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The terraces of the lower colo and hawkesbury drainage basins, New South
Wales
E. J. Hickin a
a
Teaching Fellow in the Department of Geography, University of Sydney,
To cite this Article Hickin, E. J.(1970) 'The terraces of the lower colo and hawkesbury drainage basins, New South Wales',
Australian Geographer, 11: 3, 278 — 287
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The Australian Geographer, XI, 3(1970), pp. 278-87
THE TERRACES OF THE LOWER COLO AND
HAWKESBURY DRAINAGE BASINS,
NEW SOUTH WALES
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E . J. HICKIN*
The principal aim of this study is to examine certain suggestions made by Professor G. H. Dury (personal communication)
in preliminary studies of the Lower Colo
Valley in 1965-6. This study includes a
description of the bench levels in the Lower
Colo and Hawkesbury Valleys: it has already been demonstrated that present bankfull discharge does not causally relate to
these benches and that they are in fact true
terraces. 1 In addition, some general comments are offered about the geomorphic
history of the area.
The field area
The Colo drainage system, some 1,800
square miles in area, forms part of the Sydney Basin. The Capertee River (in the headwaters of the Colo drainage basin) has incised into the uplifted Permian sandstones,
coal and shale, Lower Carboniferous quartz
porphyry and Devonian limestone. The ingrown meanders of the Lower Colo Valley2
have formed, for the most part, in horizontal
well-jointed sedimentary rock. It is likely
that the Hawkesbury and Narrabeen sandstones and shales have provided most of the
alluvium for the fill in the Lower Colo Valley.
The most obvious influence on the development of the Colo Valley has been that
phase of uplift which formed the Lapstone
Monocline and the plateau to its west. Little
is known of the pre-uplift pattern of Sydney's
rivers. The pre-uplift course of the Colo
River may probably be defined by a line of
*Mr Hickin is a Teaching Fellow in the Department of
Geography at the University of Sydney.
278
depression 600 feet deep, shown in the
plateau surface and in the Kamilaroi Coal
Measures.3 Craft suggests that the line of
depression resulted from a slight roughening
of the virtually undenuded surface during
an early phase of uplift of the Mesozoic
rock. In the light of recent work, however,
a suggested stream origin based on peneplanation and rapid uplift must be treated
with great caution.
During uplift a considerable amount of
stream derangement seems to have taken
place. River gravels on parts of the Lapstone Monocline, and at Windsor and Penrith, mark a pre-uplift course of the Hawkesbury River quite different from the present
course. Further evidence of stream derangement is found near several of the
Hawkesbury tributaries such as the Cox,
Wollondilly, and Warragamba Rivers.4
It is probable that the pre-uplift course
of the Colo River also has been adjusted to
the tectonically changing landmass. Once
uplift had begun, the Colo River, like most
rivers affected by the uplift, incised deeply
into the plateau to produce ingrown
meanders. Since the beginning of uplift,
lateral changes in the Colo River have probably been restricted to the widening of
meander belts.
Although the date of uplift is usually assigned to the Upper Tertiary (coinciding
with the Main Alpine phase of orogeny in
Europe), there is little agreement in the
period to which uplift belongs. Jensen5 suggests that the late Pliocene and early Pleistocene was the probable period of uplift, while
Craft0 suggests one phase of warping in the
Lower Tertiary, and a second phase (the
Kowmung Warp) in the Upper Tertiary.
THE TERRACES OF THE LOWER COLO AND HAWKESBURY DRAINAGE BASINS
0
5
10
15
20 Miles
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N
Upper Colo _.
divide....../
j) Leets
Vale
Q
1
Figure 1. Location map of the study area.
2
3
A Miles
279
THE AUSTRALIAN GEOGRAPHER
280
70
60
-feet
50
H higher terrace
L lower terrace
R river
MEROO BEND
60
30
Vertical exaggeration 6.65
20
10
feet
J
100
200
300
600
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20 -
0
700
800
900
1000
..
