The oldest facet of the environment of Platt National Park, and that which logically forms the foundation for everything that follows, is the physical landscape. It is the inanimate, but certainly not unchanging, stage for all biological and cultural activities. Included in its evolution is the early shaping of the earth's gross features of major geologic proportions, the development of local surface features and their attendant soils, and the origin and character of the various water forms which have helped shape the park.

ANCIENT SHAPING OF THE PLATT-ARBUCKLE AREA

The Precambrian

The sequence of events which has resulted in the Platt-Arbuckle landscape began some twelve to thirteen hundred million years ago during the period of geologic time called the Precambrian era. Very little is known about the geologic activity of that period, but it is certain that very large areas of today's continental land masses had not yet emerged from the vast prehistoric seas which covered even more of the earth's surface than the 77 per cent covered today. The only life forms that may have existed were the most simple of marine organisms, such as single-celled algae. Beneath the almost lifeless seas, however, a great deal of geologic activity was taking place. The hot core of the earth was forcing molten rock to flow from many fissures and rifts in the ocean floor. As the countless cubic miles of molten material flowed across the sea floor, it solidified into stable granite rock and formed the crustlike foundation upon which the continent now rests.

Figure 5. Formation of the earth's crust. The pressure in the earth's hot core forced molten rock, or magma, to the surface. Deep flows of magma cooled slowly to form granite.

The sequence of geologic evolution during the Precambrian period is sketchy, but it is likely that relatively little earth-building activity occurred for some five hundred million years after the continental foundation was formed. During those millions of years the changes in the terrestrial surface nevertheless continued, but at a slower pace. The erosive power of the sea relentlessly wore down the submarine irregularities which had been formed by earlier volcanic activity. Rock particles removed from the "uplands" were eventually deposited across the sea floor to form a rather uniform surface. This material, called sediment, was primarily sandy in nature and was approximately a mile thick over the Platt area and increased to as much as three miles in thickness a short distance south. As these deposits increased in depth, the great weight of the overbearing sediments and water caused the sand to become compacted and "cemented" into sandstone through a process called lithification. Eventually the sheer weight of the sandstone deposits began to deform the earth's crust, and a significant sag or downward warp began to appear under the thick sediments south of Platt.

Figure 6. Early sedimentation and subsidence.

The Cambrian

At the onset of the Cambrian period, approximately five hundred million years ago, the relatively static condition of the crustal foundation in southern Oklahoma was violently ended. Massive upheavals began to occur in the basal rocks, and the crustal sag south of Platt intensified. The degree of sag increased greatly for millions of years, while yet another of these upheavals to the north of Platt began to lift the Platt area sharply. The result of these opposing forces was a huge fault, or fracture, in the earth's crust. The Platt area was atop a massive block of the earth's crust, several hundred square miles in area, which was thrust upward far above the surface of the prehistoric sea. The sandstone cap of this elevated block, which had been part of the sea floor sediment, was subject to all the erosive agents of the sea, atmosphere, and gravity. The sand and sandstone fragments which eroded from the uplifted Platt block also made their way into the southern down-warp, which began to take on the characteristics of a major geologic basin. This basin extended from the Platt area to near the present-day border of Texas and in modern geologic terminology has come to be known as the Ardmore Basin. During the following one hundred million years the Ardmore Basin became still more pronounced as it filled with new igneous material, which flowed from hot magma chambers deep within the earth. This was rhyolite, a heavy granitelike rock, whose weight placed still greater stress on the warped granite crust and over lying sandstone sedimentary rock.

Figure 7. Faulting in the Platt area thrust a portion of land upward above the surface of the sea.

