3. 0 The Environmental Context 1 Introduction




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3.0 The Environmental Context

3.1 Introduction


The following section describes the environmental context of the Lake Burley Griffin Study Area, particularly focusing on the ‘pre-lake’ environment. The discussion of the evolution of the landscape through geological time describes how the key landscape features developed into their present forms. The vegetation which covered the pre-settlement landscape is also described. This description of the environmental setting of Lake Burley Griffin and its foreshores will allow comparisons (in later sections) between its present form and its earlier natural state, and provide a picture of how the current landscape, environment and natural values evolved.

3.2 Geology and Geomorphology

3.2.1 Earliest Sedimentary Deposition


The dominant geology of Lake Burley Griffin and its immediate catchment is of the Ordovician and Silurian Periods (480–416 million years before present). The oldest rocks are the Ordovician sedimentaries (sandstones, mudstones and shales) now exposed only in a band crossing the lake area diagonally between the Deakin and Acton Faults. The Early Silurian sedimentary deposits, belonging to the Black Mountain Sandstone and State Circle Shale formations, also occur along this band. The extent of these formations and those described below is shown at Figure 3.1.

The earliest Ordovician formations, by their fossil record, were laid down when the Canberra region (and most of southeastern Australia) was under deep sea water. The uplifting, folding and subsequent extreme weathering of these formations indicates that, for a time, the rocks were elevated above sea level. This eroded surface was again covered by the sea during the Early Silurian Period because the Black Mountain Sandstone and State Circle Shale formations show fossil evidence of a deep water environment, away from land.1


3.2.2 Limestone Plains


The next depositional phase appears to follow another period of uplift and dry land weathering because the Middle Silurian Canberra Formation (mudstone, sandstone and limestone) was laid over a shallow, submerged hilly landscape. This now underlies the centre and eastern part of the lake and environs, and is presently exposed in the Parliamentary Triangle, Civic, Manuka, Narrabundah and Fyshwick areas. It was from examples in these exposures that the name ‘Limestone Plains’ derived.

3.2.3 Volcanic Phase


Interspersed with the sedimentary layers of the Canberra Formation are tuffs and ashes, indicating the beginning of a period of volcanic activity in the region which was enough to raise the land above sea level once again.2

Extensive volcanic activity in the Middle and Late Silurian Periods formed the Ainslie Volcanics, Mount Painter Volcanics and the Deakin Volcanics. The youngest of the marine Silurian rocks, the Yarralumla Formation shales which make up Red Hill, are sandwiched by large areas of volcanic rock. The Mount Painter Volcanics are the most extensive, occupying the area west of the Deakin Fault.


3.2.4 The First Molonglo Lake Landscape


There are no rocks in the Canberra region from the end of the Silurian to the late Tertiary Period. During this time, major uplift and movement events occurred in southeastern Australia and formed the main geomorphology of the Canberra region. This is the horst-graben-horst sequence across the region corresponding to the Brindabella Range, Canberra area and Queanbeyan Hills respectively. The local topographic features of Black Mountain, Mount Ainslie, Mugga Mugga and Red Hill are the more durable rocks remaining after the weathering of the central graben area by the ancient Molonglo River and its tributaries gradually lowered the softer surrounding landscape.3

The next significant geomorphological event occurred in the Pleistocene Epoch of the Quaternary Period (around 2 million years ago). Öpik4 has pointed to evidence that scree falls from Black Mountain (which consolidated into formations called ‘fanglomerates’ during a dry interval of the Pleistocene Ice Age) dammed the Molonglo River and Sullivans Creek to form lakes along the courses of both waterways. The approximated extent of the ancient Molonglo Lake was charted by Öpik (see Figure 3.2) and closely mirrors the present Lake Burley Griffin. He postulated that after the rivers had finally broken through the obstacles, the lakes disappeared, leaving behind the sandy river flats. Subsequent work by Brown and Ollier5 supported the existence of the Sullivans Creek lake but could not confirm Öpik’s delineation of a Molongolo River lake, suggesting instead a series of changing channels and terraces over a similar area. More recently, Henderson and Strusz list the unconsolidated sands, clays and gravels of the old lake beds and the remnant sand dunes at Russell and Pialligo as evidence of ancient lake-forming processes.6


3.3 The Biophysical Environment


The pre-European vegetation of the area comprised eucalypt dry open forest on the hills and upper slopes, grassy woodlands on the lower slopes and river flats, and riparian forest (Casuarina and eucalypt) along the waterways.

