Reading — Test 6

Reading passage

Among the most visually striking legacies of the Pleistocene ice ages are the landforms carved and deposited by glaciers as they advanced and retreated across continental interiors. Glaciers shape landscapes through two complementary processes: erosion, in which moving ice scours and plucks material from the underlying bedrock, and deposition, in which the same ice releases sediment as it melts. Because these two processes operate simultaneously but dominate in different settings, the resulting terrain often displays a striking contrast between the jagged, hollowed-out landscapes of mountain regions and the smoothed, sediment-draped plains left behind by continental ice sheets. In mountainous terrain, valley glaciers produce some of the most dramatic erosional features. As ice accumulates in a mountain hollow and begins to flow downslope under its own weight, it gouges out a bowl-shaped depression called a cirque. Where several cirques form on different sides of the same peak and erode headward toward one another, they can leave behind a sharp, pyramidal summit known as a horn, of which the Matterhorn in the Alps is the archetypal example. Farther downslope, the glacier deepens and widens the valley it occupies, transforming what was originally a V-shaped stream valley into a U-shaped glacial trough with steep walls and a broad, flat floor. When such a trough is later invaded by the sea after the ice retreats, it forms a fjord, a landform that fringes the coastlines of Norway, Chile, and New Zealand. Tributary glaciers, being smaller and less erosive than the main valley glacier, typically fail to cut their beds as deeply; consequently, when the ice disappears, these side valleys are left perched high above the main valley floor, forming what geologists call hanging valleys, often marked today by waterfalls that plunge into the trough below. Where continental ice sheets rather than mountain glaciers dominate, the erosional signature is subtler but no less diagnostic. Vast sheets of ice moving across relatively low-relief terrain tend to smooth and streamline the landscape rather than carve deep, localized features. One common result is the roche moutonnée, an asymmetric bedrock knob with a gently sloping, striated surface on the side facing the oncoming ice and a steeper, more jagged face on the downstream side, where the ice plucked away blocks of rock as it passed. The gentler upstream slope forms because abrasion—the grinding action of rock debris embedded in the ice—wears it smoothly, whereas the downstream face is shaped by plucking, a process in which meltwater refreezes around joints and fractures in the bedrock and tears blocks loose as the ice continues forward. Geologists can use the orientation of these asymmetric knobs, along with the parallel scratches called striations that abrasive debris etches into exposed rock, to reconstruct the direction in which long-vanished ice sheets once flowed. Deposition, the second major glacial process, becomes especially prominent at a glacier's margins and in the terrain left behind after the ice has retreated. All the rock debris a glacier carries, ranging in size from fine silt to massive boulders, is collectively termed till, and it is dumped in an unsorted jumble wherever melting outpaces the ice's forward motion. Where a glacier's terminus remains stationary for an extended period, perhaps because the rate of ice advance and the rate of melting are temporarily in balance, till accumulates in a ridge called a terminal moraine, marking the farthest point the ice reached. Similar ridges, called lateral moraines, build up along the sides of a valley glacier from debris that falls onto the ice from adjacent slopes. Meltwater emerging from the glacier's front sorts and redeposits some of this material, carrying finer sediments away in braided streams and leaving behind stratified deposits distinct from the unsorted till left directly by the ice. Interpreting these landforms allows geologists to reconstruct not only the former extent of ice sheets but also their thickness, flow direction, and history of advance and retreat, since each landform records a specific set of conditions at the ice margin. In this sense, the landscape itself functions as an archive, preserving evidence of climatic episodes that predate any written record.