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Relative elevation is most precisely understood as the relationship between a summit and its associated key saddle (KS). The key saddle, it turns out, is central to our understanding of how mountains (defined as a region of continuously elevated terrain) relate to each other and to the rest of the continental landmass. Just what, and where are the key saddles?
contour maps and hypsometric tinting helps us visualize a terrain with
variable sea level. Imagine again
that the sea rises to the elevation of a summit's key saddle. Now picture the shape of the resulting
island for which the summit becomes the new high point; it is shaped
specific closed contour encircling the summit at the saddle's specific
elevation. We call this the
of a summit. Minor summits whose key
saddles are nearby will tend to have small prominence
islands. Major summits that are
the highest points in every direction may have huge prominence islands.
Take the extreme
example of Mt. McKinley, whose distance from its key saddle is the
farthest in the world. North America and South America are of
course connected by a land bridge in Central America. Cerro
Aconcagua in Argentina is the highest summit in the Americas, and
therefore its prominence is equal to its elevation. Mt. McKinley
in Alaska is the highest summit in North America. Its key saddle
is therefore the lowest point on the ridge connecting North America to
South America. This turns out to be a fairly low saddle of 120'
de Nicaragua (more recent excavation for the Panama Canal created an
artificial lower saddle in Panama which we shall ignore for the
purposes of discussion.) Otherwise put, if the sea were to rise
120', North America would be cut off from South America and Mt.
McKinley would become the highpoint of the new North American
Mount Whitney is another extreme example. Whitney is the highest point in the 48 states (E=14,495', P=10,075'.) Whitney's key saddle lies at 4,420' elevation in Southwestern New Mexico, 635 miles from the mountain's summit. The Whitney key saddle happens to be the low saddle on the continental divide in the 48 states, which turns out not to be a coincidence.
Imagine the shape of
North America at a sea level of 4,420', the precise point at which
Pacific waters merge across the Continental Divide in New Mexico. This is the point at which the Whitney
prominence island, covering the Western U.S. and Southwestern Canada,
separates from the Mexican section, capped by an 18,409' volcano
TABLE 3: Some major North
American summits and their Key Saddles
|PEAK||IMPORTANCE||SADDLE LOCATION||SADDLE SEPARATION|
|Mt. McKinley||HP North America||Nicaragua||N America from S America|
|Mt. Whitney||HP 48 States||S. New Mexico||Western US from Mexican Volcanoes|
|Mt. Rainier||Most prominent in lower 48||British Columbia||Western Washington from Rocky Mountains
|Mt. Mitchell||HP Eastern US||nr. Chicago, IL
||Eastern US from Western US|
|Mt. Elbert||HP Rocky Mountains||nr. Eureka Valley, CA||Rockies from Sierra Nevada|
|Pico Orizaba||HP Mexico||British Columbia||the rest of North America from Logan/McKinley|
|Mt. Logan||HP Canada||Mentasta Pass, AK||the rest of North America from McKinley|
|White Mtn. NH||HP New England||nr. Glen Falls, NY||New England from Appalachians|
placement of key saddles for major summits appears non-intuitive. In the cases in Table 3, the
geomorphologic processes that created the summit were likely quite
the geomorphologic processes that created the saddle.
On the other hand, minor summits in mountainous terrain
have saddles that
appear to be part of the same morphology. While
there is a correlation between the prominence of
a mountain and the distance to its key saddle, there is no hard rule.
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