2.3 Topographic & geologic map interpretation

Reading topographic maps

A topographic map portrays the three-dimensional shape of the land surface on a flat sheet using contour lines — lines that connect points of equal elevation above a datum, usually mean sea level. Because so much of geologic fieldwork, hazard mapping, and cross-section construction depends on them, mastering contours is essential for both ASBOG exams.

Contour rules

• Every point on a given contour has the same elevation. The contour interval (CI) is the fixed vertical distance between adjacent contours (for example 20 ft); index contours are drawn as heavier lines and labeled, typically every fifth contour.
• Contours never cross or split, because a single point cannot have two elevations — the only exception is an overhanging cliff. Contours are continuous and eventually close on themselves, though the closure may lie off the map sheet.
• Closely spaced contours indicate a steep slope; widely spaced contours indicate a gentle slope; evenly spaced contours indicate a uniform gradient.
• Relief is the difference between the highest and lowest elevations in an area of interest.
• A hachured (tick-marked) closed contour marks a depression such as a crater, sinkhole, or kettle; the ticks point downslope into the low.

Rule of Vs

Where contours cross a stream valley they bend into a V that points upstream — that is, toward higher ground. This "rule of Vs" reveals drainage direction: because the V's apex points uphill, water flows away from the apex. Ridges and spurs produce the opposite pattern, with contour Vs or Us pointing downhill. Contours form concentric closed loops around isolated hilltops.

Gradient and profiles

Gradient, or slope, equals the change in elevation divided by the horizontal distance: gradient = rise ÷ run. Using the map scale to convert map distance to ground distance, a geologist reports slope in feet per mile, in degrees, or as a percent. For example, a 200 ft rise over 2 mi is a gradient of 100 ft/mi. A topographic profile is a cross-section of the land surface built by projecting contour elevations along a chosen line onto a graph. Vertical exaggeration — using a larger vertical than horizontal scale — is common but must be reported, because it makes slopes appear steeper than they are. Steeper gradients concentrate stream energy and erosion, so slope information read from contours feeds directly into landslide, flood, and engineering-geology assessments.

Reading geologic maps

A geologic map superimposes the distribution of rock units (formations), their contacts, and geologic structures onto a topographic base. Colors and letter symbols identify formations (for example "Kb," where "K" denotes Cretaceous); lines mark contacts between units; and specialized symbols record structural attitudes and features.

Strike and dip symbols

The strike-and-dip symbol is a long line drawn in the strike direction with a short tick on the dip side and a number giving the dip angle. A horizontal bed is shown by a cross or circled-cross symbol, and a vertical bed by a strike line with a doubled tick and a 90° label. These symbols let a reader reconstruct the three-dimensional attitude of units directly from the flat map, without having to revisit the outcrop in the field.

Interpreting folds and faults

Map patterns reveal subsurface structure:

• Rule of Vs on contacts: where a dipping contact crosses a valley, its outcrop trace Vs in the direction of dip (except for vertical beds, which run straight, and beds dipping more gently than the valley). Horizontal contacts parallel the topographic contours.
• Folds: repeated, mirror-image belts of strata define folds. An anticline carries the oldest rocks in its core with beds dipping away from the axis; a syncline carries the youngest rocks in its core with beds dipping toward the axis. A plunging fold produces a nose- or zigzag-shaped outcrop pattern, and the beds of a plunging anticline close (converge) in the direction of plunge.
• Faults: offset, truncated, repeated, or missing contacts and the abrupt juxtaposition of unlike units mark faults. Repeated stratigraphic section can signal reverse faulting, whereas missing section can signal normal faulting.

Cross-sections

A geologic cross-section is a vertical slice through the map that projects contacts and structures downward to show subsurface geometry — folds, faults, intrusions, and unconformities. To construct one, transfer the contacts along the section line onto a topographic profile, then extend each unit to depth at its measured dip. The following map patterns are worth memorizing:

Read together — contours for the land surface, symbols and contacts for the rock beneath — topographic and geologic maps let a geologist deduce hidden structure, predict what lies underground, and plan drilling, further mapping, or hazard and resource assessment. This integrated map reading is one of the most heavily tested skills on the FG exam.