of the Waterhouse Range about 2200 feet, that of the James Range 

 about 2000 feet, while lastly the mean elevation of Cliandler Range is only about 

 1500 to 1600 feet. From these figures it can be clearly seen that there is a gradual 

 decrease in elevation in the ranges from north to south, each range to the south 

 constituting, as it were, a step in the descent from the iVIcDonnell Ranges to the 

 plains. 



The chief factors, in addition to the position of the longitudinal valleys 

 occupying the original troughs of the folds, that have influenced the direction of 

 the lines of denudation are (1) the lines of weakness on the crowns of the anticlinal 

 arches, and (2) the position of the bands of limestone. An example of the 

 influence of (1) is furnished by the valley of Petermann Creek, which has been 

 eroded out of an anticlinal arch, while the rocks of the corresponding synclinal 

 trough now form the George Gill and Levi Ranges. The influence of (2), as might 

 have been expected, is to be observed throughout this region, the greater number 

 of the valleys within these ranges having been, to a great extent, eroded out of the 

 limestone beds. 



The angle of inclination of the mountain slopes depends to a great extent on 

 the nature of the strata and on their dip. On one side of the ridge the slope 

 often conforms to the dip of the strata when these are inclined at fairly steep 

 angles. On the south side of the quartzite ridge, for instance, which forms the 

 northern boundary of this area, the inclination of the strata (quartzite and lime- 

 stone), as well as that of the mountain slope, is from 60° to 70". 



The north face of this ridge, however, presents for almost the total thickness 

 of the quartzite a sheer perpendicular escarpment extending from the summit 

 downwards 200 or 300 feet. Below this quartzite, as at Mount Gillen, the 

 remaining 600 to 700 feet of this face, composed of Pre-Cambrian gneiss, has a 

 slope not exceeding 30° to 40°. A similar diflerence in the inclination of the 

 slopes of a ridge is also to be seen in the case of the Mereenie Escarpment, which 

 probably extends almost continuously as far east as the Finke River. On the 

 north side there is a steep, almost perpendicular, escarpment from 500 to 600 feet 

 high, whereas on the south the slope conforms to the dip of the strata. 



In those ranges, where the .sandstone is dipping at very low angles, as in the 

 George Gill and Levi Ranges, we find very steep, almost vertical, escarpments on 

 both sides. The northern face of Levi Range, for instance, rises to an elevation 

 of about 500 feet above the valley of Petermann Creek, for nearly 300 feet of 



