SIX HUNDRED MILLION YEAR-OLD MUD CRACKS
Mud cracks are caused by the drying-out of muddy sediments which have been exposed to the action of sun and wind. The volume decrease that results from this loss of fluid gives rise to tensile stresses distributed equally in all directions within the bedding plane. The stresses are relieved by the formation of a characteristic pattern of polygonal cracks on the surface of the sediment. Arrays of polygonal cracks resulting from this process are commonly seen on dried-out surfaces in puddles, around the margins of lakes, and on tidal mudflats. The polygons are most commonly 4–5 sided, with the majority of individual cracks meeting at right angles. Slower drying is thought to create larger polygons.
Fossillised mudcracks are most likely to occur in fine-grained silica rocks, fine-grained carbonate sediments, and in some evaporitic successions. The cracks may later be filled with sediment and form fossil casts over the base. Fossil mudcracks are usually distinctive and easy to identify, but they do vary in appearance. The cracks themselves vary in width, depth, and density. Mudcrack polygons also vary in size & shape.
Mudcracks are on display at the base of a well-cemented quartzose sandstone bed - the original fine-grained muddy sediments in which the polygon cracks were developed are long gone. The features seen on the side of the cutting are fossils of the cracks surrounding the original sediment. The sandstone beds at this locaiton have been tilted to near vertical by tectonic activity. The viewer is facing toward "stratigraphic up". The mineral is ABC Range Quartzite from the lower Ediacaran (600MYA)
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- What is the average diameter of the polygons?
- What does the polygon size indicate about the drying conditions 600 million years ago?
Pichi Richi Pass railroad is famous. The cutting immediately to the south has appeared in several films, including Galipolli and The Water Diviner. The intricate block stone wall at the base of the railroad immediately to the north is locally reported to have been laid by Chinese labourers.