818 GEOORAPIIY OF BLUE MTS. AND SYDNFiY DISTRICT, 



north brought sand into the area under consideration. The 

 Manly Channel formed a quiet spot where the currents had little 

 power. Thus a great deal of the loss to the cliffs by marine 

 erosion, and to the neighbouring land surfaces by streams, fell away 

 from the centre current, and was deposited to form a shoal at low 

 tide in the passage. Wave-action was not strongly marked. In 

 course of time a shoal grew- across the channel. At high tides 

 the shoal would be completely covered, while in times of dead low 

 spring tides the greater portion of the shoal would possibly be 

 converted into dry land, and a narrow channel only connect sea 

 and harbour together at the gap. The recent elevation carried 

 the shoal some 10 feet higher, thus converting it into dry land, 

 in which, howe\'er, a slight tidal channel would still exist. 

 Marine erosion, dominant current, tide and elevation are thus the 

 key to the origin of Manl}'. 



The coastal plain continuous with the Manly flat had a similar 

 origin, although here the dominant wind had some pla}^, and the 

 conditions are at times similar to those obtaining at Bondi. A 

 couple of small creeks were dammed back by bay bars, and the 

 lagoons thus formed were silted up in part. The recent vibration 

 of elevation converted these into flats above the reach of the 

 highest tides. 



Such an island, as North Head, tied to the mainland by a sand 

 bar or flat is called a " tombolo.""^ 



Mr. L. Gundlach, Civil Enginner, informs me that Manly flat 

 is composed of sand for at least 16 feet below the present surface, 

 as revealed by sewage excavations. 



{d) Sydney Harbour. — Had the Hawkesbury outletted in the 

 vicinitv of Botany Bay or Port Hacking, the enormous amount of 

 debris carried out to sea in times of flood would be borne along 

 shore by the dominant north current and deposited between the 

 Heads in part as being a place of slight current. Thus the 



* F. P. Gulliver, "Shoreline Topography." Proc. Am. Acad. Arts and Sci. 

 xxxiv., 1899. 