The open forest areas of the uplands were unsuitable for farming and consequently many of these forest areas remain, providing indicators of previous vegetation cover. However, much of the existing vegetation can best be described as ‘modified natural’ and represents remnants of the pre-European vegetation, in a changed structural and floristic form. Other remnants are more modified, having some native species present but are substantially altered through reduced density or removal of some layers of vegetation.7

The four prominences surrounding Lake Burley Griffin, which provide its immediate ‘bushland setting’, are each of a different geological and soil type. However, their forest cover is remarkably consistent. Complete structural and floristic analyses of the vegetation communities separate them into distinct ecological entities, but the same group of species occur as dominants in the open forest and woodland communities of the area. This is an indication of the overriding effects of climate (especially the number of frost free days) and elevation in determining the vegetation of the Canberra region, and the correspondingly lesser effect of geology and soils.

3.3.1 Black Mountain, Acton Foothills and Stirling Park


Black Mountain comprises hard sandstones and has a vegetation community dominated by the red stringybark (Eucalyptus macroryncha)–scribbly gum (Eucalyptus rossii) alliance. Within this alliance, brittle gum (E. mannifera) and yellow box (E. melliodora) achieve local dominance in patches on the upper slopes. At lower elevations (including the Acton foothills), scribbly gum is replaced by Blakeley’s red gum (E. blakeleyi) and red box (E. polyanthemos). Throughout, the dominant understorey is tussock grass (Poa spp.).8

The same vegetation alliance extends across the lake to Stirling Park, which is also on surface geology of the Black Mountain sandstone formation. Not as high as Black Mountain, the ridgeline of Stirling Park lacks red stringybark and instead is dominated by scribbly gum (Eucalyptus rossii) and brittle gum (E. mannifera), with extensive stands of yellow box (E.. melliodora), Blakeley’s red gum (E. blakeleyi) and apple box (E.. bridgesiana) occurring on the slopes.


3.3.2 Mount Painter and Stromlo Foothills


The surface geology of these areas is the Mount Painter Volcanics. Only on Mount Painter does native forest vegetation survive; however, the same association would have covered the slopes to the south of Mount Painter down to the Molonglo River, before they were cleared for agriculture. The dominant vegetation is a Blakeley’s red gum (E. blakeleyi)–yellow box (E. melliodora) association, with isolated apple box (E. bridgesiana) and brittle gum (E. mannifera).9

3.3.3 Red Hill


Red Hill is on the shales and calcareous shales of the Yarralumla Formation. It supports a woodland association of yellow box (E. melliodora)–Blakeley’s red gum (E. blakeleyi), with scattered apple box (E. bridgesiana) as a sub-dominant. Apple box (E. bridgesiana) and silver wattle (Acacia dealbata) are on the eastern slopes. Other local dominants recorded include red box (E. polyanthemos)–scribbly gum (Eucalyptus rossii), with broad leaved peppermint (E. dives), apple box (E. bridgesiana) and drooping she-oak (Allocasuarina verticullata).10

3.3.4 Mount Ainslie and Russell Foothills


The Mount Ainslie volcanics consist of dacite lava (formed essentially of feldspar, quartz and biotite). The formation supported dry sclerophyll forests of red stringybark (Eucalyptus macroryncha)–scribbly gum (Eucalyptus rossii) on the higher ground, grading into yellow box (E. melliodora)–Blakeley’s red gum (E. blakeleyi) woodland with scattered apple box (E. bridgesiana) on the lower slopes. The open forest also includes brittle gum (E. mannifera), red box (E. polyanthemos), apple box (E. bridgesiana), snow gum (E. pauciflora) and hill oak (Casuarina stricta).11

3.3.5 Limestone Plains


The plains below these high points were a patchwork of woodland, grassy woodland and grasslands.

The woodland and grassy woodland were not distinct floristic or even structural formations. They were charcterised by the co-dominant canopy species of yellow box (E. melliodora) and Blakeley’s red gum (E. blakeleyi), with apple box (E. bridgesiana) along ephemeral drainage lines. The density of canopy trees varied from 10% to 30% coverage, with communities at the lower end of the density range classed as grassy woodlands and communities at the upper end classed as woodlands. The areal balance between the two would have varied as a result of fire regimes, with the woodland form being dominant at some periods and the grassy woodland at others. The understorey of both was characterised by tussock grasses of kangaroo grass (Themeda triandra) and spear grasses (Austrostipa spp).

