Contours and landforms: At what angle does a contour line intersect a ridge line on a topographic map?

Introduction / Context:
Contours are lines of equal elevation. Their patterns reveal landforms such as ridges, valleys, spurs, and saddles. Understanding how contours cross ridges and valleys enables quick, accurate terrain interpretation for route selection and drainage design.

Given Data / Assumptions:

• A ridge line is the line of steepest descent on either side (a high line).
• A contour line represents constant elevation and must cross lines of maximum slope orthogonally.
• Map contours are sufficiently close to approximate smooth surfaces.

Concept / Approach:
The gradient (steepest slope) at any point is perpendicular to the contour through that point. Ridge and valley lines align with maximum slope directions. Consequently, both ridge and valley lines intersect contour lines at right angles (90°). This orthogonality also supports construction of cross-sections and computation of slopes from maps.

Step-by-Step Solution:

Verification / Alternative check:
Construct a small slope triangle on a map: the gradient vector is normal to the contour; ridge direction is along the gradient on the crest—hence the intersection is right-angled.

Why Other Options Are Wrong:
Angles of 30°, 45°, or 60° are arbitrary and contradict the fundamental orthogonality of contours and lines of maximum slope.

Common Pitfalls:
Mistaking spur patterns for valley patterns; check the V/U shapes and elevation values to avoid misinterpretation.

Final Answer:
90°