Tamar report land Capability Survey of Tasmania k e noble 1992 Tamar Report

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Class 5 land on sands and gravels

Class 5 land on sands and gravels occur in the north of the map principally in the West

Tamar area, north of Beaconsfield.  Another area occurs as a complex with Class 4 land at

Doctors Flats (south of Lefroy).  Slopes are variable and can range up to 30% on steeper

areas.  Annual rainfalls are around 800 - 900 mm.

The sands and gravels have come from the older rocks of the Asbestos Range and Cabbage

Tree and Salisbury Hills.  They were deposited as terraces but have subsequently been

reworked and redeposited.  A large proportion of the gravel is quartz and quartzite.  In

areas nearer the coast these Tertiary deposits have been partially covered by a veneer of

windblown sand.

The quartz gravels and sand are a  major source for construction and road material.  Where

areas have been significantly disturbed by mining, they have been classified as Class 6.

Soil associations mapped in the Beaconsfield Soil Survey (Dimmock, unpublished), are the

York Town, Beaconsfield and Dalrymple Associations, which are typically podzolic or

podzol soils.

A typical profile of the York Town Association may have a black loamy sand over grey

sand; overlying a thin layer of cemented sand followed by a thin layer of dark brown

organic coatings; on mottled yellow brown clays.  Occasional pieces of quartz gravel occur

throughout the profile.

The Beaconsfield Association is generally more gravelly than the York Town Association,

and a typical profile may have a black loamy sand overlying grey sandy gravel; over dark

brown sandy gravel; over white gravelly sand.  The Dalrymple Association occurs on areas

where windblown sand has been blown over the gravel terraces nearer the coast.  Where the

sand is deep enough, these areas have been classified as Class 5 on windblown sand.

Varying amounts of quartz gravels can occur both throughout the soil profile, and scattered

over the soil surface.  The amount of gravel present severely restricts agricultural use of the

land, and this together with poor soil structure, low natural fertility and leaching of

nutrients throughout the profile, make this type of land Class 5.  Photo 13 shows an

example of this type of land.

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Photo 13:

Class 5 land on quartz gravels.  Tamar map 818459.  Clarence Point Road.

Photo 14:

Class 5 land on basanitic dolerite. Tamar map 957400.

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Class 5 land on Dolerite

Class 5 land has also been mapped on dolerite.  Slopes can range up to 40%.  Although

some slopes may be relatively gentle, the amount of dolerite boulders and presence of rock

outcrops restrict cultivation and make this land suitable for grazing purposes only.

Localities where this type of land occur are at Low Head, Clarence Point, Point Sorell, East

Arm Road, Rubicon Hills, Birralee, Four Springs, north east of Parkham, Sidmouth-Exeter,

Rosevale area, Weetah, Ecclestone Road and Rosevale.  Some of these areas also comprise

talus or boulder slopes below the dolerite bodies which mantle underlying sedimentary

strata (e.g. at Holwell, Weetah and Reedy Marsh).

Areas mapped as basanitic dolerite have also been included with this type of land because

of the amount of rock outcrops.  The composition of this basanitic dolerite has been

described as a coarse grained basalt.  It is also Tertiary in age, whereas the dolerite is older,

Jurassic in age.  These areas are around the Rowella, Batman Bridge and Murphys Hill

localities.

These areas may be sown to improved pasture species by surface cultivation on the deeper

soils around the boulder outcrops.  Where the land has so many boulders that surface

cultivation is not possible, it has been classified as Class 6 land.

The major soil association mapped in the Beaconsfield Soil Survey (Dimmock,

unpublished), is the Eastfield Association.  The soil profile depths and degree of profile

development can vary depending on the steepness of slope,  and depth to bedrock.  A

typical profile may have a grey or grey-brown fine sandy loam or silt loam A1 horizon;

overlying a pale A2 horizon often containing fine rounded ironstone gravel; over a dense,

impervious, mottled yellow-brown, grey-brown or yellow-grey clay B horizon.  Dolerite

boulders and stones are common throughout the profile.  Depth to bedrock is commonly

around 1 metre.

Drainage of these soils is variable, and in winter the dense clay B horizons can restrict

water movement, resulting in perched water tables.  Average annual rainfalls on this type of

land range between 700 and 1 000 mm.

Photo 14 shows an example of Class 5 land on dolerite, and Figure 9 demonstrates the

relationship between land capability classes on dolerite.

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Figure 9:

Diagrammatic representation of land capability classes mapped on dolerite.

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Class 5 land on Windblown Sand

Class 5 land on semi-consolidated windblown sand deposits occurs in the north of the map

sheet along the coastal platforms.  This coastal sandplain is gently undulating, consisting of

low sand dunes interspersed with wetter depressions and drainage lines.  It varies in width,

ranging up to 6 kilometres inland from the coast.

Locations where this type of land occur are at Port Sorell (both sides of the estuary),

Greens Beach, and between George Town and Beechford.  Rainfall ranges between 700

and 900 mm.

The sand deposits are variable in depth, and in some places outcrops of the underlying

older formations such as Mathinna Beds and dolerite, are evident.  The sand has been

blown inland from the coastal zone but is now stabilised and most has been developed for

grazing purposes.  The sandy soils have low natural fertility, are low in organic matter, and

have a low water holding capacity.  Nutrients are easily leached from the plant root zone,

and high fertiliser inputs are required to maintain acceptable levels in the soil.  Soils are

generally well developed with typical podzol features such as a bleached and structureless

A2 horizon and an iron-organic B horizon.  Soils depths can be up to one metre to the

organic B horizon.  The wetter swampy areas have dark organic sandy clay soils.  Soil

Associations mapped in the Beaconsfield Soil Survey (Dimmock, unpublished), are

Stockport, Dalrymple and Kelso.

Although of gentle relief (up to 10% slopes) these areas are not suitable for cultivation for

cropping because of poor soil structure, although cultivation may be carried out during the

pasture establishment phase.  The soils are prone to wind erosion and therefore

maintenance of a complete pasture cover and prevention of surface disturbance is

necessary.  The higher sand dune areas are subject to summer droughts which severely

reduce pasture growth, while in some low lying areas the wetter drainage lines and

depressions may benefit from drainage to maintain high producing pasture.  A further

problem is the occurrence of salting in some of the lower lying areas.

The relationship between land capability classes on windblown sand deposits is illustrated

in Figure 10.  An example of this type of Class 5 land is shown in Photos 15 and 19.

Photo 15:

Class 5 land on windblown sand.  Tamar map 624405.  Milldam Road.

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