The Council Trench at Bacchus Marsh

In 1873, a notice in the Victorian Government Gazette temporarily reserved a ‘six acres two roods twenty-two perches’ allotment of land north of Bacchus Marsh township on the southern slopes of Bald Hill, as a ‘site whence stone may be procured’ (Victorian Government Gazette, 1873). Quarrying records cannot be found but the allotment’s period of time as a stone quarry was probably brief, because the sandstone there was not of high quality. The disused quarry site is known today as the Council Trench.
The derivation of the name ‘Council Trench’ is not known. Chapman (1927) referred to the site as the ‘a trench in the Council Paddock’. The term ‘Council Trench’ first appeared in the literature in 1937 (Jacobson & Scott, 1937). The Council Trench is exposed as outcrop in a trench, about 40m in length and 2 to 5 m depth, cut into a low ridge, on the east side of Tramway Lane, Darley.

Plant macrofossils from the site were identified by McCoy (1892). He determined that the material was of Triassic age. Later palaeontological investigators examined further fossil evidence and confirmed the Triassic age of the outcrop (Chapman, 1919; Chapman, 1927; David, 1950; McCoy, 1898; Officer and Balfour, 1894). Douglas (1969; 1976; 1983; 1988) was more circumspect and assigned the beds to a ‘Triassic or probably Triassic’ age, while Spencer-Jones (1969) assigned them to a ‘Triassic or more generally Mesozoic’ age. M.M. Mitchell (pers. comm.) stated that work in progress on fossil material from the Council Trench confirms a Triassic age.
The Council Trench is geologically significant to Victoria. It is the only known outcrop of Triassic aged sedimentary rock in the state. In 1984 the beds were named Council Trench Formation (Roberts, 1984). The sediment is considered to be of freshwater origin (Mitchell et al., 2000; Roberts, 1988) and consists of massive, contorted, iron-stained sandstone with some conglomerate banding.

In 1980, the Geological Heritage subcommittee of the Geological Society of Australia (G.S.A.), Victoria Division, granted the Trench ‘State’ Geological Heritage status (Joyce & King, 1980). The ‘State’ heritage status was retained after a later assessment (Mitchell et al., 2000).
For over a century, the Trench has been visited and studied by academics, students and amateur geologists, often as a short side trip when visiting the nearby Bald Hill and Korkuperrimul Creek Permian glacials. It was taken for granted that this unprepossessing but important outcrop would remain in public ownership forever.

Despite its geological heritage and historical significance, the Council Trench reserved land was offered for sale in 2000 for residential development. It was part of a Victorian Government initiative to sell off all Crown Land that was not actively managed or had apparent public land value. Intensive lobbying by the Geological Heritage subcommittee, with the government body overseeing all Victorian Crown Land matters, resulted in the offer of sale being withdrawn.

The Committee of Management for the Bacchus Marsh Council Trench Reserve was appointed in early 2003. Members were drawn from the local community and from the G.S.A. Heritage subcommittee. The Committee’s role is to ensure that the Crown Land Reserve is managed and maintained appropriately and that the vegetation on the site is conserved. Weed eradication and fire control measures are management priorities. The Committee received a Victorian Government Crown Land Reserves Improvement Program grant of $5000 in 2003. A significant proportion of the grant has been spent on fencing the Reserve’s roadside boundary, cleaning debris from the Trench, providing a walking path from the roadway to the outcrop, and installing a large red gum sign at the gate identifying the Triassic Council Trench Reserve. A survey of plants on the site has been carried out. The remainder of the grant will be spent on a geological and botanical educational information panel to be placed near the outcrop, as well as on management priorities.
On Saturday October 11th 2003, the Bacchus Marsh Council Trench Reserve Committee of Management Incorporated officially opened the Council Trench Reserve. Forty-eight people, many of them local residents, were present to celebrate the occasion.
In 2000, when it appeared that the Council Trench would become a housing site, the future of the only known Triassic aged sedimentary outcrop in Victoria looked bleak. In 2004 the outlook is bright. The Trench will now remain in public hands as a Crown Land Reserve.

