Final answer:
Flat areas on a map usually represent regions with minimal elevation change, often found in plains or plateaus. These flat stretches exhibit very little topographical variation, leading to an absence of significant contour lines on a topographic map.
Step-by-step explanation:
When referencing flat areas on a map that divide different regions, it's usually an indication of minimal elevation change. This could be indicative of plateaus, plains, or valleys that do not have significant changes in topography. For example, in the description of the central U.S., it states that the Interior Lowlands Province and the Great Plains Province consist of undulating sedimentary rocks which are mostly flat, signifying minimal relief.
There's no significant elevation change that would result in contour lines on a topographic map, hence the flat areas observed. Furthermore, in the context of finding steeper areas between points A to B or B to C on a contour map, the steepness is determined by the proximity of the contour lines.
A steeper slope would have contour lines that are closer together than in a flatter area.