Hardpan Soils of the Coastal Plain of Southern Maryland
GEOLOGICAL SURVEY PROFESSIONAL PAPER 267-B Hardpan Soils of the Coastal Plain of Southern Maryland
By C. C. NIKIFOROFF
GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND
GEOLOGICAL SURVEY PROFESSIONAL PAPER 267-B
A study of reconstruction oj the environment in which soilformation took place
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1955 UNITED STATES DEPARTMENT OF THE INTERIOR Douglas McKay, Secretary
GEOLOGICAL SURVEY W. E. Wrather, Director
For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price 15 cents (paper cover) CONTENTS
Page Page Abstract..______45 The hardpan soils of the Brandywine area. _. 52 Introduction .-_____-_-_-__-______.___._ 45 General morphology of Beltsville soil.... 52 Theoretical basis__-_--.--_----_----_-___.__ 46 Red mottling of the subsoil-______53 The hardpan.______-_---___----- 56 Parent material— __-__-_-__--_____-__ 46 Hypotheses about the origin of hardpan. 57 Inherited and acquired soil characteristics. 48 Soils without hardpan.______59 Paleopedology ______49 Conclusion..______--__--_----.-_-. 61 Buried and fossil soils_--_----_-______- 50 Literature cited.______-___---_-___---. 62 Soil evolution.___-._---___-__-______._ 51 Index.______-_-__--_---.------. 63
ILLUSTRATIONS
FIGURE 35. Profile along Sunset Trail. 57
TABLES
TABLE 1. Mechanical composition of Beltsville and Chillum fine sandy loams.. ______62 2. Mechanical composition of Beltsville silt loam and Chillum loamy fine sand. 62 in
GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND
HARDPAN SOILS OF THE COASTAL PLAIN OF SOUTHERN MARYLAND
By C. C. NIKIFOROFF*
ABSTRACT Field data suggest that the hardpan may have been inherited by the present soils from the earlier stages of development Soil, a function of the environment, is a natural combination which, presumably, took place under a periglacial climate. of parent material, landform, climate, and biotic factors. The These stages should have been antedated by still earlier stages, concept of the parent material is vague, but this material is during which the initial parent material was affected by lateri the source of virtually all inherited soil characteristics. Its zation. It is suggested that laterization could have taken place alteration by atmospheric and biotic agencies imparts to the during the Sangamon interglacial stage and hardpan develop soil a series of new or acquired characteristics. Generally, ac ment during the Wisconsin stage. quired characteristics are not fixed and are subject to changes by the environment. Some of these characteristics, however, INTRODUCTION are irreversible and upon being acquired are retained by the soil irrespective of environment. Thus, certain properties which The original objectives of this study were to reex- were imparted to the soil during the earlier stages of evolution amine some peculiar characteristics of hardpan soils on may be retained throughout the later stages, during which they the Coastal Plain to determine if the earlier stages of would not develop anew. Evolution of soils consists of changes in the geographical pattern of distribution of different soils, development were indicated and to allow reconstruction rather than development of new forms or species. An area of the environment in which soil formation took place. occupied now by certain soils may have been occupied by dif Some tentative suggestions on these subjects were made ferent soils in the past and may be occupied by still others in the in a previous publication (Nikiforoff, Humbert, and future. The study of past stages of soil evolution is the field of Cady, 1948). However, the study is a problem not only paleopedology. In polygenetic soils the new characteristics are superimposed upon the remnants, of irreversible old features. for the soil scientist but also for the geologist. In some places, however, old soils are buried under younger The soils in question are developed from stratified deposits so that more recent soils develop above the ancient. Coastal Plain sediments which greatly complicate Soils that retain some features of the past stages of their analysis of genetic profiles. These profiles are super evolution and buried soils might serve as valuable stratigraphic imposed upon the lithologic units, the genesis and markers. stratigraphy of which are not yet fully understood. A Soils of the Beltsville series in eastern Maryland are charac terized by an uncemented hardpan ranging in thickness from correct interpretation of these profiles, therefore, seems about 1 foot to more than 3 feet and covered by a mantle of problematic until the pertinent geology can be clarified. mellow loamy material. This loamy layer consists largely of Hence, a detailed geologic survey of a quadrangle was silt. These soils are developed from the upper loamy member made and the problem studied jointly by a geologist of the Brandywine formation. In most places the material underlying the hardpan is marked by red mottling, the pattern and a soil scientist. of which indicates that mottling has been acquired after the Geologists, geomorphologists, and other specialists deposition of sediments. Red mottling is suggestive of lateriza- who visited the area showed considerable interest in tion. The present soils of the region are not lateritic. Appar the project. About 50 members of the Geological So ently this mottling was imparted to the soils during one of the ciety of America spent the greater part of a day in earlier stages of evolution in which climatic conditions may have been conducive to laterization. the area on a field trip preceding the 1950 annual A conspicuous feature of the hardpan in Beltsville soils is meetings. In the spring of 1951 the Friends of Pleisto the polygonal vertical cleavage resembling the pattern of large- cene held their annual convention in the same area. scale mud cracks. The soils with such a hardpan occupy the Among other visitors, were the late Kirk Bryan who undissected remnants of the upland deposits. Three hypotheses offered valuable suggestions; C. B. Hunt who is now of the origin of the present hardpan are discussed, and the working on a similar problem; L. L. Ray with a group soils are compared with the associated soils lacking hardpans. of his coworkers; C. S. Denny, L. C. Peltier, L. Leopold,
*D. S. Department of Agriculture, Soil Conservation Service. W. Armstrong-Price, and others. 45 46 GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND The interest shown by so many specialists is en Usually, soil parent materials are thought to be the couraging and much appreciated. Discussions in the unconsolidated products of the weathering of rocks, field and by correspondence are being continued. The whether hard or mellow. Hence, the soil's parent ma study now seems to be centered on problems of a more terial cannot be identified with loose mantlerock. Like general character. Therefore, a short review of the any other rock, the mantle, if it consists of transported theoretical basis for the analysis of these problems sediments and has a great thickness, is merely a source might be in order. This review forms the major part of the parent material of the soil. Only that part of the of the paper, and although no positive answer to the mantle which is directly affected by weathering in place, cardinal question has yet been found, some new data for example, the upper 10 to 20 feet, serves as parent on the original subject are given. material. Deeper portions of the mantle may or may not be similar to the top layer and may be altered by THEORETICAL BASIS other deep-seated processes which do not affect the top PARENT MATERIAL layer. Development of soil represents alteration of the man Neither is hard rock a parent material. In some tle rock by atmospheric and biotic agencies. Usually places a soil might develop from solid rock which soil is defined as a product of alteration of the so-called may appear to be a parent of the thin residual soil. "parent material" by environmental agencies. The Such cases, however, are exceptions and it may be word "product" in this definition, however, is objec argued that the rock should have been rendered loose tionable because it connotes a certain finality. After before the beginning of its transformation into an em becoming an entity, the product, as commonly under bryonic soil. Although the parent material—which is stood, is no longer dependent upon the process by which neither the initial rock nor the resultant soil but merely it has been brought into being. Earlier, and perhaps a stage of the process—is not represented now by any more correctly soil has been defined as a function of part of the present soil profile, it should have been rep environmental factors, rather than a product of their resented in the past, when the rock was pulverized but interactions (Dokuchaev, 1899). Function is defined had not yet become a soil. The state of such a material by Webster as a "quality, trait or fact so related to an during this stage of weathering is not well understood. other (fact or facts) that it is dependent upon and The inherent weakness of this theoretical concept, as varies with that other." Hence, a function is an entity a certain stage in the gradual decay of rocks, is that it only as long as the factors on which it depends continue presupposes some elusive boundary between two differ to operate. Function changes when the factors which ent processes, or between two different stages of one bring it into existence are altered. The phrase "func continuous process—formation of the parent material tion of the