[Tagging] Map a divide?
61sundowner at gmail.com
Fri Oct 5 00:44:38 UTC 2018
On 05/10/18 09:45, Kevin Kenny wrote:
> On Thu, Oct 4, 2018 at 6:26 PM Warin <61sundowner at gmail.com> wrote:
>> A land form ridge too me is a long, narrow raised part of a high edge formed by hill/mountains and there associated bits.
>> A land form of a dividing range or continental divide does not have to be narrow,
>> The 'dividing line' marks the separate water flow from one side to the other and should be 'long'.
> That divide is, ipso facto, a ridge line, because water flows downhill
> - it is a line that's higher than the basins on either side. It runs
> from peak to saddle to peak to saddle (admittedly, the 'peaks' may be
> of but little prominence) for the length of the divide.
> In any case, the place where I mean to map the divide is terrain of
> considerable complexity, topographically. Its high point is the most
> topographically prominent feature between Vermont and West Virginia.
> The divide unquestionably runs for the most part along high ridges.
> The 'ridge' nature gets lost only at a handful of saddles that are in
> relatively high-elevation wetlands - but those flat lands are still
> some hundreds of metres above the valley floors of the two rivers
> whose basins it divides. The complexity comes from the fact that the
> place is not a mountain range geologically. It is a 'dissected
> plateau', whose 'peaks' are actually arêtes between chasms excavated
> by glacial action. The direction of glacial flow was not consistent
> among the glacial epochs, so the ridges tend to run higgledy-piggledy,
> and one reason for mapping the divide is that it is otherwise tricky
> to follow visually. It wanders quite a lot.
> I live among some of the oldest exposed rock on the planet -
> surrounding me are sediments from the early Palæozoic, and to the
> north is the Canadian Shield, with its oldest rock dating to the
> Hadean Æon, with its oldest samples having an estimated formation of
> 4.2 × 10⁹ years ago. Consequently, the mountains here are all rounded
> and eroded. The ridges, except for the cols at the boundaries of
> glacial cirques, are therefore broader and flatter-topped than you
> appear to imagine - but they still rise distinctly above the
> surrounding valleys.
> Can we agree that https://www.flickr.com/photos/ke9tv/9514469053,
> https://www.flickr.com/photos/ke9tv/10031459724 and
> https://www.flickr.com/photos/ke9tv/14799708048 are all indeed views
> of a ridge? (In the last photo, ignore the mountains on the horizon,
> they're in a different range - the ridge runs diagonally away from the
> right foreground).
> Or check out the contour lines (elevations in feet; my users are
> American) along the Devil's Path
> and tell me that I'm not dealing with a ridge!
You may be dealing with a single ridge line.
Now tell me there is a single ridge line for the Australian Great Divide and I'd get you to an ophthalmologist.
Some of it is not very sharp, more flat, so not a ridge line.
There may be cliffs and ridge lines in that area .. but they do not mark the divide.
> Open question - which I'll resolve for myself - is how much
> topographic prominence to use when labeling peaks and saddles. I
> think I'll probably follow the local hiking clubs, and say, '150 feet
> (about 45 m) of prominence, and 1 km separation' for mapping peaks and
> their key cols, and at this point I care about the key cols mostly so
> as to keep the topology of the divide continuous - although I'll
> surely map the names of the saddles where I know them! (They do have
> local names, even though they aren't listed in GNIS or appear on very
> many maps. But the locals could tell you where to find Mink Hollow,
> Stony Clove, Lockwood Gap, Winisook Pass or Pecoy Notch - and yes, we
> use all those words in toponyms, reflecting the Babel of languages
> that our settlers spoke.)
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