Greetings fellow Foxies,
These hollow ironstone pipes are exposed in a gravel pit on private property south-east of Quairading. The pipes start about 2 metres below the surface and continue into the gravelly clay base of the pit. Equally amazing is the location on a granite/dolerite ridge.
This raises some questions
· How did a sand dune form on a ridge in the wheatbelt?
· How did the sand turn into gravel and what causes the ironstone pipes?
These hollow ironstone pipes are exposed in a gravel pit on private property south-east of Quairading. The pipes start about 2 metres below the surface and continue into the gravelly clay base of the pit. Equally amazing is the location on a granite/dolerite ridge.
This raises some questions
· How did a sand dune form on a ridge in the wheatbelt?
· How did the sand turn into gravel and what causes the ironstone pipes?
 Gravel pit face |  View from above |
A clue to the sand deposit lies in the large salt lake chain (The “Salt River”) to the west of the gravel pit. Millions of years ago this was a mighty river that flowed to the Helena River in Perth. Gradually the river was blocked as the Darling Range was uplifted, causing the river to back up (forming a lake?) until it was diverted north in its present course as the south branch of the Avon River. The climate was also gradually changing and becoming more variable with alternating wet and dry cycles, particularly in the last hundred thousand years. During extended droughts strong winds blew sand from the dry river/lake beds to form aeolian sand deposits (that often have Banksia prionotes and Xylomelum angustifolium vegetation today ).
We know that these are aeolian deposits because of the uniform sand grain size.
We know that these are aeolian deposits because of the uniform sand grain size.
 Salt lake chain west of the site. Note yellow sand soils. |  Pit on ridge next to a granite outcrop |
A clue to the gravel formation is the vegetation at the top of the pit face, which comprises Allocasuarina campestris (a tamma) and a grevillea species that are remnants of recent native vegetation.
This blog describes how these plants and associated bacteria create lateritic soils.
This spot is more complicated than most in being an old sand deposit over reddish clay formed from dolerite. My guess is that deep plant roots transported iron and aluminium up from depth up to the subsoil where it was converted into gravels, and deposited around the root channels to form the ironstone pipes. The loose sandy gravel flowed away from the pipes when the pit was excavated.
Despite being tough rocks, the pipes are just fine sand that has been cemented together by aluminium (white) and iron (yellow/orange/red) oxides and silica (not obvious).
This blog describes how these plants and associated bacteria create lateritic soils.
This spot is more complicated than most in being an old sand deposit over reddish clay formed from dolerite. My guess is that deep plant roots transported iron and aluminium up from depth up to the subsoil where it was converted into gravels, and deposited around the root channels to form the ironstone pipes. The loose sandy gravel flowed away from the pipes when the pit was excavated.
Despite being tough rocks, the pipes are just fine sand that has been cemented together by aluminium (white) and iron (yellow/orange/red) oxides and silica (not obvious).
 Pipe cross section |  Interior view |  Pipe with tap root surrounded by sandy gravel |  Enlargement showing cemented sand grains |
