176 PORT JACKSON SILT BEDS. 



The inner basin of the river being composed of softer rocks has 

 widened by vertical and lateral denudation, whilst the outlet in the 

 precipitous gorge formed by the hard sandstone cliffs of the 

 Harbour Heads has gradually deepened but has increased very 

 little in width. As the subsidence continued, the tidal currents 

 began to operate and chiefly contributed to scouring the channel, 

 leaving no covering on the rock near the shores and only a very 

 shallow bank in the centre in comparison with previous deposits. 

 At a certain period the erosion of the bottom ceased, either 

 because it had reached "the base level of erosion," and from 

 want of fall could not clear its bed, or because one of the hard 

 ironstone bands, which frequently till crevices in the sandstone on 

 the lines of bedding, proved too tough for the scour of the tides or 

 because of a change in the motion of the earth's crust from 

 subsidence to upheaval. The first conjecture is the most reasonable 

 because we find that by far the deepest bed of silt has been deposited 

 after the scour on the bottom of the harbour has ceased to affect 

 it, and it is largely composed of broken shells which have been 

 partly contributed by littoral currents. The most important 

 feature in the thickness or rising of these silt beds is the apparent 

 tendency of the silt to fill in the channels on both shores of the 

 harbour. This filling in took place much earlier in the centre and 

 on the north shore than on the Sydney shore, and the reason of it 

 is obvious. At first the tidal currents hugged each steep rocky 

 shore deepening the channels and leaving a rise iiT the centre. 

 This rise commenced at the entrance of the harbour between South 

 Head and Middle Head at the rocks known as the Sow and Pigs, 

 which are only a few feet below water at low tide. 



The rocky crags or spurs connected with the Sow and Pigs and 

 extending fully 600 yards down the harbour, inclining towards 

 the north shore, offered a favourable catchment ground for the silt 

 borne in or out by the tides, and gradually a bar was formed across 

 to Middle Head. This reduced the force of the current along the 

 north shore and induced the deep accumulation of silt shown by 

 the borings. For a long period afterwards the force of the current 

 running through the narrow deep channel on the south shore was 

 sufficient to scour it, but gradually the equilibrium between the 

 scour and deposit was destroyed and the channel choked which 

 led to deposition of silt in the channel on that side. Whilst this 

 bar according to the soundings on Captain Sydney's, R.N. Chart 

 is 1300 yards wide in the centre, and 1000 yards on the north 

 side, it is only 550 yards wide on the south shore. Judging by 

 the narrowchannel beyond the south end of the bar the accumulation 

 of silt appears to be gaining ground on this side, so that in course 

 of time the bar will be as wide on one side as the other. The 

 actual extent and thickness of this silt formation and its rate of 



