Section 2:    Every Summit has a Saddle

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?

Fortunately, our familiarity with contour maps and hypsometric tinting helps us visualize a terrain with a 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 like the specific closed contour encircling the summit at the saddle's specific elevation.  We call this the "prominence island" 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' near Lago 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 "prominence island."

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 Western North America at a sea level of 4,420', the precise point at which Atlantic and 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, first separates from the Mexican section, capped by an 18,409' volcano summit.

TABLE 3:   Some major North American summits and their Key Saddles

Often the 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 independent of the geomorphologic processes that created the saddle.  On the other hand, minor summits in mountainous terrain frequently 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.