Most mountain trekking is on gentle slopes made difficult by uneven paths, stones, and fallen trees. But the trails are along ridges or following streams, with the steepness of the paths gaining a few feet in altitude for every ten or hundred of feet forward.
But when the trail reaches the spots between the streams and the ridges, where it is forced to climb to reach a pass between mountain tops, the gain in altitude exceeds the gain in forward movement.
Going straight up these climbs to the pass would have shoes and horses slipping backward and cause erosion where rushing rain and snowmelt follows where feet had trod.
Enter the switchback. The same as skiing back and forth across a slope to keep your speed in check, the switchback runs across the hill at a reasonable grade, then makes a 30 degree turn to run back across the hill. Repeat as many times as needed to reach the mountain’s ridge.
Between mountain peaks there are slopes covered with shale — the rock that has broken off the granite walls thanks to hot and cold and storms and lots of time.
These broken rocks stop sliding down the slope between mountain peaks when they reach their angle of repose, where any object sliding down a slope finds its unique spot where it no longer slides on the steepness of the slope.
The rocks are called shale and all together they are called scree.
The rocks have found their place to stop sliding — but only barely. Just like an avalanche, any disturbance will get the scree moving down the slope and the larger the disturbance the more scree will get moving.
A larger disturbance like, say, a boy and a horse slipping off the narrow path of a switchback and tumbling onto the scree-covered slope.