Geology

By John Hunt and Malcolm Bruce (geology), and
Bob and Melissa Salt (soils)

The types of vegetation in the Berowra Valley National Park are strongly influenced by the underlying rocks. Different kinds of rock weather to form different drainage patterns, topography and soil types. These geological factors, combined with climate, fire regime and historical biogeography, are the major factors that have produced the native plants and native animal habitats found in the Park today.

Berowra Valley National Park is situated within a geological feature known as the Hornsby Plateau, which is part of a larger geological structure called the Sydney Basin. The Hornsby Plateau has two main sedimentary rock strata: Hawkesbury Sandstone, composed mainly of sandstone, but containing some shale lenses, and the overlying Ashfield Shale of the Wianamatta Group. In some deep valleys, interbedded sandstone and shale of the Narrabeen Group rocks are exposed beneath the Hawkesbury Sandstone. There are also minor volcanic intrusions throughout the area.

 

The sedimentary rocks

Hawkesbury Sandstone is a sedimentary rock that was laid down in the Middle Triassic period between 180 and 220 million years ago (Herbert & Helby 1980). The Hawkesbury Sandstone consists of massive and cross-bedded sheet bedforms with minor (less than five per cent) siltstone and mudstone beds, which contain fish fossils in some locations. It was deposited mainly on a vast riverine floodplain within the geological feature of the Sydney Basin. The crossbeds were formed by lateral accretion on sandbars within channels of the river system. The quartz-rich, nutrient-poor, sandy sediment was derived from the older continental area southwest of the Sydney Basin. A thick blanket, up to 274 m, of this quartz-rich sediment was deposited over the area.

The Ashfield Shale forms a cap, up to 60 m thick, to the Hawkesbury Sandstone in the central Sydney Basin. It is the lower of several shale units that form the Wianamatta Group of Middle Triassic age (Crawford et al. in Herbert & Helby 1980, and Herbert 1983). It was deposited in a freshwater lake and contains insect and vertebrate fossils (Herbert & Helby 1980). Fig. 1 shows what the Sydney Basin may have looked like during deposition of the Ashfield Shale.

The Ashfield Shale was probably derived from a volcanic source to the east of the Sydney Basin. This accounts for the different chemical properties of the nutrient-rich soils formed from the shale, compared with the nutrient-poor soils formed from the underlying sandstone. The shale is generally highly eroded and deeply weathered, changing from its dark grey colour to brown and red. Good exposures can be found in rail cuttings from Hornsby to Beecroft and Hornsby to North Sydney.

The present day surface of the area consists of alluvial soils and sands deposited on the weathered sandstone, with clayey soils developed on the weathered shale.

Volcanic rocks

Intrusive volcanic rocks, which occur in generally circular features called ‘diatremes’ or ‘volcanic necks’, are sparsely distributed in the Sydney Basin. There are about 150 known and inferred diatremes in the Sydney Basin. In and around the Park, two small diatremes are found at Pyes Creek, Dural, and Cabbage Tree Hollow, Galston. The Pyes Creek intrusion produced what was known as ‘white metal’. White metal is really metamorphosed sandstone, probably produced when the original sandstone was heated and then slowly cooled, forming blocks. The geological term for this metamorphic rock is prismatic sandstone. Another large diatreme occurs in Old Mans Valley at Hornsby.

Most diatremes are circular or oval in plan although some are dumbbell shaped. Most are less than 500 m in diameter, with some as small as 50 m across. Long dimensions up to 3 km are known. The Hornsby diatreme, about 1.5 km long and up to 400 m wide, is made up of three distinct bodies, each separated by a sandstone bridge. Quarrying for ‘blue metal’ in the largest body has produced a marvellous geological cross-section. A graphic account of the origin and formation of this type of volcanic intrusion is found in the section ‘Volcanic explosion: Hornsby diatreme’, and a photograph in the section ‘Hornsby Quarry’, in this chapter of the Guide.