environment" describes the essence of soil and the transformation of this material into soil. It more specifically than other terms, such as product or is not known precisely where this boundary is or what result. The pedogenic process is never finished, there the difference is between the two processes. fore the soil is merely a manifestation of its operation, Decay of rocks is commonly due to instability of these rather than its result. rocks under conditions other than those under which The environment of soil formation is defined as a they were formed. This decay is the essence of weath combination of natural conditions under which this ering. Formation of soil parent material is alteration process operates, including climate, landform, vegeta of rocks by atmospheric agencies, that is the same forces tion, and parent material. Of these conditions, the which operate in the development of soil. When or parent material is the most difficult to define. As the where do these agencies stop working on development words suggest, a parent material is an initial material of a parent material and begin to act as factors of soil which has been altered into soil. Naturally, this ma formation ? terial is not a "deus ex machina"; it has its own origin Many authors have suggested that the specific pedo and its own "parent", or as regards soil, a sort of file genic process begins with the introduction of biotic grandparent material. Thus, one may consider the soil agencies into the interaction between parent material parent material as representing a stage of the geochem- and its environment. Hence, there is a tendency to ical process which acts upon the outermost layer of the postulate the formation of parent materials as an essen lithosphere. During this stage a geologic body is tially abiotic physicochemical process and soil forma changed into material which is not yet a soil, but merely tion as a biochemical refining of the sterile mineral par a soil's parent material. In theory, it is not difficult to ent material. postulate such a sequence of events. These concepts, It can hardly be disputed that living matter plays however, are abstract and have little practical value. a leading part in the drama of soil formation. How- HARDPAN SOILS OF THE COASTAL PLAIN, SOUTHERN MARYLAND 47 ever, differentiation between the soil and its theoretical These different terms are mentioned to give some parent material is difficult because of the uncertainty idea about the practical handling of the problem of the of an entirely abiotic decay of rocks on the land sur soil parent material. One may conclude that this ma face at any time since the first appearance of organ terial is a certain, unconsolidated geologic surface for ized living matter—at least since the middle Paleozoic. mation. There is nothing specific in these terms which Thus, the criterion of differentiation appears to be would refer to the genetic relationship between soils purely abstract and of questionable practical value. and their parent materials. For descriptions of soils, Uncertainty as to the nature of the soil's parent ma however, not uncommonly the material of the C horizon terial apparently is so confusing that suggestions have is defined as parent material and sometimes as "unmod been made to reject this concept and enlarge the con ified" parent material, which implies an assumption cept of soil formation to include the entire range of that a similar if not identical material has been modi weathering. Jenny (1941, p. 12), states that weather fied into the A and B soil horizons. This assumption ing is "one of the many processes of soil formation." can hardly be accepted without reservations. It may be His concept agrees with the broad definition of soil true that accumulation and deposition of certain sedi by the geologist but hardly will be accepted by prac ments, such as glacial drifts, marine sediments, lacus tical soil scientists and, especially, by agronomists. trine deposits, or dune sands, had been completed be Furthermore, it hardly would solve the problem of fore these deposits were exposed to attacks of pedogenic parent material. Broadly defined, soil still would have process and that these deposits were reasonably uni its parent material which would have to be defined. form throughout their thickness. It can be assumed The soil still is a function of certain factors of its for that in soils developed from these sediments, the upper mation including the initial material. No soil can be horizons have been formed by alteration of material adequately defined or described without sufficient in similar to the C horizon. This is not true, however, of formation about the nature of its source or parent most other materials, whether residual or sedimentary. It is highly improbable that they could have been uni material. Agronomists and soil technologists believe form from the surface downward. Alteration of any that soil is the natural medium for the growth of plants. deposits by weathering, whether hard rock or a loose The thickness of this soil is only a few feet, in places sediment, changes with depth in kind and intensity. less than 1 foot. The underlying material is the sub Therefore, the mantle resulting from weathering ac soil and the parent material. The geologists' concept quires a certain profile whether abiotic weathering has of the soil is more inclusive. Geologists believe that been or has not been accompanied by the biochemical soil may represent the whole geologic layer which is pedogenic activity. Actual initial materials of the J., affected by atmospheric agencies, such as oxidation, B, and C soil horizons could be significantly different leaching, and many other alterations of the initial ma from the beginning of the pedogenic remodeling of terial. The thickness of this "soil" may amount to these materials. several tens of feet. It includes the soil proper as well However, in geologic perspective, the materials of the as what is commonly referred to as the parent ma C horizon of a certain class of soils still might serve as terial. The concept of the majority of soil scientists parent of the overlying horizons. General weathering is less inclusive than that of geologists but broader and soil formation are accompanied by solution and than that of practical agronomists. According to erosion which tend to remove a certain part of the prod ucts of the former processes and to enable the driving Marbut,1 soils range in thickness "from a mere film to forces to penetrate deeper and attack the mantle at a maximum of somewhat more than ten feet." depth. Thus each genetic horizon in the zone of Parent material is defined in general terms, such as weathering might be subject to obliteration from the Peorian loess, Mankato ground moraine, Kecent al surface and forced to encroach upon the underlying luvium, lacustrine clay, fanglomerate, Coastal Plain horizons. sediment, or residuum of some particular hard rock. A profile of weathering, especially the outermost part Usually, some specific information about the Ethology, of it, the soil profile—can be analyzed from the bottom mechanical composition, and other physical and chemi upward (Bryan, 1946, p. 56). Each genetic horizon cal characteristic of these materials is added. The could represent a stage of remodeling of an initial most common terms used in these descriptions are: material into one that is most nearly in equilibrium calcareous, well oxidized, stony, gravelly, compact, with the environment on the surface. In the natural stratified, loose, old, leached, and sorted or unassorted. arrangement in the profile, the deeper horizon repre sents the earlier stage of the process. Hence, each 1 Marbut, C. F., 1928, A scheme for soil classification: Proc. 1st Internat. Cong. Soil Sci. Comm. V, p. 1-31. horizon consists of a material which previously was 48 GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND similar to that of the next lower horizon and still Furthermore, each of these materials is altered dif earlier had passed through a series of changes repre ferently in unlike topographic and drainage situations sented by the sequence of all underlying horizons from and each of these alterations represents a separate map the bottom upward. Consequently, the C horizon might ping unit of soil. Thus a comparatively simple geologic represent the parent material of the B horizon and the map of an area becomes an intricate patchwork on latter—the parent material of the A. the soil map on which a single geologic mapping unit Obviously, this interpretation of a genetic relation may be broken into a score of soil units. ship between the different horizons applies only to soils that are subject to removal from the top. It does INHERITED AND ACQUIRED SOIL CHARACTERISTICS not apply to soils that undergo upbuilding by sedi mentation. In this latter case genetic relationship be Soil may be conceived as a sort of pedologic skin ac tween the horizons are reversed, specific properties of quired by geologic bodies in contact with the atmos each horizon are erased from the bottom, thus leading phere. The thickness of this skin averages only a few to encroachment of the lower horizons upon the over feet. It is not uniform throughout the area underlain lying ones. by any given geologic body but varies in thickness and Soils developing from stratified sediments present other characteristics with the changes in forces acting further problems. Each layer, whether several feet upon this body. or a fraction of an inch thick, represents a different Definition of soil as a function of the environment kind of parent