To prepare drawing of Cross Sections from Geological maps.

To prepare drawing of Cross Sections from Geological maps.

EQUIPMENTS REQUIRED Maps, Scale, Set Square, Papers, Pencils etc.

THEORY:

Geological Map: The geological map is the one, which reveals the geological information in terms of topography, lithology and geological structures order of superposition thickness of beds and geological history of region. A geological map is contour map over which geological formation structure etc are marked.

Civil Engineering importance: For safe, stable, successful and economical civil engineering constructions such as dams, resvoirs, tunnel etc detailed geological information us essential. A geological map provides all the details, which a civil engineer requires. This study of geological map is of great importance. In geological map, normally contours are marked as dotted line.

Symbols used in Geological maps:

Strike and slip

	strike and dip of beds other than horizontal or vertical
	horizontal beds
	strike and dip of vertical beds

Faults

Type	Symbol	Definition	Types	Symbol	Definition
normal		hanging wall down, footwall up	reverse		high-angle (45° or more dip) thrust fault
detachment		low-angle normal fault, footwall – gneiss, hanging wall – shallow-crust rocks	strike-slip		rocks on either side move horizontally in opposite directions
thrust		hanging wall up, footwall down	oblique-slip		combines horizontal and vertical motion

Folds

Type	Symbol	Definition	Type	Symbol	Definition
anticline		up fold	plunging syncline		down fold with tilted axis
plunging anticline		up fold with tilted axis	monocline		strata tilted in one direction
syncline		down fold

CROSS SECTION OF GEOLOGICAL MAP

A geological cross–section is a graphic representation of the intersection of the geological bodies in the subsurface with a vertical plane of a certain orientation.

PROCEDURE:

Step 1:

First determine the line along which you are going to draw the section, it should:

• Be representative of the study area.
• Cross all major structural features (e.g. faults and folds)
• Have appropriate data on the map or well logs to draw a complete section.
• Often be drawn perpendicular to major structural features.

Step 2:

• Now on some graph paper draw an x-axis with the same scale and length as the line of the cross section.

Step 3:

• Using a scrap piece of paper, mark on where structure contours intercept your cross sectional line.

Step 4:

• Now, overlay your scrap piece of paper with your cross section and mark on the different topographical contour points.

Step 5:

• If you join these dots up, you should end up with an accurate scaled topographic cross section that looks something like the one below.

Step 6:

• Now, with your scrap piece of paper add in where the lithological boundaries intercept your cross sectional line

Step 7:

• Now, overlay your scrap piece of paper with your cross section and mark on the different lithological boundaries.

Step 8:

• Now, extrapolate from these points the bed boundaries; if they dip, then draw the lines at the correct angle of dip for each bed.
• However for this example, as the unit boundaries followed the contours, they are evidently horizontal (remember The Law of “V’s”)

Now you have drawn an accurate, to scale, cross section.