May, 1938 The Queensland Naturalist 
89 
Railway Bridge penetrated thirty-five feet of sediments, 
the rock bottom lying 75 feet below the surface of the 
stream.* 
It might be thought that more information with re- 
gard to the thickness of these gravel deposits would be 
available through the activities of the commercial 
dredges, but , constructed as they are for relatively 
shallow dredging, mechanical difficulties and added cost 
have virtually confined their operations to depths within 
30 feet of low water. Hence the majority of the gravel 
banks are abandoned before rock bottom has been 
reached. 
Let us now inquire into the relationship of the Bris- 
bane River gravels to the other features presented by the 
stream. In particular, let us endeavour to trace their 
development in the history of the river, and thus assign 
them to their proper place in the present stream regime. 
Several facts go to show that gravels are not being 
deposited below the Seventeen-Mile Rocks as a normal 
activity at present, but that their accumulation is due 
almost entirely to occasional large floods. The normal 
deposit at present in these reaches is a river mud. This 
can be demonstrated from the facts, first that the 
dredges have to clear awav a thickness of some 6 to 12 
inches of mud before reaching the gravel deposits, and 
secondly, that, when dredges return to examine the holes 
previously dredged in the hope that, thev mav have been 
partially filled with new gravel, thov find instead that 
the holes have been filled with river mud. Still another 
piece of evidence bearing on this point is that before 
they were removed by dredging, a number of the banks 
reached well above the normal surface of the river, and 
could only have been built up during heavv floods, when 
the river was much higher than usual. It would seem 
that normally the net effect of the alternation of some- 
what sluggish ebb and flow separated by two periods of 
slack water is the gradual accumulation of a muddy 
sediment. Most of this almost certainly comes from up- 
stream, but it is the considered opinion of Mr. Fison 
that sand banks such as that removed bv dredging from 
the Botanic Gardens are replaced by mud banks built up 
from material derived from the bar at the mouth of the 
*Since preparing this address, added information 
has come to hand as a result of the putting down of trial 
bores at the proposed site of the St. Lucia Bridge. These 
show that, the sands and gravels extend to a maximum 
depth of 80 feet below low water, 