Fig. 2 shows a stylised cross-section of a diatreme. During the development of the volcano, surrounding older wallrock has slumped or sagged back into the vent owing to faulting and fracturing. In the top of the vent, older bedded pyroclastic rocks have also slumped back into the vent. In the core of the vent, younger pyroclastic vent debris is found interbedded with remnants of country (i.e. surrounding) rock. Intrusive volcanic rocks are present in the base of the volcano. The length (L) and height (H) scales are equal.

The Old Mans Valley quarry produced ‘blue metal’ from volcanic breccia.

Topography

The topography of the area was influenced by the uplift at the end of the Triassic period and subsequent erosion. The widespread volcanic activity in New South Wales and eastern Australia in Jurassic and Tertiary times possibly further influenced the topography and geology of the Sydney Basin.

The present topography of the Hornsby Plateau was produced by erosion over a long period of time following uplift of the area about eighty million years ago, associated with rifting and opening of the Tasman Sea. The plateau has been eroded by fresh water streams, which have cut a maze of deep V-shaped valleys with intervening rocky ridges into the Hawkesbury Sandstone. A few remnant shales of the Ashfield (Shale) Group occur on the ridges in the area, but most have long since been eroded. The shale ridges produced richer soil and supported richer vegetation. Hence they were attractive to the original agricultural settlers, and most of these areas are now covered by urban development.

The drainage pattern was determined by lines of weakness in the Hawkesbury Sandstone, which were exploited by incipient watercourses. As streams cut deeper into the plateau they began undercutting the sandstone walls of the valley, and blocks of sandstone fell away leaving cliff lines. Vegetation helps this process:

plant roots exploit lines of weakness in the rock, mechanically levering off blocks of stone and aiding in chemical erosion as well. A cartoon showing this sequence of events is shown in Fig. 3. Berowra Creek and its tributaries are typical of this type of topography. The process can be seen in action at the end of the track leading off from the Elouera Road Pumping Station, Westleigh.

The Berowra Valley has been subject to several dramatic changes in sea level beginning 200 000 years ago. The final stage in the development of the present topography of Berowra Valley was the rising sea level, which occurred as the Pleistocene glaciation waned about 12 000 years ago. A series of changes began in the relative levels of the land and the sea. These reduced the sea level, which moved the coastline many kilometres offshore, caused the rivers to cut down into their beds, lowering them towards the sea level, thus deepening the coastal gorges. The returning sea rose to a higher level than previously, drowning the river mouths. About 6000 years ago these changes stabilised to produce the present topography. The Hawkesbury River and Berowra Creek are, like Sydney Harbour, typical examples of these drowned river valleys.

Soil and vegetation

The type of parent rock found in an area has a major influence over the soil types in that locality. However, many other factors such as the location, climate and erosion also affect the type of soil in an area. Within the Park most of the soil has been formed from the decomposition of Hawkesbury Sandstone. This sedimentary rock is mainly composed of quartz but also contains about twenty per cent clay. The predominance of quartz results in sandy soil types such as Yellow Earth and Siliceous Sand. The moderate clay content of the sandstone has also resulted in soil types with a duplex texture trend (i.e. sand over clay) such as Yellow Podzolic soil. The clay and organic agents that bind the sandstone provide the initial soil fertility; however, these soil types are typically strongly acidic and deficient in phosphate, nitrogen, calcium and trace elements such as molybdenum. The soil types associated with the Hawkesbury Sandstone are collectively referred to as the Hawkesbury soil landscape (Chapman & Murphy 1989).

Within the Hawkesbury soil landscape the soil types on the tops of the plateau and ridges are generally lithosols, which have low fertility and are usually less than 500 mm deep (Fig. 4). These are associated with lower open woodland containing Red Bloodwood Corymbia gummifera, also known as Eucalyptus gummifera, Narrow-leaved Stringybark Eucalyptus sparsifolia, Broad-leaved Scribbly Gum Eucalyptus haemastoma, Brown Stringybark Eucalyptus capitellata and Old Man Banksia Banksia serrata.