/
1100
-"
®
/
L
30 -
10
600
H
UPPER COLO
60 -
500
/
f^^—^y
"R,
t
100
100
i
l
l
200
300
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1100
200
300
600
500
600
700
800
900
1000
1100
200
300
600
500
600
700
800
900
1000
1100
Figure 2. Terrace sections in the Lower Colo and Hawkesbury Valleys.
281
THE TERRACES OF THE LOWER COLO AND HAWKESBURY DRAINAGE BASINS
70
feet
60
50
'WATTLE'
H higher terrace
L lower terrace
R river
Vertical exaggeration 4.65
feet
100
200
300
400
500
600
700
800
900
1000
1100
100
200
300
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1100
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70
0
VI)
60 -
LEET'S VALE
50 40 30 -
10
n
R/
D
r*
100
4U
30
20
_^—-^
H
*>n
•—
200
i
t
i
i
300
400
500
600
WISEMANS
feet
i
700
i
800
i
i
900
1000
©
FERRY
-
H
10
0
1100
100
200
300
, feet
r
1
1
1
400
500
600
700
800
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i
40
MANGROVE CREEK
100
200
300
400
500
Figure 2. Continued.
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282
THE AUSTRALIAN GEOGRAPHER
The latter view is also held by David and
Browne7 who assign one phase of uplift to
the Miocene and a later phase (the Kosciusko Movement) to the Pliocene. At the
present time, all that can be stated with any
certainty is that uplift must post-date the
cessation of deep weathering about 20 million years ago;8 that is, near the Miocene/
Oligocene boundary.9
The intermittent nature of the uplift is indicated by the valley forms of the Blue
Mountains. Many of the ridges have nonstructural benches, some continuing for several miles. Yellow Pup ridge in the Cox
River basin and Perry's Lookdown in the
Grose River basin offer good examples. In
the Colo Valley, a rock-cut surface occurs
between 300 and 400 feet above river level
on at least four ridges near Upper Colo.
These are possibly the results of a short
stable period during the general course of
uplift. A much lower rock-cut surface at
about 100 feet above the Colo River level
is also possibly the result of uplift. This
lower level seems to correspond to a similarly located surface at Glen Davis and Glen
Alice. Although these lower rock-cut surfaces may not be part of one continuous
surface, a fluvial origin for all is likely.
Regional underfitness of stream channe j s io,n,i2 SU gg es ts that the Colo basin at
times during the uplift probably had a larger
runoff and a moister climate than at present.
Following the end of the major phase of
tectonic activity, the relatively stable coastline13 has been influenced mainly by fluctuations in the Pleistocene sea-level. During
these changes in sea-level, it is likely that the
Colo Valley was at times filled with alluvium
in its lower reaches and completely scoured
at other times. At present much of the alluvial fill remains in many of the valleys
of the Hawkesbury drainage net.
The terraces in section
For the greater part of its length, the alluvial
fill of the Colo Valley constitutes two welldeveloped terraces. Although the terraces are
clearly seen in the lower reaches of the valley, they are neither clearly defined, nor
indeed present, over the full length. The
main array of terrace alluvium is continuous from Mangrove Creek on the
Hawkesbury River to the Meroo bend on
the Colo River (Figure 1). Upstream of
the latter point, it occurs only on bends;
it is absent from straight reaches and
from the inflections of valley bends. At
Hungryway Creek the terraces are poorly
developed, and disappear completely a
short way upstream. Aerial photographs indicate the absence of terraces between Parr
South and Glen Davis. Upstream of Glen
Davis to the headwaters of the Capertee
River, extensive alluvium reaches a depth of
at least 180 feet. The Glen Davis-Glen Alice
alluvial fill has not been examined in great
detail; it is provisionally taken by the author
as representing a sedimentary lake, similar
to those described by Taylor14 and by
Craft,15 respectively on the Nepean and Cox
Rivers. The narrow gorges east of Glen
Davis, in association with the north-easterly
dip of the rocks, suggest that the alluvium
may have been deposited behind a dam
formed across the Capertee River during a
major phase of uplift. If this is so, the terrace
levels at Glen Davis cannot be expected to
correspond with those of the Lower Colo
Valley.
