How to read a geological map

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A geological map is a map that projects various rock formations and geological structures on a plane according to a certain proportion and represents them with prescribed colors and symbols. From the geological map, we can comprehensively understand the stratigraphic sequence and age, lithological characteristics, geological structures (folds, faults, etc.), mineral distribution, regional geological characteristics, etc. of an area. Therefore, geological maps are important materials for guiding production practice and conducting research on regional geology, geography, and natural environment. Generally speaking, the geological map refers to a plan view, but it is also often made into a geological cross-section map (actually measured or drawn in a specified direction from the plan map) in order to reflect the underground geological conditions more clearly. According to the needs of production or research, special geological maps can also be produced, such as hydrogeological maps, engineering geological maps, Quaternary geological maps, lithofacies-paleogeographic maps, mineral distribution maps, structural outline maps, geotectonic maps, etc. 1. The appearance of different rock formations on the geological map. The occurrence of rock formations includes three situations: horizontal, inclined, and upright. ； The terrain also has different conditions, including flat, undulating, and criss-crossing valleys. Due to different occurrences of rock formations and different terrain relief, the shapes of rock formations on the ground or reflected on geological maps are also different. (1) Horizontal rock formations 1. If the terrain is flat and has not been cut by rivers, only the top surface of the latest rock formations can be seen on the ground, which shows that there is only one kind of rock formation on the geological map. For example, in the North China Plain, only the uppermost layer of loose sediments is visible on the ground. 2. If the flat ground is cut down by a river, or the ground is highly undulating, you can see the older rock formations below, which are characterized on the geological map by: (1) The rock layer boundary is parallel to or coincides with the contour line ； (2) The exposure elevation of the same rock formation at different locations is the same ； (3) The thickness of the rock layer is equal to the height difference between the top surface and the bottom surface. (2) Except for changes in the direction of the rock strata in upright rock formations, the boundaries of the rock strata extend in a straight line according to the direction of the rock strata on the geological map, without any influence from the terrain. (3) Inclined rock formations 1. If the terrain is flat, the boundaries of the rock formations on the geological map extend in a straight line according to their direction. 2. If the terrain has large fluctuations (for example, there are mountains and valleys), the rock layer boundaries and contour lines on the geological map will intersect obliquely, often forming a "V" shaped bend in the valleys and ridges, which is called the "V" shaped rule. The degree of curvature is related to the inclination angle of the rock formation and the slope of the terrain. That is, the smaller the inclination angle of the rock formation, the tighter the V-shape is. ； The greater the inclination, the wider the V-shape. The greater the terrain undulations, the more complex the curved shape. ； The flatter the terrain, the less curvature it is, and even closer to a straight line. The relationship between the outcrop shape of inclined rock strata and terrain relief is as follows: (1) The inclination of the rock formation is opposite to the slope of the valley, and the V-shaped tip points upstream, but the V-shaped curvature is greater than the curvature of the contour line ； (2) The inclination of the rock formation is the same as the slope of the ravine, but the inclination angle of the rock formation is greater than the slope of the ravine, and the V-shaped tip points downstream ； (3) The inclination of the rock layer is the same as the slope of the ravine, and the inclination angle of the rock layer is consistent with the slope of the ravine. The rock outcrops on both sides of the ravine are parallel to each other. ； (4) The inclination of the rock layer is the same as the slope of the ravine, but the inclination angle of the rock layer is smaller than the slope of the ravine. The V-shaped tip points upstream, but the V-shaped curvature is smaller than the curvature of the contour line. The above-mentioned V-shaped rules all refer to the outcrop shape of rock formations in the valley. ； If it is on a sloping ridge or hillside, the V-shaped tip of the rock formation points exactly opposite to that in a valley. For beginners, the V-shaped rule is difficult to understand and master. When walking through a ravine in the wild, one often sees rock formations bending in a V-shape toward the ravine head or mouth. They always think that the formation of the rock formations has changed or folded. In fact, the formation of the rock formations has not changed, but is an illusion of the outcrop shape caused by the complex relationship between the ground slope, the tendency and the inclination of the rock formations. In other words, the outcrop shape of inclined rock formations does not equal the occurrence of rock formations (except for vertical rock formations). This law is clearly reflected in geological maps, especially large-scale geological maps. The outcrop shape of other structural lines such as fault lines also applies to the V-shaped rule. 2. The manifestation of folds and faults on the geological map (1) Folds 1. The rock layers on the two wings of the anticline and syncline appear symmetrically and repeatedly. From the core to the two wings, the rock layers become more and more new, which is an anticline. ； On the contrary, it is a syncline. 