Why Do Roads Go Through the Same Few Gaps? Where a Range Can Be Crossed

A pass is a low point in a ridge, and the low points are not randomly distributed. Most are produced by erosion, where two valleys eating headwards into a ridge from opposite sides eventually meet at a col, lowering the crest between them. Glaciation deepens and widens such gaps and can produce a through-valley where ice flowed across a divide. Faults and zones of weaker rock erode faster and create low sections. Occasionally a river cut a gorge through a rising range faster than the range rose, leaving a water gap through which a road can pass at low elevation. The resulting gaps are scarce, since a long high ridge may offer only a few crossings usable by a loaded animal or a vehicle.

Crossing a range is difficult even where a gap exists, and the difficulties are consistent:

• •Elevation, which reduces oxygen and affects both people and engines
• •Snow, which closes many passes for months and requires expensive clearing on those kept open
• •Avalanche risk, which is why galleries and sheds are built over the road in many places
• •Gradient, which limits what can be hauled and is why rail crossings use tunnels or spirals
• •Weather that changes rapidly and differs sharply between the two sides
• •Exposure, since a stranded traveller has few options at altitude

Because crossings are scarce, whoever controls one controls movement, and the consequences run through recorded history. Trade routes converge on passes, which concentrates wealth at the towns at either end and produces settlements whose whole existence is explained by their position. Tolls levied at crossings funded states and built fortifications, many of which survive. Armies are channelled through the same gaps, which is why particular passes appear repeatedly in military history across entirely different periods. Borders frequently follow watersheds and are therefore drawn along the ridges, making the pass itself a frontier post. And the isolation between valleys separated by high ground produced distinct dialects, customs and in some regions distinct languages, since the barrier restricted contact in every direction except through the gaps.

A handful of crossings carry enough history to be worth naming. The Khyber Pass between Afghanistan and Pakistan has carried invasions, trade and armies for millennia and is the standard example of a strategic corridor. The Brenner is the lowest of the main Alpine crossings and has been the principal route between Italy and central Europe since Roman times, now carrying a motorway, a railway and a base tunnel under construction. The Saint Bernard passes were crossed by armies including Napoleon's and maintained hospices offering shelter to travellers, which is where the dogs got their name. The Cumberland Gap through the Appalachians channelled settlement westward in North America. And the Karakoram Highway crosses at over four thousand metres, which is among the highest paved international crossings anywhere.

The engineering history of crossing ranges is a sequence of expensive answers to the same problem. Paved roads with engineered gradients date to antiquity, with routes over several Alpine passes maintained by Rome. Medieval improvements included bridges over gorges that had previously forced long detours, one of which opened a major route and transformed the region's economy. Nineteenth century railways attacked the gradient problem with tunnels, spiral loops gaining height inside a mountain and long base tunnels beneath the range entirely. Road tunnels followed. The longest rail tunnels now pass beneath the Alps at low elevation, removing the mountain from the journey altogether, which has shifted freight away from the high routes and left some historic crossings as tourist roads with a fraction of their former traffic.

Low points form where valleys erode headwards into a ridge from both sides and meet, and glaciation and weak rock deepen them, so crossings are scarce along any long ridge. Scarcity is why controlling one controls movement, which explains the tolls, fortifications and repeated battles. Base tunnels beneath ranges now remove the mountain from the journey entirely.