The Ordovician

Although the massive flows of Cambrian rhyolite did not intrude into the Platt area, later developments radically changed the local geology. The Ordovician period of the Paleozoic era saw the continued existence of a broad sea which covered much of the United States. An area from Texas to Wisconsin and from Missouri westward to Colorado was inundated, including the eroded fault block area of Platt that had earlier been lifted above sea level. For the next two hundred million years marine sediments again accumulated to great depths over the sea floor. These sediments were much different from the earlier sands. By that time marine life had made vast developments, and mollusks, fishes, invertebrates, and a host of other organisms were abundant. The bodies of these organisms were rich in calcium, and as countless generations of them died, their remains settled to the sea floor in thick, limy blankets. As time passed, lithification occurred as the mixture of organic debris and nonorganic sediments became cemented together by chemical action and pressure to form limestone. Intermixed with the layers of organic sediments were many layers of predominantly fine clays and silts deposited by sea currents. They were subjected to similar metamorphic activity and evolved into shale, the fine grained rock one sees in thin layers in the face of Bromide Hill. By the end of the Devonian period, some three hundred million years ago, the granite crust of the earth under Platt was being depressed by deposits of limestone and shale two miles thick.

Figure 8. Inundation of the Platt National Park area by an Ordovician sea.

The Permian

The final and most important episode in the structural formation of Platt took place between 250 and 300 million years ago. During that period both the southern edge of the Ardmore Basin and the Platt fault block were again forced upward in a sudden movement of the earth's crust. Because the upward forces were so great, the layers of sandstone, shale, limestone, and rhyolite that filled the basin were squeezed, broken, and folded in accordion fashion. The largest of the upward convex folds, called an anticline, was thrust some seven thousand feet above the sea immediately south of the Platt area and formed the Arbuckle Mountains. During those millions of years, and continuing through the Permian period, much of Oklahoma and Texas, including the Ardmore Basin and the Platt area north of the Arbuckle Mountains, remained covered by what are called Permian seas. Another deep layer of sediments was deposited on the sea floor during the Permian, but they were mostly bright-red sandstone and shales. These Permian "redbeds" form the bright-red soils one sees today while traveling through central Oklahoma. With the exception of minor local buckling and faulting, the massive earth movement which formed the Arbuckle Mountains was the last to affect the region. Subsequent changes in the geology of the area were largely the result of near-surface activity, such as erosive degradation and sedimentary aggradation, as the Permian seas retreated to the present Gulf of Mexico.

Figure 9. Anticlinal formation of the Arbuckle Mountains which occurred some 250 to 300 million years ago.

After being thrust so high into the elements, the Arbuckle Mountains, like mountains everywhere, began the slow but inevitable cycle of erosion which will someday return them to sea level. The agents of landform erosion are the elements of weather, such as heat, ice, rainfall, and wind, which mechanically attack the rock structure. There is also a subtle chemical action which causes many of the minerals that bind rocks together to deteriorate. Lastly there is the action of gravity in causing all of the decomposed or fragmented rock to be carried to a lower level, grain by grain, chip by chip. In the Arbuckles much of the erosional debris was carried by rain waters down the northern slopes of the mountains. There, logically, the heavier rock fragments came to rest first, followed by increasingly smaller particles the farther the debris flowed.

An exposure of strata of the Vanoss formation near Rock Creek Campground. Note the dip of the strata to the south (right) and the different rates of erosion. Sandstone and conglomerate both form angular, blocky outcrops, while the shale is sloped and marked by rill erosion. The outcrop near the top is conglomerate, the two outcrops below it are sandstone.

Today's Surface Geology

The surface rocks seen in Platt National Park today are remnants of the Arbuckle erosion. The surface rock which extends across the park and the surrounding local area is known as the Vanoss formation. Included in it are shales, sandstones, and conglomerates that formed in successive horizontal layers, or strata, depending upon the periodically changing rates of erosion through some two hundred million years.

The foot trail which winds up the northern face of Bromide Hill is an excellent place to see all these strata and to traverse some two hundred million years of sedimentary deposition. Near the bottom of the hill are some alternating strata of fine-grained sandstones and shales which can be easily broken off or scratched with another rock or a hard stick. Farther up the slope and capping the hill is a very thick layer of conglomerate. It looks like very coarse concrete and contains sands, gravels, and cobbles of various size, all cemented together by natural processes. It is a resistant rock that forms many ridges and outcrops in the Platt-Arbuckle area. The large size of many of the constituent materials in the conglomerate indicates that when that particular stratum was laid down the source of the rock was either very near the park site or else the transporting force was very strong.

