Mesas and Breakaways in the Narrogin District
ironstone blocks on the edge of a breakaway
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.
Local mesas are mostly remnants of an ancient lateritic plain, which developed from underlying igneous (e.g. granite dolerite) baserock 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.
Breakaways have a dense ironstone cap on top edges, which resists erosion. Water flow over the softer edges, particularly the underlying pallid zone clay gradually undercuts the ironstone surface clay to create a steep slope called a breakaway to form. ironstone boulders often tumble down the slope. Brown mallets are generally found around breakaways, which are otherwise mostly bare. The soil surface back from the edge of a breakaway often has less ironstone, and sags over millions of years to form a gentle a backslope or bowl of sandier soil.
Over millions of years soil carried to the sea by rivers leaves ironstone protected mesas standing higher in the landscape.
Local mesas are mostly remnants of an ancient lateritic plain, which developed from underlying igneous (e.g. granite dolerite) baserock 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.
Breakaways have a dense ironstone cap on top edges, which resists erosion. Water flow over the softer edges, particularly the underlying pallid zone clay gradually undercuts the ironstone surface clay to create a steep slope called a breakaway to form. ironstone boulders often tumble down the slope. Brown mallets are generally found around breakaways, which are otherwise mostly bare. The soil surface back from the edge of a breakaway often has less ironstone, and sags over millions of years to form a gentle a backslope or bowl of sandier soil.
Over millions of years soil carried to the sea by rivers leaves 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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