material. In a general description it implies the dynamic nature of this entity. Soils are may be stated that a given soil is formed from stratified not fixed but respond and readjust to any change in en parent material, the latter being treated as a unity. vironment. It is postulated that soils of any area may In a detailed description, however, it would be specified have been different in the past and may change in the that various genetic horizons of such a soil, or even future (Kossovich, 1911). The dynamic nature and various parts of a single horizon are formed from dif capacity for evolution are the principal characteristics ferent parent materials. Thus, the emphasis in defini of soils which differentiate this thin but highly acti tion of the parent material may be shifted from the vated pedologic skin from the relatively static under general geologic to the more specific lithologic character lying geologic bodies. or to the mechanical composition of this material. The general character and physical makeup of pedo Because differences in parent materials are among logic skin reflects the character of the initial material the basic criteria for differentiation between soils, it and the conditions under which its present "skin" has is obvious that taxonomic and especially mapping units been acquired. During the pedogenic process some of soils are more narrowly defined than the geologic properties of the original material are lost or altered, units. This is particularly true of soils formed from whereas new characteristics are imparted to the residue. stratified sediments. Individuality of such soils is de As a result of these changes, the pedologic skin acquires termined largely by the character of the uppermost a definite morphological individuality or a specific layer of sediments, provided that this layer is thick "profile." Such an individuality is determined by char enough for the development of the A and B soil hori acteristics which can be divided into three general zons. A single geologic formation might be the source groups: inherited, acquired, or introduced. of different parent materials. For example, according Inherited soil characteristics.—These soil character to Hack (1955, p. 10) the Brandy wine formation in istics are a carryover from the parent material. They southern Maryland consist of two members, the basal represent the properties of the initial material which gravel member and an upper loam member. Over a have not been obliterated and, presumably, could not be large part of the upland, underlain by this formation, altered by the pedogenic process. They were present the loamy mantle is removed by erosion exposing the in the parent material, hence, they may serve as criteria basal gravel. This gravel is interbedded with layers for establishment of the soil's geologic parents. Pres or lenses of sand. Either loam, gravel, or sand may ence of primary quartz, rutile, or zircon in the soil, for form the uppermost layer; and each one may be covered example, is an inherited characteristic because these by a sheet of more recent sediments ranging from gravel minerals could not form in the soil otherwise. to clay. Thus, a soil, developed from this formation Acquired characteristics.—The pedogenic process might have as its parent material the basal gravel, a imparts acquired characteristics to the soil. Some parts few feet of sand underlain by gravel, the silty loam of the soil may be bleached and the others darkened. of the upper member, or a sheet of recent overwash Different parts of the pedologic skin may acquire new covering any one of the above. colors. Content of clay present in the parent material HARDPAN SOILS OF THE COASTAL PLAIN, SOUTHERN MARYLAND 49 may increase in one soil horizon and decrease in an Differentiation is possible between the reversible and other. Secondary clay minerals that were absent in relatively irreversible acquired soil characteristics. the initial material may form in the soil. Other sub Perhaps the terms "unstable" and "stable" are better stances such as carbonates that could have been present than reversible and irreversible. Irreversible or fixed in calcareous materials may be entirely leached out. acquired soil characteristics are of particular interest These are but a few examples of acquired soil char to stratigraphers because of the possible use as strati- acteristics. None were present in the fresh parent graphic "markers." Furthermore, these irreversible material. characteristics might reveal the nature of the earlier In some instances, it is rather difficult to differentiate stages of evolution of soil and the changes in the en between the acquired and inherited soil characteristics. vironment which should have been the driving force The presence of quartz, rutile, garnet, or other chemi of such evolution. cally stable minerals is an inherited characteristic, but the relative content of these minerals may be an ac PALEOPEDOLOGY quired feature, if some changes in composition of the Paleopedology is still in its infancy but study in this parent material have been caused by the pedogenic field may become a valuable tool in geology. process. Content of titania in the soil or in some par The speed of evolution in soil depends upon the ticular soil horizon may be several times greater than in speed of changes in the environment. No matter how the parent material. Such an increase in the relative unstable a given characteristic, it cannot vanish faster content of titania is an acquired characteristic caused, than the specific condition under which it has been ac perhaps, by the destruction and solution of other less quired. Characteristics which are more difficult to stable substances. Presence or absence of carbonates eradicate might persist for some time after the change or other salts may be either acquired or inherited in environment took place. Hence, evolution in soil characteristics. Under certain conditions carbonates may lag behind the changes in environment. In cases and other salts are formed in soil developing from of fixed irreversible characteristics, the lag may extend parent material that was free of these salts. A similar indefinitely. This hypothesis is subject to scrutiny by soil, however, can develop from originally calcareous further study. material; thus, one part of its carbonates is inherited Most acquired soil characteristics are transitory. and the other acquired, and they cannot be differen They are retained by the soil under stable environ tiated. mental conditions but disappear with changes in these Introduced soil characteristics.—Soil characteristics conditions. A classic and well-understood example of absent in the initial material and not acquired under such evolution is the so-called degradation of grassland natural conditions may be introduced to the soil es soil due to climatic changes followed by the encroach pecially by man. Examples of these soil characteristics ment of forest upon the prairie. In this process, most are mechanical mixing of the uppermost soil horizons acquired characteristics of the original soil are rather by plowing, changing of the soil reaction by continuous drastically altered. In the final stage of degradation, liming, improvement of aeration of the soil by drain hardly any traces of the nature of original soil are age of poorly drained land and the subsequent enhanced discernible. oxidation of the soil material, and changes in arid soils Fixation of certain acquired characteristics is still due to a continuous irrigation. Most of these intro largely unexplored. Perhaps, one of the most con duced characteristics exist only as long as the agency spicuous manifestations of this process is the develop responsible is present. A few introduced characteris ment of hardpans. Hardpan is a secondarily indurated tics, however, might remain as lifelong scars on pedo- soil horizon. Usually, induration is effected by im logic skin. pregnation of this horizon by some fluid or colloidally Evolution may affect especially the acquired charac dispersed substance capable of irreversible solidification. teristics of the soil. These changes consist essentially The most common cementing substance is amorphous of a gradual replacement of the older characteristics. silica. Other cementing materials include ferric iron A soil may change its color, reaction, consistency, struc and other salts, especially the carbonates and gypsum. ture, chemical composition, and almost every property Hence, the hardpans might be classified as silica ce acquired during the earlier stages of its evolution. mented, iron cemented, lime cemented, gypsum ce Here, again, some acquired soil characteristics may mented, and so forth. The silica-cemented pans are, vanish in a short time, others disappear less readily, perhaps, the hardest and the most stable, although the whereas others may remain highly resistant to any iron-cemented and lime-cemented pans are probably alteration other than mechanical destruction by erosion. more common. 315913—55.