Download here the Extended Abstract of a paper given by Roger Pierson to the Australian Earth Sciences Convention, 2006 (250 kb).

See an entry on the DPI Victorian Resources Online website, with older maps and photos.

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CHAPMAN, F. 1919. A sketch of the geological history of Australian plants: the Mesozoic flora. The Victorian Naturalist, 35(10), 148-156.
CHAPMAN, F. 1927. Monograph on the Triassic flora of Bald Hill, Bacchus Marsh, Victoria. Memoirs of the National Museum, Melbourne, 7:121-155.
DAVID, T. W. E. 1950. The geology of the Commonwealth of Australia. Edward Arnold & Co., London, Vol. 1, i-xx, + 747 p.
DOUGLAS, J. G. 1969. The Mesozoic floras of Victoria, Geological Survey of Victoria, Memoir 28, Parts 1 and 2, 310 p., including 51 plates and 8 maps.
DOUGLAS, J. G. 1976. Chapter 7 Mesozoic. Triassic. In Geology of Victoria, Geological Society of Australia Special Publication No. 5, J. G. Douglas and Ferguson, J. A., eds, Geological Society of Australia, Melbourne, 143-147.
DOUGLAS, J. G. 1983. What fossil plant is that? A guide to the ancient floras of Victoria. Field Naturalists Club of Victoria, Melbourne, i-ix, + 86 p.
DOUGLAS, J. G. 1988. Chapter 7 Mesozoic. Triassic,. In Geology of Victoria, J. G. Douglas &. Ferguson, J. A., eds, Victorian Division, Geological Society of Australia Incorporated, Melbourne, 213-216
JACOBSON, R., &. SCOTT, T. R. 1937. The geology of the Korkuperrimul Creek area, Bacchus Marsh. Proceedings of the Royal Society of Victoria, 50(1), 110-156, Pl. 117 and 118, geological sketch map of Korkuperrimul Creek area.
JOYCE, E.B. J. & KING, R.L. 1980. Geological Features of the National Estate in Victoria. Geological Society of Australia. Victorian Division, 205 p.
MCCOY, F. 1892. Report on palaeontology of the Geological Survey for the year 1891. Annual Report, Secretary for Mines, Victoria for 1891 (1892), 30.
MCCOY, F. 1898. Note on an additional genus of fossil plants found in the Bacchus Marsh sandstone by Geo. Sweet, Esq., F.G.S. Proceedings of the Royal Society of Victoria, (N.S.) 10, 285-286.
MITCHELL, M. M., COCHRANE, R. M., & KING, R. L. 2000. Sites of geological significance in the Melbourne 1 : 250 000 mapsheet area, Geological Survey of Victoria Technical Record 2000/1, Geological Society of Australia, Melbourne, 190 p.
OFFICER, G., &. BALFOUR, L. 1894. Further note on the glacial deposits of Bacchus Marsh. Proceedings of the Royal Society of Victoria, (N.S.) 6, 139-143.
ROBERTS, P. S. 1984. Explanatory notes on Bacchus Marsh and Ballan 1 : 50 000 geological maps. Geological Survey Report No. 76, Geological Survey of Victoria, 102 p.
ROBERTS, P. S. 1988. Chapter 12-3. Bacchus Marsh,. In Victorian Geology Excursion Guide, I. Clark & Cook, B., eds, Australian Academy of Science, Canberra, 213-223.
SPENCER-JONES, D. 1969. Permian deposits of Victoria - a review,. In, Special Publication No. 2. Proceedings of Specialists' Meeting held at Canberra, 25-31 May 1968, D. A. Brown, ed., Geological Society of Australia Incorporated, Canberra, 47-56
VICTORIAN GOVERNMENT GAZETTE. 1873. Lands Temporarily Reserved from Sale, 1317.