# Student

PRE-LAB 7:
INTRODUCTION TO MAP INTERPRETATION: DEFORMATION STRUCTURES

INTRODUCTION

Geologic structure refers to the shapes attained by bodies of rock which have been deformed by stresses within Earth's crust. Vertical forces may raise rocks from ocean basins to the crests of mountainous areas, or may cause broad plains to subside below the surface of the sea. Horizontal forces may compress and deform thick sedimentary rock sequences adjacent to continents into major mountain systems. Such events of crustal deformation play a major role in counter balancing the effects of erosion throughout geologic time. Evidence of ancient crustal deformation include tilted strata, folds, faults, joint systems, and unconformitites. Where such features are not obvious due to erosion, soil, and vegetative cover, they may be inferred from geologic maps and shown on cross-sectional reconstructions.

Attitude of Structures

A description of the attitude (orientation in space) of faults or tilted strata may be given concisely in terms of strike and dip; this information may be plotted as a small T-shaped symbol on a geologic map.

Strike: The direction (compass bearing) of a line formed by the intersection of a rock layer and an imaginary horizontal plane (Figure 1).

Dip:   The vertical angle between an inclined rock layer and a horizontal plane.   The dip angle in degrees is always measured perpendicular to strike (Figure1).   Note that for any given strike, there are two possible dip directions that are 180 degrees apart.

Cross-Section View Map View
Figure 1:   Strike and dip of strata

Strike and dip symbols

Notation: 60˚, 25˚ S
Map Symbol:
Horizontal Strata:
Vertical Strata:

Overturned Strata:

FOLDS

When compressional forces act upon rock layers, the rock layers may respond to this force by folding.   Folding without breakage is possible because the layers slip over one another, like the pages of your lab manual if you bend them....