The terraces of the Lower Colo-Hawkesbury Valleys have been surveyed by dumpy
level at seven sites in the Colo Valley and
three sites in the Hawkesbury Valley; the
cross-sections are illustrated in Figure 2.
The first upstream cross-section showing
well-developed terraces is that on the Meroo
bend. The higher terrace at this point is
about 400 feet wide and over 50 feet above
the low-flow river level. An important property of the terrace is the landward slope of
the surface, a feature displayed also by the
terraces at many downstream sites. This
slope is unlikely to be associated with levee
building; rather it is probable that very high
discharges scoured the backs of terraces,
producing the often pronounced landward
fall away from the terrace fore-edge.
Subsequent runoff from the hillslopes
also follows the lower area behind the terrace
fore-edge until it reaches the Colo River
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THE TERRACES OF THE LOWER COLO AND HAWKESBURY DRAINAGE BASINS
283
Plate 1. The terraces on the downstream limb of the Upper Colo Valley bend.
through very deeply incised breaches in the
terrace. During heavy hillslope runoff, lakes
often form at the back of the ill-drained
terraces, and relatively fine material is deposited behind the terrace fore-edges. The
coarse to fine gradation of sediment from
the terrace fore-edge to the area at the back
of the terrace is not to be confused with the
similar sediment pattern usually attributed to
overbank flooding and levee construction.
The lower terrace at the Meroo bend is
very narrow and stands 30 feet above the
low-flow river level.
Eight miles downstream from Meroo, at
Upper.Colo, the terraces are lower but far
more extensive than those upstream (Figure
2b and Plate 1). The higher terrace is 50
feet above low-flow river level, more than
1,200 feet wide, and on the line of section
it has no landward slope. The lower terrace,
25 feet above river level and about 150 feet
wide, has an uneven surface of local drainage channels. Between the two terraces is a
40-foot-wide intermediate feature at about
43 feet above river level. This bench may
be the remains of another terrace, but seems
more likely the result of recent erosion.
Since the level occurs in but two of the ten
sections (see cross-section for Mandalay) it
has not been treated as a separate terrace,
although further work might show it to be
one.
Gross-sections at Somerset, Mandalay
and The Ruin indicate decreasing terrace
height in the downstream direction (see
Figures 2c, d, and e), although recent flood
scour and deposition, together with human
interference, have obscured the terrace levels
at some sites. Discontinuous gullying and
drainage net development are occurring behind the terrace fore-edges in many places
(see Plate 2). At The Ruin, several hundred
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284
THE AUSTRALIAN GEOGRAPHER
Plate 2. Part of a discontinuous gully developed on the higher terrace at Moran's Rock
yards downstream of the line of section in
Figure 2e, a flood channel has cut into the
terrace to a depth of 15 feet. This channel
is not indicated by the 25-foot contour of the
1:63,360 military map (1955 edition), and
may therefore have formed since the relevant survey.
At the bend named Wattle 2.3 miles
downstream from The Ruin and 3.1 miles
upstream from the confluence of the Colo
with the Hawkesbury River, the terraces are
much lower than those upstream and are in
consequence more frequently flooded.
Scouring and deposition have considerably
obscured the terrace levels. Nevertheless,
Figure 2f suggests that the higher and lower
terraces are respectively about 20 and 10
feet above the low-flow river level. Scouring
at the rear of the 250-foot-wide terraces and
deposition on the fore-edges have produced
extensive landward slopes of 0.035 on the
higher terrace and 0.025 on the lower terrace.
The Livingstone bend, 1.7 miles upstream
from the confluence with the Hawkesbury,
is the last cross-section on which the lower
terrace level is easily recognized. This is also
the last downstream site at which distinct
flood channels occur on the landward side
of the terraces. The deepest part of the flood
channel at Livingstone is only eight feet
above the low-flow river level. The lower
relative altitude of the downstream terraces
apparently hinders flood channel development.
