Southern England. Peter Friend

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      In addition to the direct raising or lowering of the surface by erosion or deposition, there is a secondary effect due to the unloading or loading of the crust that may take some thousands of years to produce significant effects. As mentioned above, we can visualise the lithosphere as ‘floating’ on the asthenosphere like a boat floating in water. Loading or unloading the surface of the Earth by deposition or erosion will therefore lower or raise the scenery, just as a boat will sit lower or higher in the water depending on its load.

      An example of this is the lowering of the area around the Mississippi Delta, loaded by sediment eroded from much of the area of the USA. The Delta region, including New Orleans, is doomed to sink continually as the Mississippi river deposits sediment around its mouth, increasing the crustal load there.

      A second example of such loading is provided by the build-up of ice sheets during the Ice Age. The weight of these build-ups depressed the Earth’s surface in the areas involved, and raised beaches in western Scotland provide evidence of the high local sea-levels due partly to this lowering of the crustal surface.

      Unloading of the Earth’s surface will cause it to rise. Recent theoretical work on the River Severn suggests that unloading of the crust by erosion may have played a role in raising the Cotswold Hills to the east and an equivalent range of hills in the Welsh Borders (see Chapter 6, Area 9). In western Scotland, as the ice has melted the Earth’s surface has been rising again.

       Vertical movements by expansion or contraction

      Changing the temperature of the crust and lithosphere is an inevitable result of many of the processes active within the Earth, because they often involve the transfer of heat. In particular, rising plumes of hot material in the Earth’s mantle, often independent of the plate boundaries, are now widely recognised as an explanation for various areas of intense volcanic activity (for example beneath Iceland today). These plumes are often referred to as ‘hot spots’ (see Fig. 32). Heating and cooling leads to expansion or contraction of the lithosphere and can cause the surface to rise or sink, at least locally.

      An example of this is the way that Southern England was tilted downwards to the east about 60 million years ago. At about this time, eastern North America moved away from western Europe as the North American and Eurasian plates diverged. The divergence resulted in large volumes of hot material from deep within the Earth being brought to the surface and added to the crust of western Southern England. It is believed that the heating and expansion of the crustal rocks in the west has elevated them above the rocks to the east, giving an eastward tilt to the rock layers and exposing the oldest rocks in the west and the youngest ones in the east. This sequence has important implications for the scenery of England’s south coast (see Chapter 5).

      HOW CAN LOCAL SURFACE MOVEMENTS BE DETECTED?

      Having just reviewed some of the processes that cause vertical movements of the Earth’s surface, it is useful to consider the practical difficulties of how such movements are measured.

      For present-day applications, it seems natural to regard sea level as a datum against which vertical landscape movements can be measured, as long as we remember to allow for tidal and storm variations. However, much work has demonstrated that global sea level has changed rapidly and frequently through time, due to climate fluctuations affecting the size of the polar icecaps and changing the total amount of liquid water present in the oceans and seas. It has also been shown that plate tectonic movements have an important effect on global sea level by changing the size and shape of ocean basins.

      Attempts have been made to develop charts showing how sea level, generalised for the whole world, has varied through time. However, it has proved very difficult to distinguish a worldwide signal from local variations, and the dating of the changes is often too uncertain to allow confident correlation between areas.

      In sedimentary basins, successful estimates of vertical movements have been made using the thicknesses of sediment layers accumulating over different time intervals in different depths of water. In areas of mountain building, amounts of vertical uplift have been estimated using certain indicator minerals that show the rates of cooling that rocks have experienced as they were brought up to the surface. However, both these approaches are only really possible in areas that have been subjected to movements of the Earth’s crust that are large and continuous enough to completely dominate other possible sources of error.

      Local horizontal movements are also difficult to estimate, although fold and/or fault patterns may allow a simple measure in some cases. Movement of sediment across the Earth’s surface by rivers or sea currents can be estimated if mineral grains in the sediment can be tracked back to the areas from which they have come. In the detailed consideration of landscapes in this book, we have to rely on using the widest possible range of types of evidence, carefully distinguishing the times and scales involved. Even then, we are often left with probable movement suggestions rather than certainties.

       CHAPTER 4 The Southwest Region

      GENERAL INTRODUCTION

      MOST OF THE BEDROCK near the surface in the Southwest Region (Fig. 35) is distinctly older than the near-surface bedrock in the rest of Southern England. It therefore provides us with information about earlier episodes, and this is all the more interesting because these episodes involved movements of the crust that created a mountain belt, the only one fully represented in the bedrock story of Southern England. Not only does this add greatly to the interest of the Southwest, but it has resulted in the presence of valuable minerals that have strongly influenced the human history in the Region.

      Bedrock foundations and early history

       Sedimentation and surface movement before the mountain building

      The Southwest Region consists predominantly of bedrock formed between about 415 and 300 million years ago, during Devonian and Carboniferous times. This bedrock records an episode during which some areas of the Earth’s crust rose while others sank, as part of a general buckling of the crust that is the first indication of compression and mountain building (Figs 36 and 37). As the rising areas became significantly elevated they were eroded, shedding sediment into the neighbouring sinking areas that became sedimentary basins. It is these basins that preserve most of the evidence of these events (Fig. 38).

      In material that has been further and later deformed, it is difficult to work out the shape of the sinking areas, but many of them were probably elongated or trough-shaped, with the troughs separated by rising ridges that ran roughly east-west, parallel to the general trend of the later mountain belt. The troughs and ridges were caused in the early stages of mountain building by the compression and buckling of the crust. The troughs were generally flooded by the sea, or on the margins of relatively narrow seaways. Muds were the commonest materials to accumulate, although sands were also in plentiful supply. Lime-rich sediments, sometimes with corals and other shelly marine animals, were locally important. There were also periodic episodes of igneous activity that contributed volcanic lavas to the sedimentary successions.

      FIG 35. The Southwest Region, showing Areas 1 to 3.

      During these episodes of Devonian and Carboniferous basin and ridge activity, the Southwest Region was just one small part of a larger belt of similar activity that extended to the west into Ireland СКАЧАТЬ