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Accès à la rubrique des textes concernant 1914-1918

1918 - Usage de la géologie dans la grande guerre Texte en langue anglaise


L'extrait ci-dessous d'une revue américaine de géologie, présente un mémoire exposé dès la fin de la guerre sur l'intérêt de la géologie pour de nombreux usages tactiques : réserves d'eau, éclatement des obus, tranchées, etc. Dans la partie concernant les manoeuvres, notamment celles des chars, l'exemple donné est une très intéressante carte géologique du secteur est de Blâmont.


SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1920.

THE USE OF GEOLOGY ON THE WESTERN FRONT
By ALFRED H. BROOKS.
(Presented in abstract before the Geological Society of America Dec. 31, 1919)

[...]
MANEUVERING.
The influence of swamps and marshes on strategy and tactics is of course well established. On many occasions skillful leaders have defeated an opponent by taking advantage of impassable ground. Present tactics and modern equipment in artillery and tanks have led to a revision of the old classification of passable and impassable ground for troop maneuvers. This revision has been necessary because of the colossal scale of modern operations, involving not only tremendous .concentrations of men but also enormous unit weight of mobile equipment, such as artillery and tanks. A land surface that would have been no obstacle to maneuvers in former wars, with their relatively small numbers of troops and light equipment, might prove almost impassable for the great bodies of men and the heavy equipment demanded by present tactics. There were, of course, plenty of exceptions to the new requirements during the late war, for much fighting was done in swampy lowland and even among rugged snow-clad ranges, where the great concentration of infantry in attack and the use of heavy equipment was impossible. The decisive battles of the war, however, were fought in areas where the passability of the ground was an important element. The question whether a certain hill slope is made up of slippery clay or of hard ground may be the decisive factor in a tactical movement. Again, the success of an operation may hinge on the quick traverse of a certain valley bottom. If the valley floor is silt or clay, it will be soft and muddy during the wet season and difficult to traverse; if it is sand and gravel it will be hard and dry and easy . of passage by troops and equipment. Again, the successful fording of a stream may depend on the physical character of the stream bottom. A knowledge of the geology of the region will make it possible to forecast the physical conditions of the surface, and this will often be of immense service in formulating plans for tactical movements.
The feasibility of a maneuver depends, as shown above, on the physical condition of the soil, as well as on the topography. Complete understanding of the topography of the scene of operations is clearly of first importance to the tactician. It has been emphasized in many publications, notably by the French, that a thorough knowledge of military topography can best be obtained by understanding its relations to the bedrock geology and to the recent geologic history of the region. This subject relates to the training of officers and, though important, need not be further discussed here. The influence of topography on tactical movements has, however, been emphasized to the exclusion of almost any consideration of the effect of soil conditions. An escarpment may afford an admirable defensive position, but a lowland underlain by clay may during a wet season be even a still greater obstacle to an attack. Ground that could be readily traversed by a light skirmish line might, after being crossed by repeated waves of infantry, be so churned up as to become practically impassable for rapid forward movement. Even if passable by infantry it might be impassable for artillery and tanks.
Another aspect of the problem is the effect of modern artillery fire, with its penetration in unconsolidated material to a depth of nearly feet. A barrage may so churn up the ground as to make it a serious obstacle to rapid infantry attacks and impassable to heavy equipment. It is evident that the shell craters will be much larger in unconsolidated material than in hard rock, and which kind of ground will be reached by the shell fire can be determined in advance by the use of geology. Moreover, underground conditions may lead to the shell craters being filled with water from below and thus add still greater obstacles to advance.
The location of outcrops of hard, brittle rock within the enemy lines, especially in their fieldworks, may also be of some local importance. Artillery fire that hit such outcrops was more effective because of the splintering of the rocks.
Whether ground will be hard or soft at any particular time depends principally on the physical character of the soil and subsoil and on the amount of precipitation. Therefore the determination of the physical conditions of the surface as a guide to planning troop maneuvers involves the use of data on geologic conditions and rainfall. .A knowledge of the areal geology is of first importance, and hence a geologic map is indispensable.
The lithology of the formations immediately underlying the surface must be known, as well as their products of weathering including soil and subsoil. An essential element to the problem is the depth to hard rock, for obviously if bedrock occurs near the surface, with only a thin cover of soil, the ground will be dry and hard at all seasons. On the other hand, if there is a heavy cover of soil and subsoil that have certain physical properties .the ground will become soft and water soaked during the wet season. If the surficial material is pervious, as, for example, in a sandy soil, the rain water will soak deep into the ground and the surface will remain dry and hard, but where there is an impervious surficial deposit, such as clay, it will hold the water and form soft, muddy ground. Other geologic factors, such as depth of ground-water table and degree of saturation of soil and subsoil at the time of rainfall, also affect the problem.