The higher terrace level of the Colo River
corresponds to the main alluvial flat on the
Hawkesbury River. At Leets Vale, 7.8 miles
downstream from the Colo/Hawkesbury
junction, the higher terrace, about 17 feet
above river level, is 800 feet wide; the increased width reflects the relative increase in
lateral activity of the larger Hawkesbury
River. The lower terrace, if present, is not
easily identifiable.
At Wiseman's Ferry, 4.5 miles downstream from Leets Vale, the higher terrace
is more than 1,000 feet wide, but is only 11
THE TERRACES OF THE LOWER COLO AND HAWKESBURV DRAINAGE BASINS
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—40
-120
-160
1
2
3
C
5
6
Higher terrace surfec*
Lower terrace surface
Prese
Mean sen level
Present river bed
Valley basement
\
10,000
2
" °°
285
0
0
*
10,000
20,000
Horizontal scale
30,000
MJOO yards
N.__
,
^~~?
-240
•280
Figure 3. Longitudinal profiles of the rock-cut basement, the present river bed, and the terraces of the
Colo and Lower Hawkesbury Valleys.
feet above river level. About nine feet below
the terrace is a 40-foot-wide mud flat which
may represent the lower terrace. Alternatively, the mud flat might represent the
existing flood plain of the Hawkesbury
River; but this interpretation would not account for the rise of the flat to a height of
five feet above the tidal stream at the Colo/
Hawkesbury junction. On balance, the location of the lower terrace on the Hawkesbury
River remains obscure.
The terraces in profile
The thalwegs of the present Colo channel,
the terraces and the rock-cut valley basement
are illustrated in Figure 3. Each terrace
cross-section has been reduced to standard
datum (mean sea-level) by one of two
methods: all cross-sections upstream from
Mandalay have been related to a surveyed
permanent bench mark at Moran's Rock,
and since the mean water surface of the
Colo River at this point is at mean sea-level,
all cross-sections downstream from this
point have been related to mean river level
and thus to mean sea-level. In addition to
the levelling of cross-sections, more than 20
surveyed spot heights have been used to construct the 80 miles of stream and terrace
profiles. The location of the present stream
bed in the lower reaches has been obtained
from the Broken Bay Admiralty chart. The
profile of the valley basement has been obtained from bore logs at bridge sites. The
profile of the present stream bed has been
generalized to provide a smooth curve; the
other profiles have also been generalized but
to a lesser extent.
Figure 3 shows that, for the most of its
length, the valley basement slopes more
steeply to the sea than do the terraces. Although the terraces and valley basement
achieve a rough parallelism for some distance upstream of Meroo bend, still further
upstream they converge. Between Hungryway Creek and Mandalay the valley basement probably has a relatively constant seaward slope of about 0.0014. The alternative
profile in Figure 3 would only apply if rock
reported in the river bed near The Ruin16
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286
THE AUSTRALIAN GEOGRAPHER
represents the maximum depth of the rockcut valley. Field inspection suggests, however, that this is an unlikely possibility. The
downstream slopes of the river bed and terraces, unlike the downstream slope of the
valley basement, are reduced from 0.006 at
Hungryway Creek to about 0.0003 at Mandalay. After this initial reduction in slope,
the river bed displays an increasing slope to
the confluence with the Hawkesbury, where
like the terraces it displays an abrupt change
to a landward slope before resuming a seaward slope further downstream. The resulting hump has very likely been caused by the
confused stream flow at the junction of the
rivers, resulting in greatly increased deposition. Farmers report that during floods on
the Hawkesbury River a negligible stream
velocity is experienced in the Colo for some
distance upstream from the confluence.
Downstream from the confluence the terraces are roughly parallel and have an average seaward slope of about 0.0003. They
appear to pass beneath mean sea-level about
4 miles downstream from Wiseman's Ferry
(lower terrace) and about 7 miles downstream from Mangrove Creek (higher terrace).
The valley basement, which at Wiseman's
and Peat's Ferries has seaward slopes of respectively 0.0004 and 0.0001, is 245 feet
below mean sea-level at the headlands of
Broken Bay.