2. The shape of the two wings and the type of folds. The inclination angles of the two wings are roughly equal, but the tendencies are opposite. They are upright folds. ； The two wings have unequal inclination angles and opposite tendencies, which are oblique folds. ； The two wings have different inclination angles, but have the same tendency, which is an inverted fold (the wing with a larger inclination angle is an inverted wing) ； The inclination angles of the two wings are equal and the tendencies are also the same. One wing is inverted and is an isoclinal fold (note the difference from the monoclinic rock layer) ； The two wings have opposite tendencies, and both wings are inverted and have fan-shaped folds. 3. Fold crankshaft The fold crankshaft can be represented by the line connecting the vertices of the turning ends of each rock layer on the plane. If the fold axis extends far away and a series of anticlines are connected synclinically, it is a linear fold.: If the fold axis is short and the rock layer is projected to be oblong or approximately round, it is a short anticline, short syncline, dome or structural basin. 4. The core width of the hub is generally unchanged, and the rock layer boundaries on both wings are roughly parallel, indicating that the hub is horizontal. ； The core is a closed curve, and the rock layers on both wings are not parallel, or have arc-shaped turning ends, indicating that the hub is tilted. ； If anticlines and synclines are connected, the rock layers will curve in a zigzag shape. ； If the core is suddenly wide and then narrow, it means that the joint is high and low in a wavy manner. ； The increasingly new direction of the rock formation along any fold-crank axis is the direction of the hinge's dip. 5. The age of folds is mainly determined based on the angular unconformity contact relationship of the strata, that is, the relative ages of the upper and lower rock layers on the unconformity surface. The age of the folds in the underlying group of rock formations is after the age of the newest formations in the group of rock formations below the unconformity and before the age of the oldest formations in the group of rock formations above the unconformity. (2) Faults 1. Longitudinal faults and transverse faults have repeated or missing rock layers, which are longitudinal faults (or strike faults). ； The rock formations are interrupted or staggered, which are transverse faults (or dip faults). 2. Ascending plate and descending plate For vertical faults, at any point on the fault line, the side of the older rock layer is the ascending plate, and the side of the newer rock layer is the descending plate. ； However, when the inclination of the fault plane is consistent with the inclination of the rock layer and the inclination angle of the fault plane is less than the inclination angle of the rock layer, the side of the older rock layer is the descending plate and the side of the younger rock layer is the ascending plate. Then, based on the inclination of the fault plane, a normal fault or a reverse fault can be determined. If a fault crosses or obliquely crosses an anticline or syncline, and the core width (or equivalent distance between wings) on both sides of the fault changes significantly, the plate that widens in the anticline is an ascending plate. ； The narrowing plate is a descending plate. In a syncline, the opposite is true. The narrowing plate is an ascending plate, and the widening plate is a descending plate. If the cores of the two disks (or the equivalent distance between the wings) are only horizontally offset without changes in width and width, it is a flat inferred layer. 3. The age of faults is determined based on the relationship between faults and unconformity, the relationship between faults and rock masses and dykes, and the relationship between faults crossing and being offset, etc., which have been mentioned before and will not be repeated. (3) Contact relationship of rock layers 1. The boundaries of the integrated rock layers are roughly parallel, and there is generally no lack of layers (sometimes there are thickening, thinning and natural pinching out of the rock layers). 2. The boundaries of parallel unconformity rock layers are roughly parallel, and there are obvious lack of layers. 