Academic Reading — Test 9

Reading passage

The Yorkshire Dales, in the north of England, contain some of the finest karst scenery in Britain. Karst is the name geologists give to landscapes shaped chiefly by the slow dissolving of soluble rock, and in the Dales that rock is Carboniferous limestone, a pale grey stone laid down in a warm, shallow sea roughly 350 million years ago. Limestone is composed mainly of calcium carbonate, a mineral that is hard and durable when dry but is gradually eaten away by mildly acidic water. Because the chemistry of the rock and the abundance of rainfall in the region combine so effectively, the Dales display a remarkable collection of surface and underground features, of which the sinkhole is among the most striking. The process begins in the atmosphere. As rain falls, it absorbs a little carbon dioxide from the air, and more is taken up as the water filters through the soil, where decaying plant matter releases the gas in larger quantities. This produces a weak solution of carbonic acid. When such acidic water meets limestone, it reacts with the calcium carbonate and converts it into calcium bicarbonate, which is soluble and is carried away in solution. The reaction is unhurried, and any single rainfall removes only a trace of rock, yet over thousands of years the cumulative effect is considerable. The acid exploits lines of weakness in the stone, particularly the vertical cracks known as joints and the horizontal partings called bedding planes. Along these openings the water widens passages, and what began as a hairline fissure may eventually become a cavity large enough to swallow a stream. Sinkholes, which are also referred to locally as shake holes, swallow holes or pot-holes, form where this dissolving process reaches the surface. Several types are recognised. The simplest is the solution sinkhole, a shallow, bowl-shaped hollow created where rainwater is concentrated at one point and slowly etches the bare or thinly covered rock beneath. A second and more dramatic kind is the collapse sinkhole. Here, dissolving has opened a sizeable void underground; when the roof of that void can no longer support the weight of the material above it, the surface gives way suddenly and a steep-sided pit appears, sometimes overnight. A third variety, common in the Dales, is the subsidence or suffosion sinkhole. In this case a layer of glacial debris left behind by ice-age glaciers covers the limestone, and fine sediment is gradually washed down into cavities in the rock, so that the ground above slumps to form a gentle depression rather than collapsing abruptly. The legacy of glaciation is central to the character of the Dales. During the last ice age, thick sheets of ice scoured the hills and then, as the climate warmed, melted to leave behind deposits of clay, sand and boulders known as till. Where this till blankets the limestone, water sinking through it carries fine particles into the joints below, and lines of suffosion sinkholes often trace the buried edge of the limestone outcrop. Where, by contrast, the rock lies bare, rainwater attacks it directly and produces the celebrated limestone pavements, flat expanses of rock divided by the dissolved grooves and blocks that walkers know as grikes and clints. Streams flowing off neighbouring impermeable rocks frequently vanish into swallow holes at the very point where they cross onto the limestone, continuing their journey underground through caves before re-emerging lower down the valley. Although the chemical and physical mechanisms are natural and ancient, human activity can hasten the appearance of sinkholes. Heavy rainfall events, which some scientists expect to grow more frequent, deliver large volumes of water that accelerate both dissolving and the washing-out of sediment. The diversion of drainage, the leaking of pipes, and the loading of the ground by buildings or roads can all encourage a concealed void to fail. For this reason, engineers and planners in karst regions survey the ground carefully before construction, since a cavity that has lain hidden for centuries may collapse once the balance is disturbed. The sinkholes of the Yorkshire Dales are therefore not merely curiosities of the landscape but a reminder that even the most solid-looking ground is, on a long enough timescale, in a state of constant and quiet change.