Passing Mysteries

OK, a little geology.

When you hike the Muir Trail, the thing that looms near the end is Forester Pass, and as a geologist it seems anomalous and so worthy if some thought.  As you go south, this is what you encounter:

  1. Donohue Pass, 11,060′ [crest]
  2. Island Pass, 10,200′ [crest]
  3. Silver Pass, 10,740′
  4. Selden Pass 10,900′
  5. Muir Pass, 11,980′ [San Joaquin/Kings]
  6. Mather Pass, 12,100′
  7. Pinchot Pass, 12,130′
  8. Glen Pass, 11,970′
  9. Forester Pass, 13,110′ [Kings/Kern]
  10. Trail Crest, 13,670′ [crest]

[Elevations from Wenk’s John Muir Trail guide; you see other numbers on signs and topo maps, but this gives the right sense].

While there is something of a rising trend north to south, everything in Kings Canyon hovers around 12,000′ and the average elevation of the range in the region is roughly constant (the biggest cluster of 14ers is the Palisades Group in the northern part of the park).  Then you get to the south edge of the park at Forester and suddenly you have to go up an extra 1000′.  What gives?

There are lots of things you might imagine, but the character of the passes as you go to the south provides a clue.  Island Pass, for instance, is a broad bench that was clearly deeply overtopped by ice.  Muir Pass also shows clear signs of ice crossing from one side to the other.  But by Glen Pass, it isn’t clear that any ice ever covered the pass: it is a ridge separating two glacial valleys. Forester, and indeed the entire Kings-Kern Divide, was well above the top of ice sheets.

There isn’t really any reason in the bedrock to suspect that pattern; generally, the metamorphic rocks stand taller (the Goddard Divide and Black Giant above LeConte Canyon and the Palisades, for instance) but these only play a role at Pinchot Pass, and even there it is a small one.

What you are seeing is a decrease in the amount of ice.  The huge ice sheet that filled the Upper Basin of the South Fork of the Kings rose up and over the wall to the north, carving out Mather Pass.  No such body was remotely deep enough at Forester to really carve a hole through the Kings-Kern Divide.  So why was there less ice?

The best answer seems to lie in the topography to the west.  The Great Western Divide begins a bit south of Kings Canyon/Cedar Grove and it captures the lions share of winter snows at this latitude.  Cut off from heavy snows, the glaciers in the Kern drainage were far smaller and less powerful than those to the north; arguably the glaciers at the southern edge of the Kings River drainage were similarly left with less snow. Less ice and snow means less erosion, and so Forester Pass was left high and dry. [The good news for hikers is that, as the last pass constructed, the trail was more fully engineered than others and so presents a more uniform grade, especially from the north]. Something similar is true at Trail Crest, although that pass was forced in order to accommodate all the hikers seeking the summit of Whitney.

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  1. Mountains that Remade America | The Grumpy Geophysicist - April 20, 2017

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