They are exposed in a series of blowouts within a vegetated Pleistocene calcareous dunefield. Obviously they were originally buried by (or formed within) the dune sands.
These are generally discrete pinnacles with a conical form, or are cylindrical with a round top. A few are hollow. They are up to 3 m high and 0.5 to 3 m wide. The broader pinnacles are composite structures with multiple peaks.
They are the dissected remnants of a cemented band. The upper part of this band is a hard pedogenic calcrete in which the primary depositional structures have been destroyed, but it grades down into a cemented dune sand where the dune bedding is still visible. At the base cemented rhizomorphs extend downward into the soft parent sand. The tops of the pinnacles show a summit conformity which would be the sharp upper surface of the original calcrete band. Where exposed, their bases may end abruptly or, more usually, grade downward into less-cemented material characterised by abundant rhizomorphs.
Those pinnacles developed in the calcrete have smooth surfaces, but those developed below have rough surfaces resulting from the fretting of the dune bedding and rhizomorphs. Where both types occur together the calcrete may form a phallic bulb at the top of the pinnacle. Sections of an earlier generation of small solution pipes (0.1 to 0.4 m wide) with a hard concentric fill are exposed in both the calcrete and the bedded material.
McNamara (1995) extended Lowry's model to suggest that some of the more cylindrical pinnacles might have formed by cementation around tap roots in zones up to 1 m wide. He also noted that some of the small pinnacles could be the cemented fill of prior solution pipes.
McNamara, K.J., 1995: Pinnacles [revised edition], Western Australian Museum, Perth. 24 pp.
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A composite, conical, pinnacle (about 2 m high) that shows the original dune bedding, and also several small solutional pipes that cut through the bedding, and predate the formation of the pinnacle. C0304-14.jpg |
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Detail of bedded dune limestone cut by small filled pipes. Exposed in the side of a pinnacle. 10 cm scale bar. C0305-12.jpg |
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A group of round-topped, cylindrical pinnacles. These are developed in the hard calcrete at the top of the indurated band. IMG00213.jpg |
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"Pinnacles" (?) in the less-cemented sand are less regular in form. These show areas of initial bedding, disrupted by solution pipes (with concentric layers of fill) and rhizomorphs (calcified root structures), and also have some remnants of thin crusts resulting from case-hardening. All fretted out by weathering and wind erosion.
Click on this picture to see the detail. "It's all rather confusing you know..." C0305-17.jpg |
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An uprooted pinnacle. Showing the upper hard part (developed in the calcrete band?), and the lower (buried) part that is in softer material with rhizomorphs. Or perhaps the upper smooth surface is a surface crust that formed after the pinnacle was exposed? This photo was taken in 1979, before I knew much about soft-rock karst. Unfortunately, I was unable to relocate it when I returned to the area in 2003. IMG00214.jpg |
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A hollow pinnacle. A small percentage (~1% ?) of the pinnacles are hollow. Possibly these are solution pipes, with thick cemented rims and softer cores, that relate to the earlier stage when the hard pan and calcrete was formed? 10 cm scale bar at base. C0304-17.jpg |
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Calcrete structures exposed in the side of a pinnacle. C0305-15.jpg |
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Calcrete structures exposed in the side of a pinnacle: A filled pipe (concentric structures) is cut by later veins. C0304-20.jpg |
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Calcrete structures exposed in the side of a pinnacle: Assorted pipes, veins and tubelets C0305-14.jpg |