———2 50 GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND It is assumed that induration of the hardpans takes relicts serve as markers to trace the pedogenic activity place under conditions which permit liberation of po in the past. tential cementing substances and accumulation of these These fixed characteristics which, presumably, can be substances in certain soil horizons to be followed by ir inherited by the recent soils from the earlier stages of reversible solidification capable of survival under con evolution are foreign in the present environment and ditions different from those under which it took place. would not be acquired under existing conditions. This Such solidification, perhaps, is a case of local fossiliza- assumption is merely a theory which has not yet passed tion of the soil. From the pedologic viewpoint pan critical examination. It may be true of Australian formation is rather a pathological development, anal- lateritic crusts. It may or may not be true in other in agous to biologic sclerosis. A normal soil is a dynamic stances. What appears to be a lack of adjustment be system in every part of which continuous movements tween certain observable features and their present are taking place, whereas induration of any part of surroundings may be our failure to coordinate the facts this system tends to stop this activity. in their proper perspective. An example of development of this kind is the so- Preservation of old acquired characteristics of the called lateritic crust, a common feature of the land soil, or what is now believed to be such a preserva scape in many parts of Australia where laterization tion, provides the theoretical basis for the concept of so- does not take place at the present time. called "polygenetic" soils, that is soils having charac Changes in relative content of certain minerals in teristics imparted to them at different times. The word other soil horizons represent the other group of fixed polygenetic is defined in Webster as "having many dis acquired soil characteristics. It is postulated that tinct sources; originating at various places or times." juvenile minerals crystallize under hydrothermal con The term polygenetic and the underlying concept ditions inside of the earth's crust and do not form in are open to criticism. All soils are subject to changes the zone of weathering where the less stable, including and it is conceivable that some records of earlier stages the majority of aluminosilicates and ferrisilicates, de of evolution are common to most normal soils, although compose. Decomposition of highly alterable feldspars we have not yet learned to recognize them. Hence, the and some ferromagnesian minerals is accompanied by differentiation between polygenetic and monogenetic a relative increase in content of the more stable minerals soils is difficult to substantiate. such as quartz or rutile. Obviously, if some particular primary minerals have been decomposed in one or an BURIED AND FOSSIL SOILS other soil horizon, then their content in this horizon will The polygenetic soils are those in which recently not be reestablished, no matter what changes occur in acquired characteristics are superimposed upon older this soil in subsequent stages of its evolution. For ex profiles developed under different conditions. These ample, if the percentage of rutile or zircon in a certain soils are dealt with in one branch of paleopedology. soil horizon has been increased by destruction of the The other branch of this science deals with "buried" less stable minerals, subsequent evolution of the soil and "fossil" soils. Unfortunately, the terms "buried" is unlikely to obliterate this change. and "fossil" are, used by some writers interchangeably Formation of some specific vadose clay minerals and as if they were synonymous. A buried soil does not their accumulation in certain horizons is acquired char imply fossilization. A fossil soil, usually just a fos acteristic which might be carried over by the soil from silized part of the soil, such as the hardpan, may or may one stage to another in evolution. Relative stability of not be buried. It could be deprived of its original individual clay minerals under variable conditions is mantle and still be a fossil. In buried soil the older profiles are not overlain by the recent ones, but are still uncertain, hence presence or absence of some par covered by some younger sediments, the outermost layer ticular minerals of this group is not necessarily a safe of which is altered into a recent soil. Hence, develop criterion of evolution of soil. A general enrichment in ment of the older (now buried) and recent soils are clay of certain soil horizons, whether due to the kaolini- separated in time and space. zation in place or by illuviation, apparently is a more The thickness of sediments above the buried soil reliable marker of pedogenic activity, provided that it ranges from a few feet to many feet. Burial of older can be ascertained that the clayey layer is not merely a soil indicates that evolution had been interrupted by mechanical layer inherited from stratified deposits. a period of sedimentation during which the rate of Collectively these examples of fixed soil characteris accumulation of fresh sediments was high enough to tics may be considered residual. Being carried over prevent pedogenic alteration until the sedimentation from earlier into later stages of soil formation, these ceased. HARDPAN SOILS OF THE COASTAL PLAIN, SOUTHERN MARYLAND 51
Chances of preservation of buried soil are poor, un ther the field data nor the data obtained by the labora less the burial was fast, took place under exceptionally tory study of the samples of these materials are suffi favorable conditions, and was accompanied by quick cient to prove such an interpretation. fixation or fossilization. It has been stated earlier in These facts are neither surprising nor discouraging this paper that soil is a dynamic system, which is the although they curtail considerably the field of paleo- function of the environment. This system operates and pedology. Soil loses its former profile as a result of maintains its organization because it is continuously burial and can retain only certain fixed properties activated by energy from the environment, especially which may be referred to as paleopedogenic bones or by the energy of the sun, whether direct or through fossils. Some of these fixed properties are invisible to the intermediary of living matter. Without this energy the unaided eye and must be identified by petrographic normal operation of the soil is impossible. Burial, and chemical analyses. Therefore, an inquiry into the naturally, cuts off the old soil from the main source problem of buried soils 2 and their significance as strati- of its energy and renders it progressively more static graphic markers should follow the lines of a study of until all normal functions of the system cease. irreversible soil characteristics, such as changes in min- Many acquired characteristics including some of the eralogical makeup of the soil—that is the chemically most conspicuous ones, are manifestations of certain inert mineral framework of it. functions of the soil. They are maintained as long as Although little has been done so far in the field of these functions are performed but eventually are lost paleopedology, this new tool of survey offers great when the system ceases to operate, unless they are fixed potentiality. Recent investigations have been made by by fossilization or some other means. Therefore many Hunt and Sokoloff (1950) in the Bonneville Lake acquired characteristics depend upon the state of the basin and by Denny (1951) in Pennsylvania. Earlier soil system and are subject to changes with the state. investigations were made by Leighton and MacClin- The state of the buried soil is fundamentally different tock (19i30), Simonson (1941), Bryan and Albritton from the state of an active soil on the surface. Specific (1943), Frye (1949), Kay (1949), and Richmond properties of the normally functioning living soil are (1950). lost in burial. Fixation of these alterable properties The value of information which might be provided before this loss is seldom possible. Therefore, perfect by paleopedological investigations will depend upon preservation of whole profiles of ancient soils is un our ability to correlate the "paleopedologic bones" with known. Not a single case of such a preservation has specific conditions under which the ancient soils de been discovered. Most reports about buried soils deal veloped. Such a correlation is the aim of paleopedol with certain marks which indicate merely that a certain ogy and finding of appropriate criteria for this cor material has been affected by the pedogenic process relation is not beyond the realm of possibility. before being covered by younger deposits. Usually, these marks consist of conspicuous local oxidation or SOIL EVOLUTION leaching of the affected material, decay of some un Unlike biologic evolution, soil evolution is not the stable minerals, local enrichment in clay, bleaching, continual development of new forms and new species. darkening or other peculiar changes in color and other General kinds of soil 3 hardly change throughout geo similar features. Sometimes the arrangement of these logic periods and probably do not change much from marks gives the impression of a faint outline of the period to period but the pattern of geographical dis soil profile, although these marks seldom are clear tribution of certain kinds of soil changes. The state enough to ascertain the correctness of such an impres ment that soils could have been different in the past sion. and may change again in the future refers to the soil Simonson (1941) described recently what he believes of a given area. An area could have been occupied by to represent fairly well preserved old soils developed soils different from those which occupy it now and from the upper part of Kansan till in southern Iowa may be occupied by still others in the future. The old and buried under Peorian loess. He states that "a light soils do not vanish entirely and the future soils are not colored band 6 to 18 inches thick at the contact between wholly new. All soils belong to all times but are dis the till and the overlying loess" marks many exposures tributed in different places; and this distribution is in southern Iowa and that, usually, this band grades subject to continuous change. Thus, evolution of soil "into an underlying layer of heavy textured clay, com 2 Recently a new name "paleosol" has been suggested for these forma parable in thickness to the B horizon" of certain present tions or relicts of ancient pedogenic alteration of geologic bodies. soils of this region. It may be perfectly true that these 3 The term "kind" is used here in a broad sense. There are probably not more than a dozen general kinds of soil such as Podzol, Chernozem, bands do represent the horizons of an old soil, yet nei and Laterite. 