810 the geographical significance of floods, 



Examples of Flood-action as observed by the Author. 



The forms to be now described occur in the northern and 

 eastern portion of New South Wales. 



(a) B o u r a 1 n g . — Here a creek, with a run of about two 

 miles, flowed along a flat of tenacious clay underlain with heavy 

 stones partly cemented together. A severe thunderstorm pro- 

 duced a rush of water which carved deep trenches and basins 

 along the flat. In one case a trench eight feet deep, ten feet 

 wide, and ten chains long, was cut by the stream. The caiion 

 thus produced had straight sides, a flat floor, and hanging valleys. 

 In another place a trench was cut showing terraces and cirque- 

 like basins. The accompanying plan and sketch illustrate the 

 occurrence. 



f^ig,6. — Trench formed at Bouralong (New England) during 

 severe storm. Length of trench, 10 chains (220 yards); width and 

 depth, 8 feet. Note the straight walls and the broad base. The 

 trench was formed by recession of a waterfall originating in a 

 marked acceleration of velocity caused by a channel-obstacle 

 situated on a decline. 



These long trenches were not excavated by a single convergent 

 thrust, but commenced rather as small amphitheatres due to 

 stream-convergence, or gravitative thrusts, determined by 

 obstructions. The waterfall thus started quickly worked its 



