The Angry Corrie 36: Apr-May 98

TAC 36 Index

Not just Glen Tilt, but Ben Tilt too?

The Ordnance Survey defines mountain heights in Britain relative to mean sea level at Newlyn in Cornwall. From memory, this level is calculated over a certain historic period, ie it is now fixed. I seem to recall a little plaque on a wall marking this datum when I was last in Newlyn. Now, if I'm not deluding myself, various things follow. Firstly, all those letters in TAC pointing out that global warming and rising sea levels will reduce the status of various Munros are wrong. This is because heights are defined by the OS relative to a fixed point (sea level at Newlyn in 1923, or whenever), and not to the current average level of the sea. So changes in actual sea level don't affect them. With me so far? Well, hold on, the best is yet to come.

Britain is tilting. The south coast (with the fixed OS datum point on it) is sinking, and Scotland is rising, as a consequence of the retreat of the glaciers after the last ice age. Obviously, the critical thing is the rate of this tilting. I have a distant memory of reading that this rate was about one foot per century at each end. Pause for implication to sink in. Yes, that's right, if my memories are correct, then Foinaven, Beinn Dearg in Torridon, etc, will be defined as Munros by the OS within only a few decades!

There are obviously a few unverified assumptions in the above, but I'd love to know if there's anything in it. And the whole argument would be altered (but not entirely disappear) if it turned out that the OS re-base their datum every now and then.

Now for another interesting question, namely what is the absolute drop from Ben Macdui? The difficulty is that the lowest point between Macdui and Nevis is, I calculate, on the surface of Loch Ericht. The reason for this is that Loch Ericht is dammed at both ends, and so the true col has been submerged.

What is the TACit Tables policy on this? The Hewitts and Marilyns of England, Note 42 (re Abberley Hill), says "man-made structures are normally disregarded", but that here they have been a "feature of the landscape for many years." A bit of an imprecise exception if you ask me, but how does it apply to Loch Ericht? It too has been a feature of the landscape for a number of years, but presumably not as many as the railway cutting. On the other hand, it would appear to be more permanent, given the end of Note 42, re the Abberley in-fill site. A further complication is that the level of the surface of Loch Ericht varies quite a lot, so it seems that the "right" answer is to work to the height of the (usually) submerged col, which I think is near the north-east end. The other assumptions would mean that the status of various hills as Corbetts or Marilyns would be potentially threatened by rogue dam builders!

Paul Prescott

TAC 36 Index