Mesas and breakaways in the Narrogin district
brown mallets on a breakaway
Mesas are lateritic gravel flat-topped hills with steep sided slopes on one or all sides called breakaways, that have mallet or powder bark wandoo thickets. A good example is the Breakaway Walk. Most mesas have a dense ironstone cap that is underlain by a lateritic pink to white clay that erodes rapidly. Ironstone protects the surface but blocks are undercut by erosion and fall down the steep breakaway.Brown mallets are generally only found on breakaways, and they prevent other plants from protecting the slope.
Mallets rarely resprout after a bushfire, but die and drop large amounts of seed that germinates to form the dense mallet 'whipstick' thickets.
Brown mallets deny water to other plants by 2 means:
Mesas were thought to be remnants of an ironstone gravel plain that was formed by high watertables accumulating iron at the surface 300 million years ago, but recent research has shown that lateritic gravels in WA are formed by existing plants and that mesas have other ways of forming.
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 deposited at sea by rivers, so more resistant parts become uplands.
Have you noticed that many hills have reddish stony laterites, reddish brown clays or dark dolerite (a mafic) rock? As dolerite weathers more rapidly than the higher silica granites, one would expect soil from these rocks to be in valleys! However much of the land has been covered by lateritic gravel, and as mafic gravels have more iron in them, they are more resistant to erosion than sandier granitic gravels. So over time they have been more protected as the surrounding areas have eroded away.
Mallets rarely resprout after a bushfire, but die and drop large amounts of seed that germinates to form the dense mallet 'whipstick' thickets.
Brown mallets deny water to other plants by 2 means:
- 1.Mallet leaves and bark make the soil surrounding them highly water repellent, keeping the topsoil too dry for other plants.
- 2. The mallet's leaves and steeply ascending branches collect rain and channels it down stems where it is rapidly absorbed at the base of the tree.Tannins in the bark are thought to 'thin' the water so it can pass more rapidly into the soil
Mesas were thought to be remnants of an ironstone gravel plain that was formed by high watertables accumulating iron at the surface 300 million years ago, but recent research has shown that lateritic gravels in WA are formed by existing plants and that mesas have other ways of forming.
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 deposited at sea by rivers, so more resistant parts become uplands.
Have you noticed that many hills have reddish stony laterites, reddish brown clays or dark dolerite (a mafic) rock? As dolerite weathers more rapidly than the higher silica granites, one would expect soil from these rocks to be in valleys! However much of the land has been covered by lateritic gravel, and as mafic gravels have more iron in them, they are more resistant to erosion than sandier granitic gravels. So over time they have been more protected as the surrounding areas have eroded away.
Adjoining mafic and granitic laterites
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Mesa on red mafic clay ridge
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These diagrams give an idea of stages of mesa formation. 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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