vertical exaggeration
contours
horizontal scale
neatness
planes, lines, points
strike, dip
trend plunge
azimuth vs. bearing
declination
DEFINITIONS
vertical exaggeration: measure of magnifying theoretical aspect of a profile with respect to the horizontal appearance. It is equivalent to "horizontally compressing" the section.
contours: are the intersection of horizontal planes at fixed regular heights with the real topography.
horizontal scale: ratio of a map distance to a true distance
neatness= clarity= ease of correction=better grades
planes, lines, points: 2,1,and 0-dimensional elements used to explain structural geological elements
strike,
dip or plunge: the angle a line makes with the horizontal 0-90°. dip is reserved for describing the inclination of planes and plunge for describing the inclination of lines
trend is the azimuth or bearing of a line
azimuth vs. bearing
Azimuth is the number of degrees a line makes with the north measured clockwise, e.g. 120°, 270°.
Bearing is the number of degrees a line makes with north or south. and is measured in degrees E or W of N or S, e.g., N 60°E, S21°W.
declination the angle between the geographic north and the local magnetic north
READINGS
Ch. 2 from Davis
AIDS
Line 8621-5 and o'head of X-sections through Timor Sea study area
o'head, 180 from Davis.
LECTURE
Why do we use a topographic map?
Geological maps will be placed on topographic maps. Therefore, topographic maps are the basis of geological maps.
Horizontal scale:
We use a horizontal scale to ensure that a large area can be included within a small two-dimensional sheet. The U.S. Geological Survey publishes both 15 minute and 7.5 minute maps scaled at 1:62,500 and 1:24,000
Q. How many kilometers does 1 centimeter represent at 1:62,500 and 1:24,000?
Why do we use vertical exaggeration?
At least 2 reasons:
(1) Without it we would not be able to see the subtleties in a geological section, such as stratigraphic terminations and topography.
insert o'head
(2) When long seismic sections are used without vertical exaggeration we would not be able to see the whole profile
In order to calculate vertical exaggeration:
V.E. = vertical scale/horizontal scale
V.E. =1, when we have a true scale
V.E. = 1:12 000/ 1: 62 500 = 52x
The
Why use contours?
In a sense they reduce the 3-dimensional topography to a group of one-dimensional lines. But, you must be able to read the map, after memorizing its rules.
At least several:
(1) the more closely the lines are together the steeper the slope
(2) whenever you look at a topographic map always ask yourself what the C.I. is!!! feet or meters?
(3) contour lines do not cross
(4) when contour lines cross a valley, the V shapes point upstream.
Measuring Geological Features in the Field
Structural Geological field work requires describing the geometry of folds and faults. We assume that these three-dimensional features can be approximated by simpler elements such as planes, lines and points.
A geological surface such as a Fault: the San Andreas Fault, can be irregular. We approximate the fault with a plane:
A plane is uniquely defined with a strike and dip:
strike is azimuth of a line on the plane that keeps the same height. But, it is not unique because the plane can dip to one side or the other and have the same strike. To distinguish we give the dip ad the direction in which the plane dips.
A line is defined by an azimuth but an azimuth in the direction in which it dips and the value of the dip (plunge)