Unkar strata

At present the Unkar Group strata are subdivided into five different formations, which include limestones, shales, quartzites and lavas. Altogether some 1770 m

(5800 ft) of sediments were laid down in very shallow seas that spread from an ocean in the west onto a continent in the east. Ripple marks and mudcracks formed in these shallow waters are still to be seen in strata, and the occurrence of red sedimentary layers shows that at times the seas retreated, exposing the surface to weathering and erosion. The eroded continental landscape of the older metamorphic rocks over which the seas flooded was low lying with very little topography. The scene would have been a bleak and desolate one of an almost featureless plain of bare rock and rock debris, with perhaps some shallow braided rivers flowing into the sea. There would have been no discernible life on the land, despite the fact that, by this time (over one billion years ago), life had been around on Earth for over two and a half billion years. What life there was lived in the shallow offshore waters.

Right at the bottom of the Grand Canyon's pile of Unkar strata there are cliffs formed by limestones between 50 and 100 m (165-330 ft) thick. Within these strata it is possible to find surfaces covered with ripple marks, mudcracks and knobbly laminated structures called stromatolites, which were made by micro-organisms. When first confronted with such familiar sights as ripple marks, it takes some moments of reflection and a leap in imagination to realise what you are actually looking at - a seabed surface formed around one billion years ago. They can be humbling moments, especially for the geologist who may well be the first human to ever have seen that particular window on the deep past. But it is the younger Chuar strata that preserve a better record of ancient life. The discovery of this fossil record is particularly significant, in that it was one of the first realisations that life might extend back into Precambrian times.

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