ANNIVERSARY ADDRESS. 57. 
has been swept through the narrow gorge of the Nepean, where it 
debouches from the Blue Mountains, about three miles above 
Penrith. 
An interesting feature in connexion with these recent alluvials 
is the fact that they appear to descend far below sea level, which 
confirms the theory that the coastal strip has subsided to a depth 
of two or three hundred feet, and so has enabled the waters of 
the Pacific to inundate the valleys of the Hawkesbury and the 
Parramatta rivers for a considerable distance inland from their 
original seaward terminations. That these alluvials in the estuary 
of the Hawkesbury extend considerably below sea level, “was 
proved by the excavations and trial borings made along the line 
of the present Hawkesbury River railway bridge. The deepest 
of these borings penetrated to a depth of one hundred and seventy- 
six feet below sea level, without completely passing through the 
alluvial deposits. The water of the estuary was here found to be 
fifty feet deep; the thickness of the alluvials at this spot would 
therefore be at least one hundred and twenty-six feet. No absolute 
proof however, was obtained that the lowest alluvials in this 
estuary were distinctly of fluviatile and of fresh-water origin; but 
as already stated, the lowest alluvials were not reached in the 
borings. It would be very important to ascertain whether coarse 
river gravels do not there underlie the estuarine clays ; and if any 
shells of Unio or Cyclas were discovered, in situ, in these alluvials, 
their original fluviatile origin might be looked upon as proved. 
The borings for the iron piers of the Parramatta railway bridge 
also show that the alluvials extend there for a considerable depth, 
eighty-nine feet, below sea level, the alluvials being at least sixty- 
three feet thick. The diamond drill bores for coal at Narrabeen 
Lagoon struck a bed of peaty loam fifteen feet in thickness, at 
a depth of over twenty feet below sea level; and a well marked 
layer of marine shells at over eighty feet below sea level. Both 
these occurrences are strongly in favour of the hypothesis that. 
either the coastal area here has sunk, or the sea level has risen to 
the amount of twenty or eighty feet at least, in late geological time. 
