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Pre-glacial Features and Evolution of Drainage A vast interval of time from the Carboniferous to the Pleistocene is unrepresented, except for the Tertiary dykes and off-shore sediments; yet during this period the outlines of the present relief were created. The amount of dissection of the area tends to obscure the fact that the present valleys have been carved into only moderate relief. Many of the rivers today are strongly discordant with structure, yet often having remarkably straight courses (some with north-west to south-east courses). The Ogwen, the Peris or Gwyrfai are good examples of this. The most likely explanation is that drainage has been superimposed onto the underlying structures. During the Tertiary era, North Wales was situated at the edge of a landmass which was undergoing uplift in relation to the development of the North Atlantic Ocean. The uplift of the landmass during the Tertiary resulted in intensive erosion of overlying rocks, exposure of the Lower Palaeozoic rocks forming the Snowdon syncline and Harlech dome, and development of an upland plateau. The uplift resulted in the dominant primary drainage directions of the North Wales area. The river systems formed part of an extensive drainage system draining the uplifted continent towards the south-east. The North Wales region was also affected by minor movements associated with the Alpine orogeny. These movements culminated in the Miocene and played a role in the continued uplift/erosion of the edge of the European continent. The later evolution of the area is far easier to be clear about. The denudation of the mountains and the carving out of the valleys took place in stages. In parts of Snowdonia, imperfect plateau surfaces occur, probably the result of erosion controlled by higher base levels (river erosion). They were primarily developed by river erosion controlled by an intermittently falling base level (rejuvenation). Some of the plateau surfaces (below 180m) are thought to have been marine cut and end abruptly as if along a cliff line. The low-level coastal platforms are definitely pre-glacial and the whole series of erosion plateau surfaces indicate a falling base level in the Tertiary period (associated with the uplift). The higher surfaces are thus older than the lower ones and are less well preserved. The effect of falling base level is clear, nick points (rejuvenation),
gorges and sections sharing many cycles of erosion are evident, despite
subsequent glaciation. Rivers in the Tertiary rejuvenation, exploited
structural weaknesses and created some river capture, which in turn
caused more rejuvenation. All evidence now points to the fact that the
Snowdon area was deeply dissected by rivers attempting to grade themselves
to a succession of lower base levels, down to levels less than 60m above
sea level. Although glaciers have considerable deepened the valleys
and later-glacial rivers also, the main valleys and mountain outlines
were already in existence prior to the Pleistocene glacial phases. |
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