THE RIVER GRAVELS BETWEEN PENRITH AND WINDSOR. 253 



The old Warragamba and Nepean rivers joined at a spot 

 not far from the westerly bend of the Nepean north of 

 Penrith. The boulder belt thence to Windsor was the 

 product of the combined stream. 



In his anniversary address in 1896, Professor David 

 suggested that the Glenbrook gravels might be correlated 

 with the gravels at St. Mary's. The writer is, however, 

 quite satisfied that the St. Mary's gravels were deposited 

 by the Nepean River, and not by the combined Nepean and 

 Warragamba, and also that the gravels found at St. Mary's 

 and in various places to the north-east, belong to an older 

 period in which the Nepean occupied a still more easterly 

 course. 



The cause of the shifting of the course of the Warragamba 

 River from the old Glenbrook channel to its present position 

 on the plains has been well explained by Professor David 

 in his anniversary address of 1896 (op. cit.) and in his paper 

 on the Kurrajong Fault (this Journal 1902). The formation 

 of the monoclinal fold slowly tilted the river out of its old 

 course. 



The diversion of the Nepean River into the Warragamba 

 River at the Basin has, however, not been explained, and 

 while the solution of the problem which is here proffered 

 has not been fully established, it nevertheless has strong 

 arguments in support of it. 



First it should be stated that the production of the 

 Lapstone monocline was very slow and probably commenced 

 as far back as the Pliocene. Secondly, we must take into 

 consideration that we have to deal with an area in which 

 there have been two causes of movement, tectonic move- 

 ment and igneous uplift. The tectonic movement followed 

 the north and south line of the monoclinal fold. Inter- 

 secting it at right angles we have a line of igneous 

 intrusions extending from the Basin in an east-north-east 