The grasslands were part of the highly variable natural temperate grasslands of the Southern Tablelands. This is an ecological community dominated by moderately tall or tall (up to 1m) dense to open tussock grasses with a significant representation of forbs (other non-woody plants). It can be treeless or have trees up to 10% cover. Enough of the ‘entirely treeless’ grassland mapped by Prior12 remain to give an indication of its floristics on the Limestone Plains. The dominant grasses were kangaroo grass (Themeda triandra), spear grasses (Austrostipa spp) and wallaby grasses (Austrodanthonia spp), with river tussock (Poa labillardieri) in the lower elevations. Common forbs were sedge (Carex appressa), common everlasting (Chrysocephalum apiculatum), button wrinklewort (Rutidosis leptorrhynchoides) and common sunray (Triptilodiscus pygmaeus).13

Riparian forest stands followed the courses of the Molonglo River and its perennial tributaries. This existed mainly as ‘gallery’ stands of trees along the banks, but sometimes extended over sand banks on the wider river meanders. In pre-European times, this forest would have comprised river oak (Casuarina cunninghamia) and ribbon gum (Eucalyptus viminalis)14, with occasional patches of apple box (E. bridgesiana).


3.3.6 Pre-European Vegetation Extent


A map of the pre European extent of these vegetation communities is provided at Figure 3.3. This map has been derived from a number of sources of data. The grasslands distribution is derived from estimates of the original extent of grasslands in the ACT Lowland Native Grassland Conservation Strategy.15 This study was a refinement of the modelled distribution undertaken by Pryor in 193816, adding survey data from all remnant sites, historical records and the mapping of cold air drainage.

The open forest, woodlands and riparian forest distributions have been derived from:



  • early photographs;17

  • the 1909 mapping by Scrivener;18

  • the 1929 aerial photographic survey of Canberra by the RAAF;19

  • species distribution notes in Pryor and Banks,20 Burbridge and Gray21, and NCDC22; and

  • field data collected from remnant vegetated areas for this heritage assessment.

The following structural communities have been delineated and mapped in Figure 3.3:

Structural Community

Dominant Canopy Species

Open Forest

Eucalyptus macroryncha - E. rossii – E.. mannifera




E. rossii – E. dives – E. polyanthemos




E. melliodora – E. blakelyi +/- E. bridgesiana

Woodland

E. albens – E. melliodora – E. blakelyi +/- E. bridgesiana

Grasslands

Themeda – Austrostipa - Austrodanthonia

Riparian Forest

Casuarina cunninghamminana – E. viminalis – E. bridgesiana


3.4 Biodiversity


It is difficult, and potentially misleading, to pinpoint the biodiversity values of the area before European settlement, since species composition and richness would have varied with the dynamic transitions between woodland, grassy woodland and grassland over time and with changing environmental conditions (and Indigenous habitat modification). In addition to resident wildlife, the area is also traversed by species migrating to and from the adjacent alpine areas.

3.4.1 Fauna Diversity


The Canberra Nature Park Plan of Management23 lists recordings of 12 different mammal species and 28 reptile and amphibian species for the combined open forest and woodland areas associated with Black Mountain, Mount Ainslie, Mount Painter and Red Hill. A comparison with similar but relatively undisturbed habitat areas (in Gould24) suggests that this is at or near natural levels for the area. The same plan of management notes that the diversity of bird fauna in the ACT is due to not only the range of natural habitats in the region, but also the habitats created by city gardens and parks, and the artificial urban lakes. Of the 245 bird species recorded for the ACT by the National Parks Association25, only a proportion would be indicative of the diversity present in the natural habitats of the Limestone Plains and adjacent high ground. Invertebrates are the major component of animal biodiversity in grasslands, but only the biodiversity ‘flagship’ species of the Golden Sun Moth (Synemon plana), Canberra raspy cricket (Cooraboorama canberrae), Key’s matchstick (Keyacris scurra), and Perunga grasshopper (Perunga ochracea) have been surveyed in any detail.

3.4.2 Flora Diversity


Black Mountain open forest and woodlands support around 500 plant species in 450ha. Aranda bushland is also considered environmentally significant, with 220 native flora species. Comparisons in Gould26 indicate that these species numbers are also at or near natural levels for the area. Red Hill, a much more disturbed habitat, currently has approximately 105 different native plant species. Similarly, a recent survey of Stirling Park27 recorded 101 native flora species.

