Answer to Question 1
In the geologic past, the mountain or ridge, typically partially eroded, became buried beneath more recent rocks. The stream developed its course on the younger strata independent of the characteristics of the buried structure. As the younger rocks were eventually stripped away by erosion, the stream, still following its original course, was set down (superimposed) onto the rocks of the older, lower structure.
Answer to Question 2
Runoff is generated when the amount of precipitation exceeds the ability of the ground to soak up moisture. Permeable surface materials, deeply weathered sediments and soils, gentle slopes, dry initial soil conditions, and dense vegetation cover (which intercepts rainfall and slows delivery to the ground surface) all facilitate greater infiltration resulting in less runoff.