197 



Valley Lake (see fig. 6). Woods regarded it as an adventitious 

 crater, but from his description it is clear that the walls were 

 not then as exposed and bare as they are now. The dip of 

 the stratified beds clearly continued uninterruptedly across 

 the area now occupied by the .depression. 



The shape and size of this depression have been com- 

 pared to those of the greater explosion craters made on the 

 western front during the war. Possibly the Punch-bowl was 

 formed by an isolated explosion late in the period of activity, 

 without any subsequent ejection of material. It seems more 

 likely, however, that it is clue purely to a small subsidence 

 occurring a little apart from the main line of collapse (see 

 fig. B, pi. x.). 



(h) The Water of the Lakes. — As already mentioned, 

 the water of the lakes is portion of that stored in the great 

 sub-artesian tertiary basin of the ancient Murray Gulf, and 

 the surface of the lakes slowly rises and falls, apparently in 

 harmony with the rise and fall of the general water-table of 

 the whole area. In some cases the rapid rise of the water 

 has caused some anxiety locally. For instance, from the 

 middle of the year 1909 the water continued to rise steadily 

 for two years, and in December, 1910, stood 9 ft. higher than 

 in June, 1909. Such increases in level can scarcely be appre- 

 ciated in the general appearance of the deep and steep-walled 

 lakes, such as the Blue Lake and the eastern portion of Valley 

 Lake ; but in the shallower portions of the Leg of Mutton 

 and Crater Lakes this variation in level causes considerable 

 alteration in their shape and appearance. 



Fig. 9. 



Graph showing the variations in level of the surface of 



the Blue Lake for the years 1896-1921, as recorded in the 



office of the State Hydraulic Engineer, and supplied by 



the courtesy of that officer. 



. During the past 25 years the limits in the variation of 

 water level have been only about 14 ft. Further back in 

 time, this variation may have been much greater. It has 

 been mentioned that there is a local record that about 1841 

 and subsequent years the Crater Lake was dry. Writing in 

 1862, Woods says of the Leg of Mutton Lake that "the water 

 at the bottom has only made its appearance, as I am told, 



