Landform Interpretation

Objectives of Lecture

1. Map Reading: making sense of contours
2. Map Analysis: measuring slope gradient
3. Map Interpretation: shapes and textures of geomorphic processes

Making Sense of Contours

Contour Interval

vertical spacing between contour lines

Index contour

provides (more frequent) reference to numerical value, every 5 or 10 contour levels...

Default rule

nested contours are "up" (at next contour interval)

EXCEPTION: "depression contours" (hash marks on down side)

Notches

contours bend inwards where a stream or valley cuts into a hill

Slope (see readings on page 352-355)

Terminology:

Muehrcke uses "slope" for the rate of rise or fall along a line; gradient for the steepest slope at a given point. Uses slope orientation for direction of slope.

More common usage (in GIS):

Slope is a composite of gradient and aspect (orientation of steepest slope), measured by the orientation of a plane tangent with the surface.

Aspect: often a key factor in vegetation in mountain environments (deserts too)

Estimating slope on a contour map

• Spacing between contours approximates slope gradient
• Count contour lines over a specified distance on the map (make the calculation simple, pick a distance that divides neatly into the contour interval...)
• Short distances work fine in steep terrain, gentle slopes require longer distances, but then smooth out local variations.

Other topographic analysis

Trace ridges, delineate watersheds (nested as streams merge)

Interpreting Landforms from Contours

The Easy Stuff (often infrequent)

Depressions are rare:

• Volcanic craters, meteor impacts
• Karst (landscape formed by disolving limestone, forms caves, water disappears down sinkholes, reappears randomly...)
• Sand dunes

The Regular Clues

• Texture
• Smoothness (regularity of contour spacing, rounded vs jagged)
• Relationship of landforms to streams

Some favorites

• Drumlin fields (ice advance)
• Chaotic drainage left after glaciers melt
• Linear features (San Andreas Fault, etc.)
• Karst (try to delimit watershed here!)
• "Mesa Country" (sharp lines between erosion and deposition)
• Finding dry lake beds and lake shore features along the valley walls
• Scablands (Eastern Washington dramatic flooding events from ice dam breaks)
• ... and on and on,

Version of 24 January 2000