Mesas and Breakaways in the Narrogin District
Mesas are lateritic gravel flat-topped or gently sloping upland plateaus with steep sided slopes on one or all sides called breakaways. A good example is the Breakaway Walk.
They are remnants of an ancient lateritic plain, which developed from underlying igneous (e.g. granite dolerite) over millions of years to form a three layered surface: lateritic sand or gravel over pink to white clay over weathering granite and dolerite. More recent geological uplift and climate fluctuations have eroded the plain.
There were very dry, cold and windy arid periods, which favoured laterite destruction. There was less ground cover to protect the soil, and flash floods created gullies and soil erosion. During wetter periods returned, there was more plant cover and renewed laterite formation on uplands and soil deposited on slopes - until the next arid period.
Over time a dense ironstone cap forms on the eroded edge of a plain,which protects it from further erosion,but erosion of the softer underlying clay causes a steep slope called a breakaway to form. Brown mallets which are generally found around breakaways, keep the slopes bare. Sand and gravel continues to form on the inner part of the plain, which usually sags like a shallow bowl (which is known as a backslope.
Over millions of years soil is carried to the sea by rivers leaving ironstone protected mesas standing higher in the landscape.
They are remnants of an ancient lateritic plain, which developed from underlying igneous (e.g. granite dolerite) over millions of years to form a three layered surface: lateritic sand or gravel over pink to white clay over weathering granite and dolerite. More recent geological uplift and climate fluctuations have eroded the plain.
There were very dry, cold and windy arid periods, which favoured laterite destruction. There was less ground cover to protect the soil, and flash floods created gullies and soil erosion. During wetter periods returned, there was more plant cover and renewed laterite formation on uplands and soil deposited on slopes - until the next arid period.
Over time a dense ironstone cap forms on the eroded edge of a plain,which protects it from further erosion,but erosion of the softer underlying clay causes a steep slope called a breakaway to form. Brown mallets which are generally found around breakaways, keep the slopes bare. Sand and gravel continues to form on the inner part of the plain, which usually sags like a shallow bowl (which is known as a backslope.
Over millions of years soil is carried to the sea by rivers leaving ironstone protected mesas standing higher in the landscape.
diagram of a mesa and breakaways
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Soil layers on a lateritic mesa
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Have you noticed that many ridges and hills in the district have reddish stony laterites?
Mafic rocks such as dolerite contain more iron, which makes the gravels more resistant to erosion than adjoining sandier granitic laterites. Over millions of years mafic areas have become higher in the landscape as surrounding granite laterites and associated soils eroded away.
Mafic rocks such as dolerite contain more iron, which makes the gravels more resistant to erosion than adjoining sandier granitic laterites. Over millions of years mafic areas have become higher in the landscape as surrounding granite laterites and associated soils eroded away.
Adjoining mafic and granitic laterites
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Mesa on red mafic clay ridge
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These diagrams show how mafic rock areas have become higher in the landscape. for more information see the file below
| vegsoil_booklet.pdf |
Resistant gravel forms on mafic dyke, adjoining sandy soils weather more
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Soft lateritic clay under the ironstone erodes causing the breakaway slope
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