3. Angular unconformity. The younger rock layer covers the boundary line of the older rock layer. The bottom boundary line of the newer rock layer is the unconformity line. The occurrence of the rock layers on both sides of the unconformity line is different. The rock layer boundary line on the side of the newer rock layer is roughly parallel to the unconformity line. The rock layer boundary line on the side of the older rock layer intersects with the unconformity line. There is a significant lack of layers between the new and old rock layers. (4) Igneous rock mass 1. The boundaries of rock base or rock mass often pass through different boundaries of surrounding rocks. If the scale is large and the shape is irregular, it is called bedrock. ； If it is smaller in scale and more regular in shape, it is a rock plant. 2. The boundary line of the rock mass is consistent with the trend of the surrounding rock, and the shape is round or more regular. 3. The bed rock mass is in the shape of a long strip, extending in the same direction as the surrounding rock. 4. The rock mass of the rock wall is in the shape of a long strip and passes through different rock layers. 3. Steps and methods for reading geological maps 1. Looking at the map title, map code, scale, etc. The map name and map code can tell us the geographical location of the map. A geological map is generally named after the largest residential area, major rivers, major mountains, etc. in the area included in the map. The scale tells us the degree of reduction and the accuracy with which geological phenomena can be represented on the map. In addition, attention should also be paid to the publication time of the figure, the author of the figure, etc. 2. Look at the legend. Through the legend, you can understand which strata are exposed in the mapped area and their new and old order. The legend is usually placed on the right side of the frame, and the strata are generally represented by colors or symbols, arranged from top to bottom, from newest to oldest. Each legend is a rectangle, with geological age indicated on the left, lithology indicated on the right, and stratigraphic codes indicated in the square. The legend for igneous rocks is generally below the legend for sedimentary rocks. Structural symbols are placed under rock symbols, and the general order is folds, faults, joints, occurrence elements, etc. 3. Section lines are sometimes drawn as black straight lines through two points on the relative frame of the geological map, with words such as AA′ or II′... marked on both ends. Such straight lines are called section lines, indicating that a section has been made along this direction. 4. Analyze the terrain features in the map. If it is a large-scale geological map, it often has contour lines. Based on this, you can analyze the general direction of the mountains, where the watershed is, the highest point, the lowest point, the relative height difference, etc. If it is a small-scale geological map without contour lines, the characteristics of the terrain can generally only be analyzed based on the distribution of water systems. For example, the main stream of a huge river always flows through lower-lying areas, while the tributaries are distributed in higher-lying areas. ； The terrain gets lower and lower as you go down the river, and it gets higher and higher as you go upstream. ； A watershed area halfway between two rivers is always higher than a river valley area, and so on. Understanding topographic characteristics can help understand the distribution patterns of strata, the relationship between landform development and geological structures, etc. 5. The geological content should be analyzed from the whole to the part and then to the whole. First, understand the general geological conditions in the map, such as: (1) The distribution of strata, where the old strata are distributed, where the new strata are distributed, whether there is unconformity between the strata, etc. ； (2) What are the general characteristics of the geological structure, such as whether the folds are continuous or isolated, the size of the fault, where it develops, what is the relationship between the fault and the fold, whether it is parallel to the fold direction, perpendicular or oblique, etc. ； (3) The distribution of igneous rocks and the relationship between igneous rocks and folds and faults. 6. On the basis of mastering the geological outline of the entire region, analyze each local structure: (1) It is best to start with the old rock formations in the picture and gradually expand outward to avoid being confused. ； (2) Detailed analysis of each structural form, including folds, faults, unconformities, igneous rock bodies, etc. For example, fold type, fault type, structural combination relationship, etc. 7. Connect various parts to further understand the internal connections and development rules of the entire structure, mainly including: (1) Based on stratigraphic and structural analysis, restore the geological development history of the entire region ； (2) The relationship between geological structure and mineral distribution ； (3) The relationship between geological structure and landform development, etc. The above are just general steps and methods for reading pictures. As for how to specifically analyze a certain geological map and each structure in it, it must be gradually mastered through practice.