Academic Reading — Test 194

Reading passage

Visitors gazing up at the sheer, fortress-like cliffs of Table Mountain, which rises abruptly above the city of Cape Town, are often told that the peak must be the eroded stump of an ancient volcano. The comparison is understandable: the mountain's flat summit, steep grey walls and isolated silhouette resemble landforms found at volcanic centres elsewhere in the world. Geological investigation, however, tells a different and in some ways more remarkable story. The rock that forms the upper bulk of the mountain, known as Table Mountain Sandstone, is not volcanic at all. It is a sedimentary rock, built up from particles of sand that were laid down in shallow water hundreds of millions of years ago and only later hardened into something resembling stone armour. The story begins with a much older foundation. Beneath the sandstone lies a basement of two distinct rock types: dark, folded sedimentary rocks of the Malmesbury Group, and pale granite that forced its way upward while still molten and then cooled slowly underground. This granite, visible today at sea level around the base of the peninsula, did not erupt onto the surface; it solidified at depth and was only exposed later, after the rock above it had been stripped away by weathering and erosion over an immense span of time. By around five hundred million years ago, this combined basement had been worn down into a gently undulating, almost featureless plain, broken here and there by low hills. It was onto this eroded surface that the sand that would eventually become Table Mountain began to accumulate. Geologists believe the sand was deposited by braided rivers and within shallow coastal waters along the margin of an ancient continent. Layer upon layer of quartz-rich grains settled out as conditions shifted between river channels, tidal flats and beaches, sometimes leaving behind ripple marks and pebble bands that can still be traced in the rock today. Over millions of years the accumulating pile grew to a formidable thickness, in places exceeding a kilometre, as the land slowly subsided to make room for fresh layers of sediment above the old. The minerals within these sands were unusually pure, consisting overwhelmingly of quartz, a mineral that resists chemical breakdown far better than most. This purity would later prove decisive in determining how the rock responded to the forces of erosion. Burial beneath subsequent layers subjected the sand to enormous pressure and a modest rise in temperature. Water carrying dissolved silica percolated through the spaces between the grains and crystallised there, cementing individual particles together into an interlocking, exceptionally hard mass. In places this process advanced far enough to convert the sandstone into true quartzite, a metamorphic rock of even greater toughness. It is this hardness, rather than any volcanic origin, that explains why Table Mountain has endured so prominently while the softer rocks that once surrounded it have long since been worn away. Where the city's softer Malmesbury and granite foundations weather into rounded hills, the resistant sandstone above stands as cliffs and plateaus. Millions of years after deposition, the entire region was caught up in a continental collision that crumpled the layered rocks of the Cape Supergroup into a series of folds and ridges known today as the Cape Fold Belt. Table Mountain itself sits on a relatively flat-lying portion of this folded sequence, which is partly why its summit remains so level rather than tilted. Subsequent erosion by wind, rain and the chemical action of groundwater patiently picked apart the surrounding landscape, removing weaker rock and leaving the resistant sandstone block standing proud above the surrounding plain. The dramatic gorges, gullies and rock faces visible today are the product of this long erosional sculpting rather than of any explosive geological event. Understanding the true sedimentary character of Table Mountain Sandstone matters for more than academic interest. The rock's structure, with its network of fine cracks and porous sandy texture, allows rainwater to penetrate and be stored within the mountain, feeding springs that have supplied Cape Town's residents since the earliest colonial settlement. The same purity of quartz that gives the rock its strength also means it weathers into notably infertile, acidic soils, a condition that has shaped the extraordinary and highly specialised plant life found nowhere else on Earth in such concentration. Far from being a freak of volcanic violence, Table Mountain is better understood as a patient record of ancient rivers, slow burial and the relentless, age-long work of erosion.