On the side slopes and benches the soil and vegetation slowly change. The soil is discontinuous, and sandstone outcrops and boulders may cover fifty per cent of the ground surface. In this area the uniform sand soil types such as Yellow Earth and earthy sand are interspersed with the Duplex Yellow Podzolic soil. Soil depth is usually less than 70 cm, but along joint lines it can exceed two metres. On the more sheltered side slopes a dry sclerophyll open-forest predominates. This forest contains Silvertop Ash Eucalyptus sieberi, Sydney Peppermint Eucalyptus piperita, Sydney Red Gum Angophora costata and Black Sheoak Allocasuarina littoralis.

Within the landscape, drainage lines are either on bedrock or have deposits of gravel or loose quartz sands. In sheltered gullies, deep and moderately fertile Yellow Podzolics and Earths are associated with wet sclerophyll closed forests of Blackbutt Eucalyptus pilularis, Sydney Blue Gum Eucalyptus saligna, Water Gum Tristaniopsis laurina, Coachwood Ceratopetalum apetalum, and Black Wattle Callicoma serratifolia. In some locations, Common Ground Fern Calochlaena dubia and/or Bracken Pteridium esculentum forms a closed scrubby understorey.

Old Mans Valley contains not only the Hawkesbury soil landscape but a range of soil types and vegetation. One such variation is the Hornsby soil landscape (Fig. 5), which was produced by weathering of the Jurassic volcanic diatremes in Old Mans Valley, Pyes Creek and Cabbage Tree Hollow. This landscape contains soil types which are generally deep, clayey and moderately to highly fertile and can support a richer flora than can be supported by the Hornsby landscape. Yellow and Red Podzolic soil and Yellow-Brown Earth predominate in this area where tall to tall open-forests (Specht 1970) are dominated by Sydney Blue Gum Eucalyptus saligna. Additionally, Blackbutt Eucalyptus pilularis, and the rainforest species Coachwood Ceratopetalum apetalum and Rough Treefern Cyathea australis can also occur. These associations can be seen on the Joes Mountain section of the Blue Gum Track.

This understorey can be seen along the Blue Gum Track on the western side of Old Mans Valley. The natural ecosystem around the drainage lines, however, has been altered. Development on the ridge-tops has led to nutrients being washed into the drains and creeks from soil erosion and rubbish, fertilisers, sewage and other pollutants. These pollutants have enriched the soil along watercourses and drainage lines, allowing the exotic weed seeds carried by the water to establish and flourish. Many creeks are now infested with a vast army of invading exotics.

Other soil landscapes such as Lucas Heights, Faulconbridge and Lambert are found on the ridge-tops and upper slopes around the park. These soil landscapes support varied dry sclerophyll vegetation ranging from heath to tall open-forest. Species include Banksia, Angophora, Turpentine Syncarpia glomulifera and Grey Ironbark Eucalyptus paniculata. These associations may be found in the area of the Quarry Road fire trail.

Further details on all of these landscapes and their associated vegetation can be found in the Soil Landscapes of the Sydney 1:100 000 Sheet, a map and book produced by the Department of Land and Water Conservation (Chapman & Murphy 1989).

References

Chapman, G.A. & Murphy, C. L. 1989, Soil Landscapes of the Sydney 1:100 000 Sheet, Soil Conservation Service of NSW, Sydney.

Charman, P.E.V. & Murphy, B.W. (eds) 1991, Soils: Their Properties and Management: A Soil Conservation Handbook for New South Wales, Sydney University Press in assoc. with Oxford U.P. (Australia) & the Soil Conservation Service of New South Wales. Sydney.Herbert, C. (ed.) 1983, Geology of the Sydney 1:100 000

Sheet 9130, New South Wales Department of Mineral Resources.

Herbert, C. & Helby, R. (eds) 1980, A Guide to the Sydney Basin, Dept. of Mineral Resources, Geological Survey of NSW, bulletin no. 26.

Specht, R.L. 1970, ‘Vegetation’, in The Australian Environment, 4th edn, ed. G.W. Leeper, CSIRO & Melbourne University Press, Melbourne, pp. 44-67.