The dip of the strata determines in a large measure the areal distribution of the formations and this of course holds true of the soils derived from them. In areas of horizontal strata there will evidently be less variation in type of soil from place to place than in regions of close folding.
In the practical application of these principles no great refinement will be necessary. Having a geologic map of an area with which he is familiar and data on precipitation, an experienced geologist will be able to forecast the soil conditions of any formation with a sufficient degree of accuracy for use in maneuvers.
The whole problem is closely tied to the quantity and seasonal distribution of rainfall. A certain formation may afford hard footing and hence be easily passable during the dry season, only to become soft and practically impassable during wet weather. Therefore monthly data on precipitation are absolutely essential, and information about evaporation is desirable. In northern latitudes the seasonal distribution of ground frost may have a bearing on the passability of the terrane at times.
The permanent ground frost of polar and subpolar regions introduces another element. In these regions the soil and subsoil thaw only to a depth of 1 foot to 3 feet during the short summer. This condition produces a swampy character in the ground even on hill slopes, and as a result much of the country is almost impassable for modern armies.
An area in which the character of the soil and the precipitation are such as to form muddy ground during the wet season may be artificially drained. Locally the plain of Flanders has been so drained as to give relatively hard ground, but the great plain of western Russia, with similar natural physical conditions but not drained, is wet and muddy.
In arid and semiarid regions the considerations above set forth are. of less importance, for the terrane, except for talus slopes, is likely to be firm at all seasons. As the greatest concentrations of the population of the world are in the humid regions, however, these are likely to continue to be the scenes of important wars. Therefore the use of geologic maps in forecasting the physical conditions of the surface of the ground deserves attention by tactical leaders. This military use of geology need not be overemphasized, for evidently in many regions it will have little practical application. Moreover, its important effect in military maneuvers will be confined chiefly to the wet season of the year.
Though geologic maps may be only occasionally of service to the tactician, yet he can not afford to ignore any source of information about the physical character of the terrane in the scene of his operations. Advance knowledge that the soil conditions in this or that part of the battlefield will slow down an infantry attack or make the advance of artillery or tanks impossible may give a decisive advantage in a given movement. Some instances from the late war, showing the influence of geology on troop maneuvers, can be cited.
In the Lorraine sector of northeastern France strongly contrasting physical conditions were found between the areas underlain by hard limestones (Jurassic) and sandstones (Triassic) and those underlain by clays and marls (Liassic and Jurassic). The• former afford hard footing at all times; the latter during the wet season were in many places impassable for heavy equipment, such as artillery and tanks or even for quick movements of large bodies of infantry. The wet country may be provided with hard roads, but in time of advance the enemy will destroy the roads in whole or in part, so that they are likely to be impassable. Moreover, a general attack is made over a broad front, and the advance of heavy artillery or tanks must be possible without counting on the use of roads.
The Germans had planned their withdrawal from the St. Mihiel salient in October, when the water-soaked clays of the Woevre lowland are in their worst state so far as the movement of troops is concerned. Captured documents showed that they had formulated very elaborate plans for the demolition of roads and bridges. Fortunately, our attack. was made in September, both before the physical conditions were at their•worst and before the plans for demolition could be accomplished. Had the attack been deferred until October and the roads been destroyed as planned, a part of the new positions would have been almost impregnable to attack until after the rainy season. Evidently the Germans had taken into account the physical conditions of the surface formation and its seasonal variations.
The physical character of river bottoms and banks may also be an important factor in carrying out any particular movement. A shallow watercourse bottomed by mud or silt will be impassable for large bodies of infantry, not to mention artillery and tanks. The presence of a bedrock reef on such a stream, affording a hard bottom, will provide a feasible crossing. For example, the streams traversing the Woevre lowlands, underlain by the Oxfordian clay shales, had muddy bottoms that could not be crossed by artillery, tanks, or large bodies of infantry. In places, however, as shown by the geologic maps, the streams traversed limestone reefs where crossings could be made. Geologic information of this kind was used in the planning of the St. Mihiel attack, principally in choosing routes for tanks. Again, the gravel-filled valleys of the streams whose sources are in the hard rocks of the Vosges Mountains afford dry, firm footing, but the Meuse and its tributaries, traversing areas of soft limestones and shales, are bottomed with silt and mud and are very difficult to traverse during the wet season. The ground underlain by the Cretaceous chalk usually remained hard during all seasons. Many of the Tertiary formations became water soaked during the wet season and furnished a difficult terrain for troop movements. Much the larger part of the success in the use of tanks during the war was attained in the chalk areas, but attempts to take tanks across areas underlain by clay formations, such as the Oxfordian, in the St. Mihiel salient, failed.
