Precambrian

In addition to the above terms, which all have a time connotation, geologists have developed a hierarchy of rock-types (litho-stratigraphical) terms for bodies of rock which have distinctive characters, but which may not necessarily all be of the same age everywhere they occur. Groups, formations and members are the terms used. For example, the Swerford Member is a sand unit in the Chipping Norton Formation of Oxfordshire; the Chipping Norton Formation is a unit of sands and limestones which occurs at the base of the Great Oolite Group of central England; and the Great Oolite Group can be recognized throughout Britain and northern France as a group of limestones, clays and sands occurring below the Oxford Clay Group.

Fossil sequences do not always reflect time. If, in a particular region, the sea was getting progressively deeper, a succession of fossil communities will be present which reflect progressively deeper environments. And if deep water conditions occur in some parts of the region earlier than in other parts, the deep water fauna will appear earlier in these areas. Before the development of Palaeo-ecology in recent years, this progression of communities was not often recognized, and many zonal schemes based on benthic fossils have had to be revised. On the other hand, zones based on pelagic animals (graptolites, ammonites and globigerinid forami-nifera) have stood the test of time very well; slowly migrating communities do not pose the same problems when zones are based on these free-swimming and drifting animals. The revision of zonal schemes based on benthic animals involves a search for changes that are independent of changing environments. These can be found by examining the evolution of particular genera. Many (possibly most) benthic lineages appear to change very little once they have been established. But evolution with time can be demonstrated in some benthic lineages (for instance, the progressively weaker ribs in the Silurian brachiopod Eocoelia discussed in the Silurian chapter) and in several pelagic groups (like the Mesozoic ammonites).

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