Geology for conchologists - Introduction

For the benefit of conchologists having no special knowledge of geology, a series of articles is to be produced to explain the fundamentals of geo¬logical knowledge of greatest interest to them. Two sub-sections of the science are important - stratigraphy and palaeontology. Stratigraphy is concerned with the relative order of deposits and palaeontology with the study of fossilised remains of former living organisms.

Perhaps by now, you may be wondering what this has to do with conchology.

Any one moment in time can be regarded as a photograph. A series of them taken in chronological order would constitute a cinematographic film - a complete record of the earth's history. The study of living species of mollusca (conchology) is like the snapshot. To find how they came to be thus (evolution) and why they are found where they are today (distribution) we require knowledge of past 'photographs'. The rocks and their fossils give us these required 'snapshots'. A more correct analogy of geological study is with a cine film taken on a partially fogged negative so that definition is irregular (imperfection of fossilisation) and which has subsequently been badly'cut' (imperfection of geological record). The elucidation of the order of the strata and recognition of gaps (unconform¬ities) both large and small is the province of the stratigrapher. A sequence of systems (groups of geological beds) has been elucidated which is based mainly on unconformities and is given in the table. The recent science of geochronology has been able to give tentative absolute dates to the ages of the rocks. These are subject to variation as methods improve or a determination is proved false. The relative ages are now well established.

Classification of Sedimentary Rocks

The major unconformities between systems are not world-wide but localised (e.g. to Europe west of the Urals) and smaller unconformities can be present even within systems in localised areas. In another large area, e.g. Australia, no unconformity may be present and rocks may gently grade from one system to another, while beds present here represent unconformities there.

The stratigrapher's task is also made more difficult by mountain building which folds and distorts the rocks. Thus the negatives of our analogy have been torn up and the remnants crumpled up]

After the stratigraphy has been elucidated the palaeontologist can begin to work out the former distributions of species and the effect of changes of climate, etc. on these distributions.

Of course in the history of geological research no branch of geology can be isolated from another as I have tried to do in the preceeding para¬graphs but both have progressed together.

Conchology, not only has benefited from geology, but has, and still is, proving of great assistance to geology.

The identification of fossils and recognition that they are kindred to particular living species indicates the type of environment they lived in e.g. on broad lines whether the rock bed is marine, freshwater or land. Land deposits, however, apart from the Quarternary Period rgrely contain land shells or any fossils.

An example is the Bembridge limestone of the Isle of Wight which contains myriads of Lymnaea and Planorbis showing the deposit was formed in freshwater, not marine, a great help* as most rocks have been formed under marine conditions. This fact was recognised very early in geological investigations. .he freshwater nature of these beds was found in the late 18th. century when geology as a science was first emerging.

In this series of articles each will be devoted to a part of the geological column. In geology it is often more convenient to work back in time , as the older the rocks, the more distortion that has taken place.

The first few articles will be devoted to the Quarternary era. This is one of the more perplexing groups of sediments - it was once said (about 1930) that more papers had been written on this era than on all the others together,' This is due to the very fragmentary nature of its deposits and the relatively short time of geological history it represents. It is divided into two systems, the HOLOCENE and PLEISTOCENE, though some regard all the deposits as one system, the Holocene being called the 'Post-glacial'