Figure 10. The last two hundred million years has seen periodically changing rates of erosion shape the Arbuckle Mountains.

DEVELOPMENT OF SURFACE FEATURES AND SOILS

The process which shaped the earth's crust and created the major geologic features of the Oklahoma landscape slowed down some 250 million years ago, but change is ceaseless. If there are any laws or regularities which the earth adheres to as it journeys through space and time, one of those laws is the inevitable and unending change which affects all things, not the least of which are the physical features of the earth's crust. The processes which act upon the surface landscape are called gradational processes, because they mechanically shape, or grade, major geological structures into the landforms one sees today. The over-all process of change is termed geomorphism, and has five geomorphic agents which do the shaping work: streams, underground water, waves and currents, glaciers, and wind. Of these five agents, streams have played the most significant role in shaping the post-Permian landscape of the area around Platt National Park.

Thinly bedded strata of shale, sandstone, and conglomerate of the Vanoss formation on the northern face of Bromide Hill. The angularly fractured rock on which this visitor's hand is resting, and that at his hip level, is sandstone; shale is sandwiched between the sandstone and appears again at knee level. The conglomerate which caps the hill begins just above the man's head. Photo by Chester Weems.

The Formation of Rock Creek

Although there is no beginning or end to a landscape, it is likely that the present surface features of the local area, including the park and the city of Sulphur, are the result of the last few thousand years of the Quaternary geologic period. Until that time a stratum of conglomerate rock one hundred or more feet thick capped the series of sandstones and shales which were earlier described in Bromide Hill. The area was originally higher than much of the surrounding terrain by virtue of being close to the Arbuckle Mountain remnants. Additionally, conglomerate is a rock very resistant to nature's erosive forces, and it was more slowly worn down than the rocks of some nearby areas. Nevertheless, the rains which fell during those thousands of years could not soak into the rock but had to find or make a pathway which would eventually lead to sea level. The persistent runoff from a large area north and east of the park site formed a stream which meandered across the conglomerate landscape, continually seeking a lower level. For some unknown reason, perhaps because of a fault or a weak area in the conglomerate, the stream made a niche which it continued to expand. That stream is today called Rock Creek, and its niche is now a substantial valley which extends for many miles and is over 150 feet deep. Much of Rock Creek's valley has a fairly common appearance, but there are some landforms which catch one's attention and require an explanation. Bromide Hill is the most notable example, but the reasons for its formation can be applied to many similar hills or bluffs in the vicinity.

When the major earth movements stopped in the Permian period, the large area of conglomerate and its underlying strata of sedimentary rocks were not left in a level position but were slightly tilted in a south-southwesterly direction. It appears that the high ground around Sulphur was the northernmost extension of the conglomerate cap. From the city northward for some distance the Vanoss formation shale was exposed to the surface. This set the stage for a process known as differential erosion, which is responsible for a great portion of the earth's landforms.

Differential erosion occurs where two or more earth materials of different resistance to erosion are in contact. The Vanoss conglomerate is more resistant to erosion than underlying strata for a number of reasons. Most importantly, it is an extremely porous rock which catches rainfall and lets it permeate downward through the rock rather than forcing the full volume of precipitation to flow across its surface and cause mechanical erosion. The conglomerate is also made up of relatively hard rocks which are often larger than baseballs and are tightly cemented together. As a result, it is very difficult for wind and water to break down and transport the rock materials. On the other hand, the sandstones and shales are made up of sand and fine, silt-sized particles which were formed by only light cementation and pressure compaction and were thus vulnerable to quick destruction. The shale which predominates is also a very compact, nearly impervious material which does not readily pass water. When water falls upon the shale formation or flows across it in a stream, virtually all of it races across the surface and creates a drainage system of rills, gulleys, and eventually large valleys. The fine grains of the rock surface are then easily dislodged and carried away by the running water.