52 GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND refers first to evolution of the geographical pattern of parently is fairly well sorted and consists largely of distribution of certain types of soil and second to the silt and very fine sand. Content of clay is low in the sequence of replacement of one kind of soil by another upper part of the layer which corresponds to the A at any given point or area. horizon of the soil. The clay content increases marked In one place or another, all these soils, past, present, ly in the lower part representing the B horizon, just and future, exist in all times and in each particular above the pan. place are adjusted to their own environments. For The mechanical composition of the pan is not uni example, it is unlikely that during the Sangamon inter- form. Some pans consist largely of slit, others of het glacial stage somewhere 011 land there were some pe erogeneous materials containing an appreciable amount culiar soils which do not exist now, or that there are of sand, whereas others are rich in clays. In most some recent soils which did not exist in Sangamon time. places, the pans are mostly free of gravel, although Hence there is opportunity to examine all kinds of soil solitary pebbles and even small pockets of gravel im and their coordination in the respective environment, bedded in the earthy matrix are not uncommon. without which paleopedological studies, probably, The material below the pan is varied. It may be would not be too profitable. Correlation of various sand or clay with or without pebbles or largely gravel paleosols with the corresponding recent soils has not in either sandy or clayey matrix. This poorly stratified, even been attempted as yet. heterogenous material represents the upland deposits described by Hack (1955). THE HARDPAN SOILS OF THE BRANDYWINE AREA The term "upland deposits" is used tentatively by GENERAL MORPHOLOGY OF BELTSVILLE SOIL Hack to include the gravelly and loamy deposits which underlie the highest, plateaulike surface of the Coastal In this study attention has been centered on the hard- Plain. These deposits have been subdivided by geolo pans in soils of the Beltsville series. Originally these gists into several formations, including the Brandy- soils were regarded by Marbut (1935, p. 33-34) as wine, Sunderland, and Wicomico formations in south members of the Leonardtown series which included all ern Maryland. The definition of these formations has hardpan soils in the northern part of the Coastal Plain. been subject to much controversy, and as they are litho- Similar hardpan soils in Tennessee, Kentucky, and some logically similar, the more inclusive term "upland de other southern States are grouped into the Grenada posits" is used here. The old surface of the southern series. Recently, the old Leonardtown series has been Maryland seaboard was fairly smooth but subsequently divided into the Leonardtown and Beltsville series, the dissected by many steep-walled valleys and ravines. latter including the better-drained and more oxidized Most of this dissection appears to be Pleistocene or members of the older group, and the former—the im younger. At the close of accumulation of the Coastal perfectly drained and less oxidized members. In Plain sediments and before the inauguration of the re earlier publications, the soil which is now referred to cent cycle of erosion, the older sediments, which range as Beltsville was called Leonardtown. The hardpan in age from Cretaceous to Miocene, were covered by in these soils is not typical and some doubt exists as a 40- to 60-foot thick layer of gravel, sand, and other to whether it should be called a hardpan. This par poorly sorted material. According to Hack this man ticular pan is not cemented as believed by Marbut and tle has been formed over a period of time beginning other earlier workers. Its hardness is due to compac in the Miocene and ending in the Pleistocene. tion and, presumably, interlocking of mineral particles. Pleistocene and later erosion removed a large part ot The hardpan is almost impermeable to water and plant these younger sediments and in many places uncovered roots, but chunks of it disintegrate in water. the underlying Miocene and older formations and The Beltsville hardpan ranges in thickness from spread the gravel over other areas including some well- about 1 foot to probably more than 3 feet although in marked terraces in the valleys of major rivers. most places the thickness is hardly more than 2 feet. The deposits of gravel undisturbed by the subsequent The upper boundary of the pan is sharp if not abrupt, erosion are preserved over the areas which escaped dis whereas the lower boundary is gradational; hence, section, such as the conspicuous Brandywine flat, as values of the thickness are somewhat arbitrary. No well as on the flattened tops of knobs and hills repre where is the pan exposed on the surface. It is typically senting the remnants of the original upland in the now covered by a mellow loamy layer, which ranges in dissected areas. thickness from about 16 inches to more than 2 feet. The Brandywine formation and other upland de In most places, however, thickness of this loamy man posits generally consist of two members, the basal tle ranges from 18 to 24 inches. This loamy cover ap gravelly layer and the overlying loamy layer. The loam HARDPAN SOILS OF THE COASTAL PLAIN, SOUTHERN MARYLAND 53 rests on the gravel along an irregular and in many but nowhere is the hardpan formed entirely from the places gradational contact. This loamy layer is, or gravelly layer, regardless of the texture of its matrix. has been particularly susceptible to erosion by wind The hardpan is also absent in soils developed from and water so that nearly everywhere some of this ma materials having a relatively high content of sand, terial has been either removed or piled on top of the especially in the upper soil horizons. Compaction of gravelly layer. Over much of the area this loamy layer the soil material to pan consistency apparently is con is stripped entirely, leaving the underlying resistant ditioned by a certain mechanical composition. It is gravel exposed. The thickness of the loamy layer aver probable that the presence of not less than a certain ages about 15 to 20 feet and ranges from less than 1 minimum of silt and, perhaps, clay and not more than foot to a maximum of about 30 feet. The extremes, a certain maximum of gravel and sand were among the however, are rare. essential conditions which determined whether the The basal member of the upland deposits is generally hardpan could have been formed under an otherwise a sandy gravel but has a wide range in texture and in similar environment. thickness. Beds of gravel as much as 40 feet thick are common although the average thickness ranges prob BED MOTTLING OF THE SUBSOIL ably from 10 to 20 feet. The matrix of the gravel ranges In most places the layer underlying the hardpan is from sandy to clayey and in places the gravel is inter- marked by a conspicuous red mottling. Usually, a few bedded with lenses of material free of pebbles and usual faint mottles appear directly below the pan. They ly consisting of assorted fine sand or silt. range in size from less than 1 to more than 2 inches in Stratigraphically and, perhaps, lithologically the cross section. The color in the central parts of the Brandywine formation and similar upland deposits in mottles is deeper. Toward the periphery the red gradu Maryland and Virginia probably are similar to the ally fades and merges with that of the matrix. The beds in Kentucky and Tennessee which are referred mottling becomes denser with depth. Individual, usu to as the Pliocene and Pleistocene sediments by Roberts ally, somewhat lobate mottles increase in number and and Gildersleeve (1945). It is true, however, that the become larger and more brilliantly red. At the same upland deposits in Maryland are underlain largely by time the material between the mottles becomes bleached. the Miocene and locally by the Eocene and older for About 1 to 2 feet below the level at which mottling be mations, whereas the Pliocene and Pleistocene sedi gins the neighboring mottles join to form an intricate ments in Kentucky rest commonly on Eocene and older reticular pattern of brilliantly red network in which are deposits. The Miocene deposits are conspicously ab enmeshed blotches of nearly white or bluish-white ma sent. The Pliocene and Pleistocene beds in Kentucky terial. In some places the entire material is solidly red. are also capped by the loamy mantle, which is better Mottling and reddening have been seen in clayey and sorted than the corresponding mantle in Maryland and sandy sediments and also in the matrix of gravel beds. is believed by certain investigators (Smith, 1942) and In the gravel beds most of the pebbles are heavily others to be an eolian loess. If such is the case, then coated with red, purple, yellow, or brown films. An the age of this loamy mantle and the underlying gravel especially striking reticular coloring, however, occurs are not the same. About 40 or 50 years ago the loamy in fairly compact clayey material. layer of the upland deposits of southern Maryland also The pattern of red mottling and its reticular form was referred to by some geologists as loesslike. Hack strongly suggest that this peculiar characteristic has (1955) states, however, that no geologist "has ever been imparted to the upland deposits subsequent to proposed that any of the material of the upland actually the deposition of sediments. In other words, mottling might be loess." Hack considers the loamy layer an and reddening represent a conspicuous alteration which aqueous deposit. took place after the accumulation of these sediments. Soils of the Beltsville and Leonardtown series, that Because the upland deposits accumulated during a pe is almost all hardpan soils of the region, are developed riod of time "beginning in the Miocene and ending in typically from the upper or loamy member of the up the Pleistocene" (Hack, 1955, p. 37), it appears that land. The minimum thickness required for full devel such an alteration should occur not earlier than the opment of the Beltsville soil profile appears to be about latter part of the Pleistocene. 