Mesas and breakaways in the Narrogin district
diagram of a mesa and breakaways
Mesas are lateritic gravel flat-topped hills with steep sided slopes on one or all sides called breakaways. A good example is the Breakaway Walk. Most mesas have a dense ironstone cap that is underlain by an erodible lateritic pink to white clay. Ironstone protects the surface but blocks are undercut by erosion and fall down the steep breakaway. Brown mallets which are generally only found around breakaways, keep the slopes bare
Mesas are the remnants of a widespread undulating lateritic gravel landscape which formed hundreds of millions of years ago when native plants and associated microbes formed lateritic gravel soils, which resisted soil erosion.
This was favoured by a moist climate like now.
Over millions of years there were large changes of climate. In arid periods there were less plants to protect the soil, and occasional very heavy rains created gullies into the plain. In wetter periods, plants formed more laterites on uplands and on material washed down slopes.
As this area has not had mountain building events or been under the sea for a huge period of time, the land has been lowered by erosion at a slow rate with material transported by rivers to the ocean.
Over millions of years soil from valley and slopes washed away to the ocean, and ironstone protected mesas stood higher in the landscape.
Have you noticed that many ridges and hills in the district have reddish stony laterites, reddish brown clays or dark mafic rock such as dolerite?
Mafic rocks contain have more iron, which makes mafic gravels more resistant to erosion than sandier granitic gravels. Over millions of years mafic areas have become higher in the landscape as surrounding granite laterites have eroded away.
Mesas are the remnants of a widespread undulating lateritic gravel landscape which formed hundreds of millions of years ago when native plants and associated microbes formed lateritic gravel soils, which resisted soil erosion.
This was favoured by a moist climate like now.
Over millions of years there were large changes of climate. In arid periods there were less plants to protect the soil, and occasional very heavy rains created gullies into the plain. In wetter periods, plants formed more laterites on uplands and on material washed down slopes.
As this area has not had mountain building events or been under the sea for a huge period of time, the land has been lowered by erosion at a slow rate with material transported by rivers to the ocean.
Over millions of years soil from valley and slopes washed away to the ocean, and ironstone protected mesas stood higher in the landscape.
Have you noticed that many ridges and hills in the district have reddish stony laterites, reddish brown clays or dark mafic rock such as dolerite?
Mafic rocks contain have more iron, which makes mafic gravels more resistant to erosion than sandier granitic gravels. Over millions of years mafic areas have become higher in the landscape as surrounding granite laterites have 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 thelandscape. 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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