Earlier this year a colleague asked me if I knew what caused hollow ironstone pipes that were 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.
I was pretty sure that it is a laterite formed from sand and checked out the site with Dr Bill Verboom.
Bill’s ground-breaking research has revolutionised the understanding of soil formation, and underlies much of the following.
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?
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 (present Banksia prionotes yellow sand deposits).
We know that these are aeolian deposits because of the uniform sand grain size.
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.
I have briefly described how these plants and associated bacteria create gravel soils, and provided more information in these references.
This spot is more complicated than most in being an old sand deposit over reddish clay formed from dolerite. My guess is that iron, aluminium and silica released by special plant roots has been converted into gravels, and being pumped up from mineral rich depth by tap roots has also been deposited around the roots to form the ironstone pipes. Because the gravel is quite loose due to little clay being present in the sand, it has flowed away from the pipes when the pit was excavated.
Thumbnails below show detailed images of the pipes. Click each to expand the image. You will notice that despite being tough rocks they are just fine sand that has been cemented together by aluminium (white) and iron (yellow/orange/red) oxides and silica (not obvious).
Ain’t nature wonderful?