The study of the physical conditions affecting troop movements must sometimes take account of the effect of barrage fire. In the Tertiary formations of the British front there were places where an artesian water-bearing sand was capped by impervious clay. As a result of a heavy barrage the ground was pitted with innumerable shell craters filled with water from the underlying strata. Advance over such ground was exceedingly difficult for infantry. The designation of the areas where these conditions existed, so that they might not be chosen as scenes of attack, was one of the duties of the British geologists. On the other hand, where an impervious clay overlay a dry sand the effect of shell fire was to drain the surface. Such types of terrain might be of local importance in affording a series of dry shell craters to organize as defensive positions.
The use of geologic maps to forecast the physical conditions of the surface was of slow development during the war. In this field the French did the pioneer work. A map of a part of the Reims sector, making a classification of surface deposits according to degree of passability, was issued by the Fifth French Army in July, 1917. It is called a soil map and is on a hachured base on a Scale of 1:80,000. The physical character• of the surface outcrop of each formation in its relation to infantry and artillery movements is described for both the wet and dry seasons. This appears to have been the first and probably was the best of this type of geologic maps. The French also issued "general information maps," on some of which the soil was outlined and classified in a crude way according to passability. Toward the end of the war the Service geographique started on a project to make a series of maps for the use of tanks, but none of these were issued.
As has been shown, the British geologists were consulted on questions relating to physical conditions of the surface, especially in regard to the effect of barrage fire. They also prepared some maps, giving a broad classification of their entire front with reference to the use of tanks.
No special maps giving a classification of soils as affecting the maneuvering of troops were issued by the American Expeditionary Force. Information of this kind was, however, given on some of the geologic engineering maps. In these the explanations included a description of the surface conditions of each formation for both the wet and dry season. An example of this type of map, the original of which was in colors, is given in Plate XVI.
It also fell to the geologists of the American Expeditionary Force to collect and make available information about river crossings. This had largely to do with the fluctuations of stream volume but also took into account the character of the river bottom. The description of the topography and geology of the enemy's lines, prepared by the geologists of the American Expeditionary Force, included mention of the areas in which hard or soft footing would be found on the uplands and slopes as well as in the valley bottoms. Just before the armistice the Tank Corps of the American Expeditionary Force developed a plan to make use of a geologist in the selection of area suitable to the maneuvering of tanks. By that time experience had shown that a proper use of tanks is possible only if heed is given to the physical conditions of the soil.
It appears that in the German Army the use of geology in forecasting the conditions to be encountered in an advance was only a late development. Not much space is devoted to this application of the science in the German secret manual of war geology published in January, 1918. Through the courtesy of Dr. A. Renier I saw at Brussels a German map giving information of this type, issued in June, 1918. This was called a military geologic map and is published on a scale of 1:50,000. The legend is of the same general type as those of the French and American maps already described. Its purpose is evidently to guide the tactical commander in planning a forward movement. While intended primarily to describe the surface formations, it also contains geologic information bearing on fieldworks. The areas of natural and possibly artificial inundation are indicated. Like all the other German military geologic publications, the map is confusing because of the attempt to show too great a variety of facts on a single base. Dr. Renier told me that a number of other maps of this type had been prepared, but this is the only one captured, so far as I know. In one respect it was different from all other German geologic maps that I have seen, inasmuch as it covered areas held by the Allies and was therefore in part based on compiled data.
The sources of information needed for geologic maneuvering maps will be field observations within an army's own lines and compiled data on areas occupied by the enemy. Geologic and soil maps will be the best sources of information. The facts furnished by these maps must be interpreted in terms of firmness and stability of. soil at different seasons of the year. A proper interpretation will be possible only if the geologist has a personal familiarity with the region, and he should also know by observation the effect on the soil of a given type of the passage of large bodies of troops and heavy equipment. He should also know the unit weight of the heaviest equipment which is used.
River and stream crossings should be marked on the same maps, with a statement of whether bottom is hard or soft and the seasonal fluctuations in depth. Maps of this type should be made on a contoured base and on a scale not larger than 1: 50,000. It is very desirable that the same maps also show distribution of forests, which is important to maneuvers. [...]

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