The general scheme of relationships,
therefore, is a rock-cut valley basement sloping towards the sea more steeply than the
stream channel and terraces. The terraces,
roughly parallel and sloping more steeply
towards the sea than the Colo/Hawkesbury
low-flow river surface, pass beneath mean
sea-level several miles inland of the coastal
headlands. Seaward extrapolation of the terrace profiles would probably place the higher
and lower levels at respective depths of
about 10 and 20 feet below mean sea-level
at the headlands of Broken Bay.
Interpretation
Although the geomorphic history of the Colo
Valley is extremely complex, on the basis of
the terrace character a few general remarks
can be offered about some of the major
geomorphic events.
Since the Hawkesbury basement is 245
feet below sea-level at the present coastline, the river which formed it must have
flowed to a sea-level much lower than the
present one. Although the basement must
relate to at least a —245-foot-level, it could
well relate to a much lower one at a former
eastward coastline. The level of the Hawkesbury basement at the present coastline accords with the levels of the surrounding
valley basements of 181 feet (Seaforth), 203
feet (Parsley Bay), 250 feet (Georges
River) and 268 feet (Hunter River at
Stockton), (data from D.M.R. bore logs
at bridge sites).
The latest period of cutting of the Colo
Valley basement is likely to have been during the last glacial (Wiirm) lower sea-level.
According to Fairbridge17 this sea-level stood
at about —100 metres (—385 feet), but
more recent work indicates a former sealevel possibly as low as —450 feet.18 At the
time of the last glacial maximum the coastline of New South Wales was probably at
least 12 miles east of the present coast.19
Since the valley basement profile in Figure
3 can only be extended to —450 feet at the
former 12 mile coastline by increasing its
seaward slope, it probably corresponds to a
river which was working to a sea-level above
this low level prior to the beginning of the
Recent.
Since the beginning of the Recent the
sea-level has risen rapidly but intermittently
to reach a level close to that of the present
approximately 5,000 years ago.20 The intermittent nature of the Flandrian Transgression, as this rise in sea-level is called, is reflected in the pattern of alluviation in the
Colo Valley. While sea-level was still rising,
the Colo River built its bed to a maximum
height corresponding to the level of the
higher terrace. The higher terrace formation,
although of Recent age in the upper reaches
of the Lower Colo, may well be underlain by
varying depths of Pleistocene alluvium in
the wider Hawkesbury Valley. The deposition of the higher terrace material was followed by a period of incision. This was sue-
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THE TERRACES OF THE LOWER COLO AND HAWKESBURY DRAINAGE BASINS
ceeded in turn by a further period of alluviation, during which the lower terrace deposits
were laid down. Deposition of the lower
terrace material was followed by a further
phase of incision and subsequent alluviation
to the level of the present river bed. Since
the last major stage of incision, a slight
submergence has taken place, drowning the
Lower Hawkesbury and Colo Valleys. This
submergence is probably the result of late
eustatic adjustments in sea-level.
Although the terrace levels in the Colo
Valley correspond to certain levels of the
sea, it is not suggested here that they are
causally related to them. It is more likely
that the climatic changes which accompanied
changing sea-levels were responsible for initiating phases of incision and alluviation.
The correspondence of the Colo River
terraces and lower sea-levels conflicts with
the interpretation of many workers. For example, Taylor, Hall, Craft, and Walker and
Hawkins use similar terrace evidence as an
indication of a higher (+20 feet) sealevel.21'22'2324 These interpretations, in the
light of the present study, seem likely to be
incorrect. The present evidence supports the
more recent suggestions of Langford-Smith
and Thorn23 and Dury20 relating the so-
287
called +20-foot-level to a lower stand of
the sea.
Conclusions
Detailed analysis of coastal valley forms can
provide much information about the climatic, eustatic, and tectonic environment in
which they have evolved. The present study,
although indicating a set of terraces related
to lower sea-levels than the present one, has
examined the evidence from only one valley
system. There is a real need for similar
studies of all the New South Wales coastal
rivers to provide the necessary amount of
data to test the tentative chronology presented in this study. It is important to note
that river terraces may only be related to
relative levels of the sea (i.e. lower or higher
than the present). Unless the exact locations
of former coastlines are known, it is impossible to relate river terraces to specific levels
of the sea.
