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Page 46 - A Geology Walk up the Clyburn Valley

Published by Ronald Caplan on 1994/8/1 (241 reads)

of that sea.) Yes, indeed. You can be a thousand miles back from the ocean, and the river will still be responding to it. Let me give you a good example of it. The Mediterranean Sea evaporated away once, or maybe several times. And when it did, the rivers that flowed into the Mediterranean Sea suddenly were not flowing in at sea level, but were flowing over a great cas? cade, probably, down to a much lower lev? el. Now, when the Mediterranean Sea evapo? rated away, it evaporated to a depth of over 600 metres, an3rway, of seawater were removed. The River Nile has cut a gorge underneath the Nile Delta today and under? neath the city of Cairo, some hundreds of metres underneath the city. Then, when the Strait of Gibraltar broke open and the At? lantic Ocean went cascading into the Medi? terranean Sea and filled it up again, sud? denly the sea level rose back up to its original level. Perhaps it took a couple hundred years to fill it. And the silt and sediment from the Nile started filling up the great canyon it had cut, and then filling out across the delta to the posi? tion it has today. And underneath all the big rivers that flow into the Mediterrane? an, there's a big canyon. Underneath the Po and the Nile and the Ebro in Spain, and so on--each one of them has got the same deep chasm underneath them, which all tes? tifies to the drying up of the Mediterra? nean. They all flow into the Mediterranean. Here, the amount of sea level adjustment's not nearly as much. We're only talking about maybe a couple of hundred metres, not a couple of thousand metres. But even still, the rivers were adjusting to that constantly. And as sea level continues to rise, with the greenhouse effect and the warming of the earth--it could be on the rate of a couple of metres a century. The rivers are going to continue to silt up 'GIANT MacASKILLMusEUM with Craft Shop and Confectionery/Snack Shop CLOTHING FURNITURE PHOTOGRAPHS INFORMATION GENEALOGY OPEN: 7 Days a Week • 9 a.m. to 6 p.m.l 2.5 km. from ENGLISHTOWN FERRY on Route 312 ' CftapeC gdCery of 'int Art ??paintings f stainecCgCass J jewettery Sun Men Tue Wed & Sat : 10-5 Thursday & Friday : 10-10 fftrstMCovt • Cctpe (Breton Istand their mouths as the water level rises back up the rivers with the rising sea level. Although we think of the sea level as something which is very stable and very permanent, it's an on-going and constant phenomenon. In the life scale of a single person, it may be constant. But in the life scale of a river valley, it's far from constant. The unit that we've come into now is a slate unit that we don't quite know what to do with it. When we first mapped this area, we thought that it was part of the Precambrian rocks that are exposed up in the central Highlands. But they're not metamorphosed as intensely. But included within them are some volcanic layers and some volcanic ashes. And in fact, right at the end of the trail, the slate is black, not green like you see in these boulders here. And included in it are two 4-inch- wide white bands. And those bands are bands of pure volcanic ash. But other than that, it's a black, graphite-rich slate. When we tried to date these volcanic rocks, they turned out to be Silurian in age. I don't remember the exact number, but let me say 430 million years, just as a guess. If they were 430 million years old, then they would have to have been intruded into the surrounding rocks because we thought they were Precambrian--older than 560 mil? lion years. They're not. They're ashes. They were laid down along with the slate. So we had to re-think the geology of this area. The rocks turned out to be some 200 million years, or 150 million years, young? er than we thought they were! So we're not quite sure what to do with it at the mo? ment. Is it still part of the Bras d'Or terrane? If it is, it's too young. Is it part of the Aspy terrane? If it is, it's the right age, but it's not metamorphosed See Extraordinary St. Ann's Bay.. Drive the Great Circle around St. Ann's Bay
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