H. H. Read

The Geological Record

The rocks which make up the earth’ s crust are the principal documents of geological history. Every aspect of these rocks, their composition and physical properties as well as their distribution and mutual relationships, is indicative of their origin and it is the function of the geologist to establish from observation a basis for the interpretation of earth history. The crust as a whole has been changing since the earliest geological times and its present-day make-up is only intelligible when examined in relation to its past evolution. The historical approach, the distinctive trait in geological thinking, is therefore fundamental to the understanding of the earth sciences.

The Cordilleran and Andean Mobile Belts and the North American Crato

When viewed in relation to the tangle of island arcs and small seas which characterise the western Pacific margin, the continental border-zone flanking the eastern Pacific arrests the eye by its simplicity (Fig. 11.1). From Alaska to Mexico and from the Caribbean to Cape Horn two broad mountain-belts 8000–10000 km in length rise abruptly above the narrow continental shelf to separate the ocean from the large cratons of North and South America. These mountain-belts mark a continental margin which has been mobile throughout the Phanerozoic eon, but a margin characterised by features which can hardly be matched in other mobile belts past or present.

European Shield-Areas

We will preface the account of the shield-areas of each continent with a short summary of the make-up of the continent as a whole, illustrated by a diagrammatic map for reference in later parts of the book. Accordingly, Fig. 2.1 shows the principal structural units of Europe in which the main events in the history of the continent are recorded.

The Cratons of South America and Antarctica

To deal with the early histories of the continents of South America and Antarctica in the one chapter is not a matter of pure expediency. It has long been recognised that the Andean fold-belt comes ashore in Grahamland (the Antarctic Peninsula) and, with the increase of exploration, many fundamental similarities in the constitution of the two regions have become manifest. Before embarking on our account of the Precambrian rocks of the two continents, we shall as usual give summaries of their geological histories, from which these similarities will become apparent. Geological knowledge of these difficult terrains is necessarily uneven so that our generalised descriptions must be provisional. For the same reason, we shall carry our descriptions on to include the latest Precambrian and earliest Palaeozoic rocks, rather than leaving these to be treated separately in Part II.

North Atlantic Shield-Areas

The scattered land-areas which lie within and on the borders of the northern Atlantic fall into place as portions of a simpler structure that have been dispersed by continental drift in comparatively recent geological times. The continental fragments which form the bulk of these lands, when reassembled to make allowance for the effects of drift, form three great structural entities (Fig. 3.1): (a) The Caledonian— Appalachian system of Palaeozoic mobile belts. (b) The more ancient shield and platform areas which constituted the stable foreland on the western side of the Caledonian belts. (c) A fragmentary cover of younger rocks superimposed on these structures and largely concentrated along the Atlantic margins of the continents; these margins are fringed in some places by Mesozoic or younger sedimentary successions and in others by late Mesozoic—Tertiary lava-plateaux and associated igneous intrusions — the components of the classic North Atlantic or Thulean igneous province.

Gondwanaland in Late Proterozoic and Early Palaeozoic Times

The object of this chapter is to outline the geological cycle which began in late Precambrian times in the continents of the southern hemisphere. It reached its terminal phases in early Palaeozoic times and was followed by very extensive stabilisation of the crust. It is referred to by many names: Katangan, Mozambiquian, Pan-African, Indian Ocean, Adelaidean are all in common use for what are, in essentials, equivalent cycles.

The Caledonides and their Forelands

The last few hundred million years of Precambrian time saw the initiation of a world-wide network of mobile belts most of which were to remain active until well into the Phanerozoic eon. In Laurasia (Fig. 2.1) this system was built around a fairly small number of cratons; most parts of the system remained mobile at least until the end of Lower Palaeozoic times and some are still in being at the present day. In Gondwanaland, the mobile belts enclosed a rather larger number of smallish cratons; much of this southern network was effectively stabilised well before the end of the Lower Palaeozoic and only a few branches remained active in later Phanerozoic times.

New Themes in Earth History

The geological records of the 500–600 m.y. which make up the Phanerozoic eon are not only more detailed than those of earlier periods: they also cover a wider range of geological happenings, notably those connected with the history of the ocean basins, in addition to the continents, and with bodily movements of continental masses as well as relative movements within such masses.

Precambrian of Asiatic Laurasia

The geological constitution of so vast a land-mass as Asia can only be dealt with here in the broadest outline, to provide a background for the discussion of topics of special interest which are considered in later pages. The continent of Asia as it now exists is composite; part is derived from Laurasia, part from Gondwanaland, the two portions being separated by the broad Himalayan mobile belt (Fig. 5.1).

The Australian Craton

The continental mass of Australia (Fig. 8.1) is made up of four great structural units, to which we may add a fifth represented in the fringing island arcs to the north and east. The oldest and most extensive unit is the Precambrian craton which comprises most of the central and western parts of the continent where Precambrian rocks are exposed beneath an intermittent Phanerozoic cover.