A large block of conglomerate which long ago tumbled into Rock Creek after being undercut by the stream.

The Carving of Bromide Hill

As Rock Creek sought the easiest path to sea level, it established its course across the landscape very near the northern edge of the conglomerate cap. Through the years its ceaseless erosive action began to erode vertically and create a V-shaped valley, and the stream channel also started lateral erosion. Such lateral erosion of stream banks is normal, but in this case it progressed in one predominant direction—the south—and into the present park area. The reason for this southward migration of the channel is the force of gravity "pulling" the water downward along the easily eroded slope, or dipping plane, of the shale strata. Thus Rock Creek is incising the Bromide Hill mass much as a road grader cuts at the side of a large earth hank along a country roadway. The result of this migration is a typical asymmetrical valley with a shallow gradient on the north next to Sulphur and an abrupt rise, known as a bluff, on the southern bank of the stream. The most obvious example of the undercutting action of Rock Creek is near Bromide Spring.

As such undercutting proceeds, the overhanging rock will eventually become too heavy for existing support, and it will break off and tumble into the stream channel. Room-sized remnants of one such rockfall are visible in the creek about one hundred yards east of the entrance to Rock Creek campground. Along other parts of the hill the undercutting is less spectacular because gravity works through mass wasting to continuously transport smaller portions of rock and soil down the face of the hill, where they are removed by the streams and deposited elsewhere. If this erosive action of Rock Creek continues without a major disruption, Bromide Hill will someday he totally removed.

Travertine Creek

The eastern end of the park encloses the valley of Travertine Creek. It is a more common symmetrical valley which for many thousands of years was shaped solely by periodic, or ephemeral, runoff, which resulted from local rain storms. The unusual point of this valley is not easily recognized by eye although it is discernible from maps, aerial photographs, or detailed walks through the area. This point of interest is the sudden broadening and flattening of the valley floor which occurs west of Buffalo and Antelope springs. The springs, of course, are responsible for the sudden increase in size because of their equally sudden appearance and erosive capability. East of the springs the valley remains V-shaped because it is still a channel for only storm drainage, and even some of that is trapped behind the dams of ranch ponds outside the park's boundaries.

An exposure of the deep, dark alluvial soil typical of the park's lowland. Its high clay content allows it to tolerate such vertical cuts. Note the surface litter and tree roots which add organic matter to the soil. Photo by Chester Weems.

Soil Development

Another facet of the park's landscape is the mantle of soils and alluvium, or regolith, which blanket the geologic landforms. The development of this soil blanket is significantly affected by all the factors of environment: vegetation; the landform where the soil is forming; the rock, or "parent material," which furnishes the raw materials; the climatic conditions; and the period of time over which these factors work. Once a visitor is aware of these soil-forming factors and their effects on the regolith, soils become one of the clearest indications of changing environment. As one walks through or visits different areas of the park, one should match the soil to his other surroundings.

The lowland areas of the park along the streams have developed the deepest soils and those which one would normally think of as fertile and valuable for agriculture. They are generally clay loams that are generally seven to ten feet deep. For the most part these soils were formed of sediments which were moved from one area to another by stream action. Such parent materials are called alluvium, and they can be either rich in nutrients, which is the case in the park, or they can be generally "worthless," as they are along sandy streams. Since most of the lowland alluvium was carried from the exposed shale strata north of the park, the soils tend to be rich in silt, clay, and organic matter. These soils are also like the parent shale in that they absorb a great deal of water but hold it in the soil structure instead of allowing it to drain through, as do sandy soils. Although new by the scales of geologic time, these are fairly old and stable soils in human years. Consequently, they have also had the opportunity to develop specific site characteristics.