3 to 4 feet, allowing for the formation of at least the Reddening of the outer layer of the mantle rock sug upper part of the pan from this "loamy" layer. If the gests laterization. Lateritic type of weathering is typi loam is less than 3 feet, then the pan was not formed. cal of tropical and subtropical climates. Laterization In a few places the hardpan extends for a few inches affects the mantle from the surface to a variable—in into a gravelly material underlying the loamy layer, places very considerable—depth. Occurrence of beds, 315913—55———3 54 GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND which show signs of having been laterized, beneath a accept an interpretation of the development of Red cover that shows no evidence of having been so acted Podzolic soils through the degradation of hypothetical upon, usually indicates either burial of the laterized lateritic ancestors. Red Podzolic soils have some char material by younger sediments or a subsequent altera acteristics that presumably a*re acquired due to lateriza tion of the upper part of the previously laterized ma tion and others that clearly indicate podzolization. A terial. In either case—especially where there has been simultaneous operation of laterization and podzoliza subsequent alteration—there should have been a change tion is difficult to conceive. This difficulty led to the in kind of weathering, from laterization to a different assumption that laterization probably antedated the type. This change would require a change in the en more recent podzolization which imparted certain new vironment, first in climate, without which a change in properties to the original lateritic soil. the environment could hardly have been possible. At the root of laterization and podolization is a Burial of old laterized by younger unlaterized sedi thorough decomposition of aluminosilicates and the ments, probably would be indicated by a stratigraphic rupture of chemical bonds between silica and alumina break at the plane of contact between the two strata and iron oxides. The principal difference between the and a rather sharp boundary between these strata. two processes lies in what happens to the free oxides Evolution of the lateritic soil, in all likelihood, would after their separation from one another. In lateriza be characterized by the gradational boundary between tion, silica is leached leaving behind hydrated oxides the affected and unaffected parts of the profile. The of iron and alumina whereas, in podzolization, iron gradational enhancement of red mottling with depth oxides and alumina are removed leaving behind the below the hardpan seems to indicate the probability of relatively immobile silica. Hence, the silica-sesqui- evolution of an old lateritic soil and, consequently, the oxide ratio tends to be low, usually less than 2, in existence of a sufficiently long period of time during lateritic soils and high—more than 2—in Podzolic soils. which the outermost layer of the upland deposits should Theoretically, the difference in behavior of free silica have been subject to effective laterization. and sesquioxides in laterized and podzolized soils is due The present Gray-Brown Podzolic soils of the region to the difference in reaction of the medium and, per are not laterized. If such a soil develops from a parent haps, in the ratio between the rates of liberation and material which has not been previously affected by leaching of free bases. lateritic weathering, then it carries no sign of past or Whatever may cause the difference in the effects of present laterization. If, however, soil develops from laterization and podzolization, the essence of lateriza a laterized parent material and, especially, if it regen tion is a relative enrichment of the soil in iron and erates from an old lateritic soil, then the carryover of alumina, whereas podzolization is an equally con some characteristics which could have been acquired spicuous relative enrichment of the upper soil horizon— before the inauguration of podzolization is to be the Az—in silica. Such characteristics apparently are expected. mutually exclusive and preclude the simultaneous op The probable conversion of an old lateritic soil into eration of laterization and podzolization. This, how podzolic soil should be analagous to the conversion of a ever, would be true only if all silica were supplied only typical grassland soil, say Chernozem, into a secondary by the decomposition of aluminosilicates. .Mineralogi- Podzol—a process better known as "degradation." cal analyses of the high-in-silica materials of the pod General causes, mechanism and other details of desrada- ' O zolized soil horizons show that these materials consist tion of Chernozems are fairly well known and, usually, chiefly of primary quartz. Hence, a relative increase the term "degradation" is applied specifically to the conversion of this particular soil into Podzolic, although in content of silica in the upper soil horizons may be degradation of many other soils is not uncommon. due largely to an utter destruction of aluminosilicates Nothing is known so far, however, about the degrada and removal of all products of their decomposition in tion of lateritic soils. cluding the combined silica leaving behind a residue of Theoretically, all soils are subject to inevitable primary quartz. Presumably, such an accumulation of changes, whether degradation or aggradation, when residual quartz and hence, the development of the high- their environment changes and lateritic soils are no in-silica horizon accompanies laterization as well as exception. A large group of the so-called Red Podzolic podzolization, provided quartz was present in the parent soils may consist largely of the degraded old lateritic rock. Free silica liberated in the decay of alumino soils. These soils are the commonest regional soils silicates might be added to the residue of primary throughout the Piedmont and Coastal Plain in the quartz and enhance the rate of silicification of the A southeastern States. Many soil scientists do not horizon of a Podzol, while it would play no part in HARDPAN SOILS OF THE COASTAL PLAIN, SOUTHERN MARYLAND 55 silicification of the corresponding horizon, of a later- developing at a former time when climatic conditions itic soil. were different. . . . This suggestion does not corre Therefore, the presence of the high-in-silica—more spond with the true situation." His reasoning in sup correctly high-in-quartz—horizon capping a lateritic port of such a conclusion is not convincing. Elsewhere soil is not necessarily the result of degradation of an in the same work, Marbut states, "it is apparent that old lateritic soil or replacement of the lateritic weather the soil is first converted into a laterite by the lateritic ing by Podzolic. It might be a normal feature of process and after that it is attacked by the podzolization laterization. process" and that podzolic process "can operate . . . The complete profile of a Gray-Brown Podzolic soil, after the lateric process has completed its work." Mar- which is characteristic of the region, is underlain by but further recognized that podzolization operates in laterized material and is quite different. The Gray- the temperate climates and laterization in the tropical. Brown Podzolic soils are characterized by a rather light Hence, if a soil had been first converted into a laterite buff or grayish-brown throughout their profiles. They and later transformed into podzolic, then a marked have a loamy pale or yellowish-gray A horizon which change in climate can be postulated. ranges in thickness from 6 to about 10 inches and rests More recently the genesis of Eed Podzolic soils has on the more clayey yellowish-brown B horizon. This been discussed by Simonson (1950) in a paper dealing horizon is about a foot thick and grades downward into chiefly with formation of clay and its distribution in a light-brown or mottled brownish-gray subsoil. Red the profiles of Red-Yellow and Gray-Brown Podzolic mottling and red of any part of the profile are not soils. Probable evolution of these soils is mentioned. essential features of these soils. Normal Gray-Brown It is stated that "Red-Yellow Podzolic soils have been Podzolic soils are free of these characteristics, which considered more lateritic in character than Gray-Brown occur locally and become conspicuous in the regions in Podzolic soils." Referring elsewhere in this paper to which the Podzolic process apparently is overlapping the occurrence of Gray-Brown Podzolic soils on "the the earlier operation of lateritic weathering. youngest land surfaces in the region of Red-Yellow Mottling and general reddening may begin at the Podzolic soils," Simonson states that "these (gray- depth of only a few inches. In places the B horizon brown) soils appear to be in the early stages of horizon of the soil is distinctly red. In other cases, for ex differentiation. It is believed that with the passing of ample, in the hardpan soils of southeastern Maryland, time they will next assume the characteristics of the mottling begins at the depth of about 4 or 5 feet. Red-Yellow Podzolic soils on young land surfaces and Throughout the region there are scattered isolated ultimately take on the profile features of the Red-Yel low Podzolic soils on older land surfaces." Thus Simon- areas ranging in size from less than an acre to several son conceives of an evolutional process developing from square miles and underlain by the red or red mottled an earlier podzolic stage toward a more advanced sediments. Soils of these areas might be red from the lateritic stage of Red Podzolic soils. This hypothesis top to the bottom or bleached but slightly underneath is opposite that assumed by Marbut and others. Simon- the leafmold. Whether Podzolic process in these areas son visualizes an ultimate defeat of the podzolic process is chronically subdued or the rate of surface erosion is by laterization. high enough to prevent the development of the normal- Dealing with a transitory object that leaves no rec t o-the-region soil—is still an open question. ord of changes in the past, it is difficult to ascertain Marbut 4 pointed out that the A horizon of the red its general trend of evolution. An intermediate soils of the southeastern States is yellow or gray which state might represent a stage of either degradation or appears to be due to podzolization, "a condition that aggradation. Whether the first or the second is to be these soils did not always possess." Marbut stated, inferred, circumstantial evidence is the tentative basis "There is every reason to think . . . that the red color for determination. Recent investigations in Pleisto now dominant in the B horizon originally extended to cene stratigraphy, ecology, and climatology (Peltier, the surface or much nearer to the surface than at the 1949) lend support to the earlier views which claim present time." However, Marbut thought that the laterization antedated the current podzolization that is change of the original red of the surface soil to yellow normal to the region. This theory may explain the or gray could not be due to the change in climatic presence of gray-brown soils, apparently unaffected by conditions and stated that "it has been suggested that their (soils') red color ... is an inheritance from a laterization, on "the youngest land surfaces," as re color that was impressed upon them when they were ported by Simonson, but not on the older, which could have been in existence when laterization still was the * Lecture 16, p. 7, of mimeographed series given in the Graduate School of the U. S. Department of Agriculture in 1928. dominant feature of weathering. 56 GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND Returning to the discussion of hardpan soils of south the lower boundary of the pan. Thus, it appears that ern Maryland we recall the assumption that the earliest the cracks which broke the pan were widest at the top time, during which the supposed laterization of the and gradually narrowed with depth. Coastal Plains deposits could take place, should be in All cracks are closed now but not all are sealed, so the late Pleistocene. Now we may ask if there is any in places adjacent blocks can be separated from one evidence which would permit us to correlate this event another along the planes of cleavage. The fresh facets with the late Pleistocene chronologically. of blocks appear to be covered by glistening dark-gray It has been pointed out that the red mottling of the colloidal films, frequently with the imprints of thin sediments begins below the hardpan. The hardpan rootlets. Commonly, however, the blocks are firmly usually serves as a roof of the zone of red mottling. No fastened to one another and lumps of the hardpan break traces of probable laterization of the parent material across the old cleavages. are preserved in the pan itself. Hence, if this material The hardpan soils of the region (fig. 35) typically had been laterized, then the effects of such a process occupy smooth level areas. They are conspicuously should have been lost before or during the development absent in regions that are hilly, rolling, or strongly of the pan. In any event the cumulative laterization undulating. The locality shown in figure 35 is the same of the deposits should end before the formation of the as shown by Hack (1955, fig. 22). Marbut (1935, pan. Therefore, if the geologic age of the pan can be p. 33) states that ascertained, then the end of a period, during which in the Coastal Plain region . . . certain remnants of an old laterization of the upland deposits took place, would be smooth land surface, antedating the inauguration of the exist established. ing cycle of erosion, are still intact. A soil, Leonardtown silt THE HARDPAN loam, has been developed on these areas under the influence of a ground water surface lying, it seems, at about 3 feet beneath A peculiar characteristic of the hardpan in Beltsville the surface of the ground. An indurated layer has developed soil is the pattern of its cleavage. Throughout its thick at this level. ness, the pan is split into large irregular blocks ranging These hardpan soils are very well developed through from about iy2 to 2 feet in horizontal diameter. Planes out a fairly extensive flat on which the town of Brandy- of cleavage are marked by strong bleaching of the walls wine is situated. Throughout this flat the hardpan of fissures which produces conspicuous streaks on the forms a nearly continuous sheet broken in places by exposures of the hardpan. In vertical planes, these ir sandy ridges or swampy depressions, in which the hard- regular light-colored streaks are roughly parallel, pan is absent. Along the margins of this flat, the land whereas in horizontal planes, they form a striking is dissected and the hardpan is found only in relatively polygonal network. elevated small flat-topped areas which presumably rep Examination of these streaks on horizontal and verti resent remnants of the once larger old flat. Some of cal cuts of the hardpan shows that the pan had been these areas, now detached from the main body of the broken by cracks, similar to mud cracks, which were flat, are very small, hardly more than 200 feet in diame subsequently filled with mud and sealed. The crack ter, and yet their flat tops are underlain by the typical fill consists chiefly of a rather dark clay. Walls of the hardpan. We may conclude that such a pattern of dis cracks are bleached laterally for a distance ranging tribution of the hardpan soils lends support to Marbut's from a few millimeters to more than an inch back from basic assumption that the pan formation has been as the faces of the blocks. Beyond these bleached zones sociated with an old smooth surface, now largely dis there are yellow to orange oxidized zones so that on sected by recent erosion. cuts the bleached streaks are enclosed between rust- Therefore, it can be assumed that the formation of colored bands. the hardpan should be older than the present erosion. The streaks begin abruptly at the upper surface of The other essential horizons of the hardpan soils, A and the hardpan. Nowhere do they extend into the soft B horizons, have according to Marbut (1935, p. 34), layer overlying the pan. The uppermost part of the "the usual characteristics of the podzolic soils of the pan, ranging in thickness from 1 to about 3 inches, region. They are normal A and B horizons." Essen usually shows a coarse platy structure having thin hori tially the same horizons characterize soils occupying zontal layers of similarly bleached silty material. At the present undulating or gently rolling and hilly post- the upper end, just beneath the platy layer, the streaks erosional surface. Hence, these horizons most likely are widest and most conspicuous. They become nar represent the most recent adjustment of the soil to rower, more irregular, and less conspicuous in color present environment. This assumption was the first with depth. Few extend into the C horizon beyond the to suggest that Leonardtown and especially Beltsville base of the hardpan, whereas the majority vanish near soils might possess certain characteristics representing HARDPAN SOILS OF THE COASTAL PLAIN, SOUTHERN MARYLAND 57
I I H m -=.,--n g
m Feet Feet Feet
14-
