The greater the rainfall, the more effective streams become as agents of erosion. So, “common sense” suggests that very wet mountain areas should be eroded more quickly and develop a more profound relief than those that are drier. With the advent of detailed digital elevation models that cover the world, it is very easy to calculate slope angles and relief over huge areas, and match them with rainfall records. Geomorphologists from the Universities of Montana and California have done this for the wettest and most rugged area in the world, the Annapurna area of the Himalaya (Gabet, E.J. et al. 2004. Climatic controls on hillslope angle and relief in the Himalayas. Geology, v. 32, p. 629-632). The main agent of erosion there as streams cut downwards is by landslides. The region also shows a profound gradient in annual rainfall from about 1000 mm in the High Himalaya to 4500 at the front of the range, where the monsoon rains hit hardest. “Common sense” is wrong, for the slopes decrease from an average of 35º to 25º as rainfall increases. The authors believe this is due to the influence of deeper weathering in more humid parts that reduces the strength of slope materials so that they must stabilise at lower angles than those in dry areas. Their other finding is that relief (elevation difference in small segments of an area) and slope angle have a strong positive correlation, so that relief itself is inversely related to rainfall. They are able to comment interestingly on various ideas about mountain evolution. Their main conclusion is that in any particular area, a transition from dry to wet conditions lowers mountain ridges faster than valley incision can shift the debris, whereas during drying, ridges are barely lowered, while streams cut unhindered into bedrock, thereby sharpening up the landscape.
Formation of gorges in tectonically quiet areas
The flanks of the North Atlantic probably became tectonically inactive in Mesozoic times, yet rivers large and small have cut large gorges, often through highly resistant bedrock. But they also have developed broad valleys over millions of years, and it is into them that the gorges are incised. Slow upward flexing caused by sediment loading on the continental shelves, a general lowering of sea level since Antarctica first formed a permanent ice cap, and isostatic response to gradual denudation help explain the full extent and shape of the rivers drainage basins. The gorges are young, and must have developed rapidly. Old ideas focussed on W.M. Davis’ theories of landscape evolution, particularly rejuvenation associated with changing base levels of erosion, but with no quantitative backing. The development of means of dating eroded surfaces using the decay of short-lived radioactive isotopes that cosmic-ray bombardment creates now offers an opportunity to test hypotheses rigorously and come up with others. Quite a few published works on cosmogenic dating applied to landform development seem to add little to geomorphological knowledge, so it is a relief to find one that does (Reusser, L.J. et al. 2004. Rapid late Pleistocene incision of Atlantic passive-margin river gorges. Science, v. 305, p. 499-502). The authors, from the Universities of Vermont and Maryland, the USGS and the Lawrence Livermore National Laboratory, focus on impressive gorges in the lower reaches of the Susquehanna and Potomac Rivers as they drain the eastern US into the Atlantic, and a series of higher surfaces which they cut into to leave as rocky straths. The oldest ages occur on the highest of these straths, as expected, and age decreases on successively lower ones to the rocky flood plain of the modern rivers just above their current channels. The highest levels are between 85 and 97 ka, the most prominent strath formed between 30 and 33 ka, succeeded by one at 19 ka and the lowest level seems to have formed between 13 and 14 ka. Interpreting the periods of intense erosion that cut each level must involve late Pleistocene climate change, sea-level shifts, and the bulging effect due to the North American ice sheet which reached its maximum extent in the northernmost part of the Susquehanna basin. It seems that during the early part of the last glacial episode, incision was slow, although probably faster than during the Holocene. But around 30 to 33 ka ago it accelerated rapidly to half a metre every thousand years, some 1 to 2 orders of magnitude greater than at present. This was at a time when ice loading was only half that at the glacial maximum around 20 ka, so it seems likely to have been initiated more by increased storminess and torrents, and indeed correlates with an abrupt increase in sea-salt content in the Greenland ice cap brought in by winds at that time. Lasting through the glacial maximum, increased frequency of flooding combined with more rapid sea-level fall, also beginning at around 32 ka, were probably the main driving forces for gorge incision. This still leaves a puzzle. Both drainage basins had been in existence since well before the cycles of glacial and interglacial periods began on the flanks of the North Atlantic around 2.5 Ma ago. Similar periods of accelerated incision must have been repeated, at least during the last 6 or 7 glaciations which were the most extensive. Did earlier topographic features exert any control over later ones, and do any relics of them remain?