NNPC FSTP

Geoscience

Course Code:Lesson

Geological Maps and Cross-Sections

Contents

• A map is a two dimensional representation of any part of the earth surface.

• It is a device for transferring selected information about the mapped area to the map user using signs and symbols.

• Such features may include mountains, valleys,lakes, rivers, deserts, vegetation, faults, joints, folds, lithological spread, roads, towns,airports, dams etc.

• Photographs taken from orbiting space satellites or at lower altitude from air-crafts can better the accuracy of maps.

• Maps are used on ground as data base for planning, location of particular features and as a working tool in general.

Maps

• Geological maps show the lithological spread of an area.• Topographic maps show the morphology of an area.• Basic map is a topographic map usually made on a large

scale than any previous map of the area.• Special map is a rather nebulas term that covers all types

of maps other than topographic maps.• Thematic maps illustrate a theme such as rainfall, land

use, population density etc.

Types of Maps

• Cadastral Maps are those that show property boundaries and areas.

• Charts include navigation maps such as Nautical charts-not covered by topographic maps- and show much invisible detail such as depths, currents.

• Plans cover only small areas at a large scale. They do not usually give relief information.

Types of Maps

• Geological maps like other maps are 2-dimensional representations of any part of the earth surface.

• The conventional geological map is a combination of topographic and lithologic maps.

• Maps must have the following:• Title Boundary Co-ordinates• Direction Scale Legend

• Scales can be:Fractional Scale=1/25000Statement of scale=1’ rep. 1KmLinear or Bar Scale= Km

Geological Maps

Conventional Signs & Symbols

Conventional Signs & Symbols

Conventional Signs & Symbols

Conventional Signs & Symbols

Spot Heights and Topographic Maps

Spot Heights and Topographic Maps

Spot Heights and Topographic Maps

Spot Heights and Topographic Maps

Lithologic Maps-show the lateral distribution of the different rock types underlying an area

Topographic/Contour maps are maps showing areas of equal height above sea level expressed as contours.

Topographic/Contour Maps

• All points on the same line are at the same elevation• All contour lines close somewhere, although it may be

outside the map at hand.• Contour lines never cross, except when representing an

overhanging cliff in which case those beneath the overhang are dotted. On a vertical cliff several contour lines may become superimposed.

• Contour lines never divide.

The General Characteristics of Contours

• Contours are far apart on a gentle slope.• Contours are close together on a steep slope.• Contours bend upstream in valleys and cross streams at

right angles.• On level ground there are no contours.• An isolated closed contour has the same elevation as the

next adjacent contour.• All points inside a depression contour are lower than that

line.

The General Characteristics of Contours

• Geological maps = a combination of lithologic and topographic map in one.

• Geological cross sections-show the subsurface configuration of rocks based on the altitude of outcrops or through the use of wire logs.

• Geological History = Geochronological accounts of geological events that have occurred in an area.

• Geological Map Interpretation requires the application of the three cardinal principles of geology.

Definition Terms

• The Principle of Uniformitarianism which states that the present is the key to the past. That is, the process or processes by which geological features are being formed today are the same as those that formed ancient geological features.

• The Principle of Superposition which states that in a given stratigraphic sequence of rocks, the basal lithologic unit is the oldest rock in the sequence provided that there has been no tectonic over turning of the rocks in the area.

• The Principle of Correlation which states that all rocks with similar fossil assemblage are of the same age and were laid down at the same time.

The Three Cardinal Principles of Geology

Steps For Preparing a Topographic Cross-Section.1. Choose a section line AB if one is not provided.2. Choose a vertical scale e.g. 1cm=200’.3. Examine the contours along the section line noting the highest and the

lowest values.4. Place a strip of paper along the section line and mark off all the points at

which the contours intersect the line. The same contour may intersect the line more than once.

5. Write the height of each contour line against the mark made on the strip of paper.

6. Move the strip of paper to the base line of your graph.7. Project each point upwards to the corresponding height on the vertical scale

and point at the appropriate position.8. Join all the points with a smooth curve.9. The curve you have produced is a Topographic Profile which shows the

geometry of the intersection of the land surface with a vertical plane.

Preparation of Cross-Sections

Steps For Preparing a Geological Cross-Section.• Follow steps 1-9 listed previously.• In a similar manner mark the point at which the

line of section cuts the boundaries or bedding planes.

• Project the points vertically upwards to touch the topographic profile.

Preparation of Cross-Sections

• Prepare the Topographic Profile of the map provided.

• Prepare the geological cross-section as stated above.

• Write a brief account of the geological events recorded in the area in a geochronological order. This is the Geological History.

• You can only perfect all these acts through practice and every map must be interpreted on its own merit.

Practice

THE USE OF STRIKE LINES IN THE PREPARATION OF GEOLOGICAL CROSS-SECTIONS.