In general one can expect the deepest, darkest soils on the flat lowland. There may be some sand mixed into the upper few inches from periodic flooding, but it will become a heavy reddish-brown clay a few inches or so beneath the surface. Because the lowland soils are under a moderately heavy growth of vegetation they are shaded and tend to remain moist during all seasons. The forest floor litter, which is composed of decaying foliage and wood, produces mild organic acids which permeate the upper horizon, or layer, of the soil and characterizes nearly all forest soils. Many kinds of plant roots penetrate these soils to several inches or feet and some of the larger lowland trees, such as sycamore, will have roots extending to several yards in depth. This soil is also rich in micro-flora and fauna.

A natural levee which was deposited by flood waters in 1970 can be seen in the right foreground of this photograph of Rock Creek's bank.

Within a few tens of yards of Rock Creek's channel one can often find a great deal of light-colored sand either in the channel or atop the banks. This sand came primarily from the decomposition of local sandstones and is too heavy to he carried very far by normal or flood-stage stream water. Whenever there is high water, it therefore comes to rest near the stream bank, while finer and lighter silts are carried away. Such ridgelike sand accumulations, whether obvious or indistinct, are known as natural levees. The last such deposits in Platt National Park are the result of serious flooding in the autumn of 1970.

The farther one proceeds from a stream bottom, the shallower and drier the soils will become. In the first place, it is harder for the streams to make distant deposits of alluvium; second, the soils which do form on the site are subject to greater erosion because of the increasing slope. Soils which form in place from weathered parent material are often created no faster than they are carried downslope, and so they maintain a relatively constant depth which decreases with the steepness of slope. Slopes also encourage drainage, which consequently reduces soil moisture.

An interesting exception to this general rule is found along the foot trail up the face of Bromide Hill. On some of those fifty-degree slopes the shale strata have weathered into fairly deep soils. These soils constantly slip and wash down the slope, but the dense vegetation cover and its network of roots protect and anchor the soil to an amazing degree. This slope also stays unusually moist because it is shaded and sheltered from the afternoon sun and prevailing summer winds.

Dark soil, several inches in depth, is retained on the steep and wooded slopes of Bromide Hill by a dense network of tree and grass roots shown above the pick handle.

The soils of the park's uplands are universally formed from the coarse Vanoss conglomerate. None of these soils have been deposited by water but have formed where they are as residual soils. Most of the upland soils on all sides of the park are therefore very shallow soils of gray-brown color that are mixed with a great deal of gravel and cobble. The general term applied to this category of soil is "rough-stony." Because the conglomerate weathers into soil-sized particles so slowly, these soils are commonly only two to eight inches deep. Even that depth is largely filled with rocks, and without a substantial cover of grass or shrubs these soils either blow or wash away as quickly as they form. As circumstance would have it, the rapid drainage and the upland exposure of the conglomerate make it frequently too dry for such vegetative protection. This combination of soil-forming factors results in the alkali or base rich soils which characterize semi-arid grasslands.

Between these two markedly different soils there is a transitional spectrum which can best be observed along one of the foot trails that traverse the valley side. Perhaps the best trail for such a study begins just east of the Travertine Nature Center on the south side of Travertine Creek.

This roadcut along the Perimeter Drive exposes part of the conglomerate stratum and shows the formation of a thin and stony layer of soil. Photo by Chester Weems.

THE PARK'S WATERS

The waters of Platt National Park have always been of interest to the visitor not only because of their refreshing qualities but also because of the mystery which springs have always held for many people. It is hoped that the facts of their origin and characteristics will be equally interesting.

At least thirty springs of both fresh and mineral water are found within the park's boundaries. There are also several more "seeps" where water oozes from the rock or soil, several of which can be seen along various foot trails. Most of the springs are found in one of two areas. The fresh-water springs are centered in the extreme east end of the park, and the mineral springs are clustered around Bromide Hill. Some of the springs produce impressive quantities of water, but none approach the size of many in Arkansas and Missouri, which produce tens of millions of gallons a day.