The broad framework of this study was suggested
by Professor G. H. Dury. The author would also
like to express his gratitude for Professor Dury's
constructive criticisms during all phases of the
study.
REFERENCES
1. E. J. Hickin, 'Channel morphology, bankfull stage,
and bankfull discharge of streams near Sydney',
Austr. Journ. Science, Vol. 30, 1968, p. 274.
2. G. H. Dury, 'Incised valley meanders on the lower
Colo River, New South Wales', Austr. Geogr., Vol.
10, 1966, pp. 17-25.
3. F. A. Craft, 'The coastal tablelands and streams of
New South Wales', Proc. Linnean Soc. N.S.W., Vol.
58, 1933, pp. 437-60.
4. F. A. Craft, 'The physiography of the Cox River
Basin', Proc. Linnean Soc. N.S.W., Vol. 53, 1928.
pp. 207-54.
5. H. I. Jensen, 'The river gravels between Penrith
and Windsor', J. Proc. Roy. Soc. N.S.W., Vol. 55,
1911, pp. 249-57.
6. F. A. Craft, op. cit., 1933, pp. 437-60.
7. T. W. E. David, The Geology of the Commonwealth
of Australia, W. R. Browne (ed.), Edward Arnold,
London 1950, Vol. 1, p. 586.
8. N. F. Exon, T. Langford-Smith, and I. McDougall,
'The age and geomorphic correlation of deep "weathering profiles, silcrete, and basalt in the Roma-Amby
region, Queensland' (in press, 1969).
9. B. F. Funnell, 'The Tertiary Period', Quart. Journ.
Geol. Soc. London, Vol. 120S, 1964, pp. 179-91.
10. G. H. Dury, op. cit., 1966, pp. 17-25.
11. E. J. Hickin, unpublished data.
12. R. W. Young, 'Some Aspects of the Fluvial Morphology of the Lower Shoalhaven Basin', thesis submitted for the degree of Master of Arts, 1968, Geomorphology, Chapters 2 and 6.
13. E. C. F. Bird, Coastal Landforms: an introduction to
coastal geomorphology with Australian examples, Australian National University Press, Canberra 1964, p.
23.
14. G. T. Taylor, 'The warped littoral around Sydney',
J. Proc. Roy. Soc. N.S.W., Vol. 62, 1923, pp. 58-79.
15. F. A. Craft, op. cit., 1933, pp. 437-60.
16. G. H. Dury, op. cit., 1966, pp. 17-25.
17. R. W. Fairbridge, 'The changing level of the sea',
The Scientific American, Vol. 202, 1960, p. 76.
18. R. J. Russell, 'Techniques of eustacy studies', Zeits.
fur Geom., N.F. Band 8, 1964, p. 39.
19. T. Langford-Smith and B. G. Thorn, 'New South
Wales Coastal Morphology', in G. H. Pacham (ed.),
The Geology of New South Wales, Geol. Soc. Aust.
(in press, 1969).
20. J. R. Hails, 'A critical review of sea level changes in
Eastern Australia since the last glacial', Austr. Geogr.
Studies, Vol. 3, 1965, pp. 63-78.
21. G. T. Taylor, op. cit., 1923, pp. 58-79.
22. L. D. Hall, 'The physiography and geography of
the Hawkesbury River between Windsor and Wiseman's Ferry', Proc. Linnean Soc. N.S.W., Vol. 51,
1926, pp. 555-93.
23. F. A. Craft, op. cit., 1933, pp. 437-60.
24. P. H. Walker and C. A. Hawkins, 'A study of river
terraces and soil development on the Nepean River,
New South Wales', J. Proc. Roy. Soc. N.S.W., Vol.
91, 1957, pp. 67-84.
25. T. Langford-Smith and B. G. Thom, op. cit., 1969.
26. G. H. Dury, op. cit., 1966, pp. 17-25