B FIGURE 35.—Profile along Sunset Trail in Cedarville State Forest, west of the west fork of Butler's Branch, showing the relationship between the characteristic soils of the Brandy wine upland. A., profile showing (I) Beltsville silt loam, (II) Chillum fine sandy loam, (III) Evesboro gravelly sand; g, lower gravel member of Brandywine formation; I, upper loam member of Brandywine forma tion ; h, the hardpan. B, Sections I—III. Arabic numbers show depth in feet. Letters designate genetic horizons : a, leached zone ; bj zone having the highest clay content; li, hardpan ; c, unmodified parent material. at least two different stages of their development. The is questionable, especially in the areas occupied by problem, however, is not as simple as it might appear. Beltsville soils. Parent materials of these soils are well Other soils, which are developed from comparable oxidized to considerable depth without leaving marks but younger sediments, with similar mechanical and of reducing conditions which inevitably would accom lithologic composition, acquire analogous A and B pany a high water table. At present the lejvel of horizons, without the hardpan, even if they are formed water table is fairly high in local depressions scattered in similar topographic position. Absence of the hard- throughout the Brandywine flat. The soils in these pan in these soils suggests that formation of the pans depressions are affected by strong gleization but have took place under conditions which do not now exist. no traces of hardpan. Marbut's (1935, p. 33-34) assumptions that these pans are cemented by silica and were formed in the HYPOTHESES ABOUT THE ORIGIN OF HARDPAN presence of ground water cannot be accepted. The lack An important point to be considered in discussion of of cementation is proved by a very simple experiment the development of hardpan is the relationship between mentioned elsewhere (p. 52). A high content of silica in this material, mentioned by Marbut, is due to a very the indurated pan and the mellow material above the high percentage of quartz, not to amorphous silica. pan. At least three different conditions of such a re Furthermore, were the hardpans formed due to a high lationship are possible: (1) The hardpan could have water table, as suggested by Marbut, then cementation been compacted on the old upland surface and later most likely would be mostly by iron oxides rather than covered by loose material. (2) The hardpan could silica. The probability of a high water table, however, have developed at a certain depth so that the overlying 58 GEOLOGY AND SOILS OF THE BRANDYWINE AREA, MARYLAND soft material is at least as old as the pan itself. (3) The fact that cementation or increase in effectiveness of hy existing hardpan might represent the remnants of drolytic action should take place at a certain depth an originally thicker pan, the upper part of which has rather than on the surface does not require that harden been altered by weathering into the soft material cover ing by compaction at such a shallow depth should follow ing the uiideconsolidated part remaining. The last of the trend. these three hypotheses was suggested by the late Kirk The general character and especially the cleavage of Bryan during his last visit to the area only a short time the hardpan might be used as an argument against this before he passed away. hypothesis. Strong polygonal cracking of the soil at If the hardpan has been covered by loose material a depth of about 2 feet without any effect upon the over after being formed, then there should be a stratigraphic lying layer is hardly possible. It is true that fragmen break at the contact between the pan and the overlying tation of clayey B horizons and development of coarse soft layer. Such a break would indicate discontinuity blocky or prismatic structure in these horizons are of sedimentation for a certain length of time sufficient quite common. Hence, fairly strong cracking of the for the development of the pan, whether in the form soil below the structureless or even laminated upper of an indurated crust or merely compacted and ready horizons is not unusual. It takes place, however, only for hardening at some later date, perhaps after burial in horizons having a high content of clay and overlain under the soft sediments. Certain characteristics of by friable silty and sandy materials. The cracking the pan, especially its polygonal cleavage resembling begins and ends at the boundaries of the clayey hori the pattern of mud cracks and the platy structure of zon. This is not true of Beltsville soil. Content of the material at the contact between the pan and the clay in the B horizon of this soil, just above the pan, loamy layer, strongly suggest the probability of such usually is much higher than in the pan and yet for a break. Some mineralogical data support a prob some unknown reason cracking does not affect this ability. These data, however, are too meager to be de horizon. Moreover, cracking which takes place in the cisive, whereas the data on mechanical composition of clayey B horizons, including vertical cracking, usually the pan and overlying soft material do not show any breaks the soil into much smaller clods or prisms. marks of a break at this level. Strong vertical cracking which breaks the heavy soil The other strong argument against the hypothesis into blocks of comparable size and shape is also fairly of a stratigraphic break at the contact between the pan common but, as a rule, these cracks are open on the and its soft mantle is provided by a conspicuously con surface. stant thickness of the loamy sediments above the pan. The third alternative, for which we are indebted to Wherever the pan is present, it is covered by about 18 the late Kirk Bryan, tends to compromise the contra to 24 inches of mellow material. It seems highly im dictions inherent in other hypotheses. Bryan suggested probable that the recent sediments would be spread that the soil
Page Page Abstract ______45 Induration of hardpans______50 Acknowledgments ______45—46 Inherited soil characteristics- ______48 Acquired soil characteristics__—______48 Introduced soil characteristics- ______48 Age of upland deposits______52 Lateritic crust______—————————————— 50 Amorphous silica__———______49 Laterization ______53, 54, 56 Beltsville, series—______52, 53, 56, 57, 60 Leonardtown series, composition of hardpan—————————— 52, 53, 57 boundary of hardpan______52 division..______52 content of clay in__—______58 silica content of hardpan______————————————— 52 mechanical composition______60 Marbut, C. F., quoted______—— 47,55,56 thickness of hardpan—______52 Mechanical composition of soils______—_——-— 52, 53, 60, 62 loamy layer——————————————______52 Brandywine formation—______48,52,53,59 Origin of hardpan, hypotheses. ______57 basal gravelly member____—______48, 52 Parent material, definition______————————————— 47 thickness—————______53 description of__——_——_———___————————————— 47 upper loamy member______48, 52 effect of weathering on————__———————————————— 46 thickness—______53 formation by atmospheric agencies_————————— —— —— 46 "Buried" and "fossil" soils, difference between-—__——____ 50 part of mantle that serves as_—————————————————— 46 Cementing materials______49 theoretical concept of______—————————————— 46 Chillum series______60 Pattern of cleavage of hardpan————————————————————— 56, 62 content of sand————————______60 red mottling_—_—_—___—_—————————————————— 53 mechanical composition______60 "Pedologic skin"______——————— 48, 49 Classification of hardpans according to cementing materials____ 49 Podzolization————— _..————————————————————————— 54, 55 Comparison of cleavage of Beltsville and Grenada hardpans__ 62 "Polygenetic" soils______——————————————————— 50 Content of titania in soil, significance of______49 Profile of weathering—______————————————— 47 Definition of soil______—______46,48 Recent investigations in paleopedology—— ————— ——————— - 51 Degradation of grassland soil______49 Red mottling___——______—————————————————————— 53, 56 Difference in the effects of laterization and podzolization______54 Red Podzolic soils-————————————————————————————— 54, 55 Differentiation between reversible and irreversible character Red-Yellow Podzolic soils___————————————————————— 55 istics —————————______49 Soil(s) : Distribution of hardpan soils______56 definition ___..______47 Elongated ridges of sand______59 formation-______47 ______48 origin —————————————————————————————————_____ 59 from stratified sediments- .______47 range in elevation——______59 horizon (s) A______47, 48, 52, 57, 58, 60 Environment of soil formation, definition______46 B______47, 48, 51, 52, 55, 56, 57, 58, 59, 60 Evesboro series-—————_—______61 C______..______47, 48, 56, 60 Evolution of lateritic soil______54 range in thickness———..—————————————————————— 47 present landscape, formation of hardpan______61 pedogenic mileposts in—______61 Sunderland formation——————————————————————————— 52 Theoretical concept of parent material, abiotic physicochemical "Function of the environment"______46 process———————————————————————————— 46 Genetic relationship between different horizons- 48 biotic agencies_——————————————————————————— 46 Gradation from hardpan to pan-free soil____. 60 inherent weakness—————————————————————————— 46 Gray-Brown Podzolic soil, color__—______. 55 Upland deposits——————————————————————————————— 52, 53 description—————————————______55 thickness ______55 Wicomico formation______——————————————————— 52 63
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Geology and Soils of the Brandywine Area Maryland
GEOLOGICAL SURVEY PROFESSIONAL PAPER 267
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1955 UNITED STATES DEPARTMENT OF THE INTERIOR Douglas McKay, Secretary
GEOLOGICAL SURVEY W. E. Wrather, Director CONTENTS
[The letters in parentheses preceding the titles are those used to designate the separate chapters] Page (A) Geology of the Brandywine area and origin of the upland of southern Maryland, by John T. Hack.______1 (B) Hardpan soils of the Coastal Plain of southern Maryland, by C. C. Nikiforoff-____--__-_-___--___-__---_----_----_ 45 in