A a gap between two hills used for transport and trade
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Answer:
A gap is a low area between two higher-elevation landmasses, such as mountains. Gaps are similar to passes, but more rugged and difficult to navigate.
The most rugged gaps are often called "notches." Notches are rarely crossed, and usually marked by steep cliffs on either side.
Another name for a gap is a "saddleback," because wide gaps often have the shape of a saddle. There are peaks on almost every continent called Saddleback Mountain: Saddleback Mountain, Maine; Saddleback Mountain, Arizona; and Saddleback Mountain, Australia, are just a few.
Gaps often help indicate a peak's prominence. Prominence is an expression of a peak's independence, or how isolated it is from other elevations. Prominence is the vertical distance between a summit and the lowest contour line (point of equal elevation with another peak). A U-shaped gap can indicate this contour line in a mountain ridge.
Outside the U.S., gaps are often called cols. A key col, in fact, is a mountain's highest gap and an effective measure of that peak's prominence.
The South Col, for example, is the gap between Mount Everest and Lhotse, the highest and fourth-highest mountain peaks in the world. The South Col bridges Nepal and the Chinese region of Tibet, and is the most popular site for the final camp of mountaineers climbing Everest. The South Col is also an entry into Everest's infamous "death zone," where altitude sickness can impair judgement and most climbers need supplemental oxygen.
The South Col and most other geologic features in the Himalayas were created through the ongoing process of crustal collision. Crustal collision is a form of tectonic activity, where massive continental plates crash into each other. In this case, the Indian plate is crashing into the Eurasian plate. Material above the crashing plates continues to be uplifted, creating monumental, jagged peaks and rugged gaps.