The origins and nature of the park's springs are classic textbook models. Virtually all the soils and rocks which mantle the earth's crust contain varying amounts of moisture known as ground water. The ability of any portion of these upper layers of the earth to contain ground water, and the amount they hold, depends on a few elementary physical conditions. First, since all ground water was originally precipitation, the greater the rainfall in a given area the greater the potential ground water source. Likewise, the more porous the soil or rock the greater its capacity for storing or transporting ground water, much like a sponge or a wick. Lastly, the force of gravity moves the water from one point to another.

As we have said, Platt National Park is situated in an area of ancient sedimentary rocks which were tilted and folded several hundred million years ago. The high ground in the park is capped with a permeable limestone conglomerate, but under the conglomerate are alternating strata of shale and sandstone, which are the rock elements essential to the existence of Platt's springs. The park is situated on the lower northern slope of one of the massive rock down-folds, or synclines, which was formed in the Permian period. Since that time erosional processes have degraded and truncated the syncline to expose a broad cross-section of the various strata a few miles east and south of the park.

Figure 11. The origin of the park's springs.

The vertical sequence of the exposed strata happens to be favorable for the formation of springs because the permeable conglomerate and the porous sandstone are closest to the surface, where they can absorb and transport a maximum of the precipitation or surface water they receive. Such conductive rock and soil strata are known as aquifers. Directly under the sandstone is a massive layer of impermeable shale which acts as an aquiclude and blocks further vertical infiltration of the ground water. Without this impervious stratum the water might pass into deeper strata and be unavailable as a spring source. Gravity forces the ground water vertically downward until it reaches the shale aquiclude. From there the water moves down the dip or slope of the shale until it finds a surface exit or moves into the deeper rock past the park.

In Platt National Park some of the water was provided with a surface exit by the action of surface erosion. Rock Creek and the ephemeral waters that flowed over the eastern end of the park area carved their channels progressively deeper into the sandwiched Vanoss formation until both streams cut into the water-filled sandstone strata. The resulting springs now add their waters to both channels.

Fresh-Water Springs

Antelope and Buffalo springs are the largest of the springs in the park. They and the other fresh-water springs have their origin in a catchment area several miles east of the park and flow through sandstone of the Pontotoc group. Since water from these springs has no distinctive odor or taste, it is called "fresh" water, but in fact it contains a great deal of calcium carbonate in solution. The Pontotoc sandstone is apparently high in calcareous content, and this is dissolved as the water in filtrates toward the park. It is this heavy load of calcium carbonate which precipitates to form the travertine rock, or calcareous tufa, mentioned in Chapter 1. Because of the depth of the water-bearing sandstone beneath the surface and the many years the water rests in the rock, the springs have a year-round water temperature of sixty-six degrees.

The major fresh-water springs have either run dry or been severely curtailed in flow several times in the last century, and always after a long drought. Once rains come again, the aquifer is recharged, and the volume returns to normal. A more serious threat to the springs recently has been heavy water-well pumping in the area east of the park. The wells remove water from the sandstone aquifer faster than it is replenished by rainfall. It may be a matter of only a few years before man disrupts the springs and, in turn, the ecology of this valley and others like it elsewhere.

Mineral Springs

All the park's mineral springs have a great number of compounds in solution, but it is the bromine and hydrogen sulphide compounds which are so predominant that they lend their names to most springs. The water for these springs comes from a catchment area approximately two hundred feet higher, twelve miles southeast of the park. It is an area of exposed sandstone of the Simpson group. This particular sandstone stratum formed adjacent to a stratum of oil-bearing sands in ancient geologic times. When the Permian folding took place, the two beds were squeezed together and compressed so that the petroleum was forced into the porous spaces of the Simpson sandstone to form natural asphalt. Until a few years ago that asphalt was commercially mined, and some of the abandoned workings are still visible west of Arbuckle Dam. It is from the petroleum compounds in the Simpson sandstone that the slowly permeating water absorbs its many minerals. Mineral waters are not commercially pumped around Platt National Park, and, although some of the mineral waters flow intermittently, park officials say that there is no foreseeable danger to their continued existence.