Alluvial Terraces

The Quaternary extends from about 1.8 mya to the present and is subdivided into Pleistocene and Recent (10 000 years ago to the present). There were several ice ages during the Quaternary which caused considerable rises and falls in sea level. This was because as the ice caps grew in size, the water levels in the oceans dropped.

The last ice age was about 17 000 to 20 000 years ago and resulted in a sea level fall of about 150 m and a land bridge between Australia and Tasmania. As well as being cold, the climate was dry and windy.

The Latrobe, Macalister, Thomson, Avon and Mitchell Rivers cut deep valleys into their earlier flood plains, which then became partly in-filled as the sea level rose to its present level. This has resulted in a well-defined break between the old flood plain (upper terrace), and the present flood plain (lower terrace). Less extensive terraces formed on the Nicholson and Tambo River flood plains.

A number of sets of terraces were formed during the Pleistocene period in Gippsland, but whether they were formed as a result of sea level change associated with ice ages or uplift or a combination of both is uncertain. Geologists have identified five terraces with Qp1 being the oldest and Qp5 the youngest. An additional terrace, Qp6, has been recognised during this study.

Qp1
Minor occurrences of this terrace occur in the upper reaches of the Mitchell River and Perry River valleys.

Qp2
This terrace is the most extensive terrace, stretching, almost without interruption, from Stratford to Paynesville. It is very flat with an even SSE slope of about 1 in 200, with a maximum elevation of about 160 m near Briagolong and 125 m at The Fingerboards, and a minimum elevation of 20–25 m at its southern margin. However, some geologists believe that more than one terrace may be present.

Sand hills and dunes form a discontinuous mantle over the plain east of Blackall Creek, and these become more extensive east of Perry River. The sand dunes in these areas were most likely formed from sands blown from these streams by the prevailing westerly winds, particularly during periods of aridity associated with past glacial periods.

Qp3
This is the least extensive terrace, and is represented by three isolated terraces near Stratford, Wuk Wuk, and Lindenow South. Its elevation is 20-30 m lower than QP2.

Qp4
This occurs as wide, flat terraces in the Avon, Mitchell, Thomson and Macalister river valleys. This has a much gentler southward gradient of between 1 in 300 and 1 in 450. Thus, the difference between Qp2 and Qp4 is about 60 m at Briagolong, but decreases steadily to 15 m near Stratford.

Qp5
This terrace has a similar distribution to Qp4 and is about 20 to 30 m lower. An extensive area between Toongabbie and Nambrock West is regarded as Qp5. Minor areas of Qp5 terraces occur on the alluvial plains associated with the Mitchell, Nicholson and the Tambo Rivers.

Qp6
This is included under Qra on the geological maps. The soils generally have a deep surface soil overlying a reddish sub-soil and are regarded as excellent irrigation soils. They mostly occur around Sale and Nambrock. Small areas also occur east of Bairnsdale.

Qra
This includes all modern flood plain deposits of the various rivers. The soils on these flood plains mainly comprise fine sand, silt and clays.

Subdivisions of the East Gippsland alluvial terraces, coastal plains and coastal dunes