Natural temperate grassland is very diverse. Recent surveys indicate that a total of more than 500 native and herbaceous species occur in these ecosystems in the Southern Tablelands.28 However, in surveys undertaken in ACT lowland native grasslands since 1991, 50 species of grasses and over 200 species of native forbs (including sedges and rushes, lilies and orchids) have been identified.29 The varying nature of species diversity in grassland is further illustrated by the recording of about 50 grassland species at Yarramundi Peninsula in 1996.30


3.5 Conclusions


This discussion of the environmental context of the Lake Burley Griffin Study Area shows that the principle of designing with nature which was a central theme of Griffin’s work on Canberra is borne out by the evidence of the landforms and landscape. The built features of Canberra and its central lake were not imposed on the landscape, but have been fitted into it. The lake closely mirrors the shape and extent of ancient lake or channel deposits on the same site. The forested hilltops and wooded slopes bounding the lake are mostly intact visually and, on the basis of structure, floristics, and biodiversity, are also largely intact biologically. Only the grasslands and riparian forest, which once covered the central parts of the Limestone Plains, have been severely disrupted by later development. Surviving remnants of these communities around Lake Burley Griffin will therefore have particular natural heritage value, while the creation of the lake has in turn created a new ecosystem. The natural values of the lake as an ecosystem are discussed in Section 8.0.



Figure 3.1 Geology and geomorphology.



Figure 3.2 The hypothesised extent of an ancient Molonglo Lake.



Figure 3.3 Pre-European settlement vegetation (as discussed in Section 3.3.6).

3.6 Endnotes


1 Bureau of Mineral Resources 1981, The Geology of Canberra, Queanbeyan, and environs. 1:50,000 geological map and explanatory notes, Australian Government Publishing Service, Canberra.

2 ibid.

3 ibid.

4 Öpik, AA 1958, The Geology of the Canberra City District, Bulletin No. 32, Department of National Development, Bureau of Mineral Resources, Geology and Geophysics.

5 Brown MC and CD Ollier, 1975, Geology and scenery of Canberra, Australian Geographer, 13, p97–103.

6 Henderson, GAM and Strusz, DL 1982, Rocks and Fossils around Canberra, Bureau of Mineral Resources, Australian Government Publishing Service, Canberra.

7 Environment ACT, 1999, Canberra Nature Park Management Plan 1999, Department of Urban Services Conservation Series No.14, Australian Capital Territory Government.

8 ibid.

9 ibid.

10 ibid.

11 ibid.

12 Pryor, L 1938, The botany, forestry and zoology of the Australian Capital Territory on an ecological basis, in Handbook for Canberra, ed, K Burns, ANZAAS Meeting, Canberra, January 1939, Canberra: Commonwealth Government Printer.

13 Environment ACT 2005, A Vision Splendid of the Grassy Plains Extended—ACT Lowland Native Grassland Conservation Strategy, Action Plan No. 28, Arts, Heritage and Environment, Canberra.

14 Pryor, LD and Banks, JCG 2001 (2nd Edition), Trees and Shrubs in Canberra, Little Hills Press, Canberra.

15 Environment ACT, op cit.

16 Pryor, op cit.

17 NCA Photographic Collection, and in Öpik, op cit.

18 Reproduced in 2004, The Griffin Legacy, Canberra the Nation’s Capital in the 21st Century, National Capital Authority, Canberra.

19 Reproduced in The Griffin Legacy, op.cit.

20 Pryor, LD and Banks, JCG, op cit.

21 Burbidge, NT and Gray, M 1970, Flora of the Australian Capital Territory. ANU Press, Canberra.

22 NCDC, 1984, The Ecological Resources of the ACT, National Capital Development Commission, Canberra.

23 Environment ACT, op cit.

24 Gould, L 2004, Picaree Hill—Integrated Conservation Management Plan, Murrumbateman Landcare Group.

25 Taylor, M and Day, N 1999, Field Guide to the Birds of the ACT, National Parks Association of the ACT Inc.

26 Gould, op cit.

27 Muyt, A, 2006, Stirling Ridge, Yarralumla: Vegetation Survey Methods, Management Issues & Flora and Fauna Species Lists, report prepared for the National Capital Authority, Canberra (unpublished).

28 Eddy, D, Mallinson, D, Rehwinkel, R, and Sharp, S 1998 (3rd reprint), Grassland Flora—a field guide for the Southern Tablelands (NSW & ACT), Crown Copyright.

29 Environment ACT, op cit.

30 Rowell, A 1996, Yarramundi Grassland Vegetation Mapping and Management Recommendations. Report to the National Capital Planning Authority. Canberra: ANUTECH Pty Ltd.

Lake Burley Griffin—Heritage Assessment—Final Report, October 2009




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