5.1.4 Ouachita Physiographic Province

5.1.4 Ouachita Physiographic Province

The location of the Ouachita Physiographic Province within the United States is shown in Figure 5.1-1. The Ouachita Province occurs in eastern Oklahoma and western Arkansas. It lies south of the Ozark Plateaus, north of the Gulf Coastal Plain, and east of the Central Lowlands.

A very small-scale shaded relief image of the United States in Figure 5.1-2 shows the rugged terrain in the western third of the United States. The Ouachita Physiographic Province, located in eastern Arkansas and southeastern Oklahoma, represents some of the the highest elevations in the central United States. The general boundaries of the Ouachita Province, while not readily apparent everywhere, can be seen by the contrasts of the relatively smooth terrain elsewhere in its surrounds.

The Ouachita Physiographic Province has two physiographic sections: The Ouachita Mountains Section and the Arkansas Valley Section. These are delineated in Figure 5.1.4-1, with 'A' being the Ouachita Mountains Section and 'B' the Arkansas Valley Section. The Boston Mountains Section of the Ozark Plateaus Province is in the area of 'C' and the Mississippi Embayment Section of the Atlantic Coastal Plain is to the south and east at 'D'. The area to the west, 'E', is in the Osage Plains Section of the Central Lowlands Province.

The two sections of the Ouachita Province are roughly coextensive. The Province has a structure much like the Valley and Ridge Physiographic Province with long collinear ridges and intervening valleys. The major difference between the two sections of the Ouachita Province is made on a structural basis. The Arkansas Valley Section is structurally a synclinorium (i.e., a composite synclinal structure of regional extent composed of lesser folds) and the Ouachita Mountains Section is an anticlinorium (i.e., a composite anticlinal structure synclinorium of regional extent composed of lesser folds). The Arkansas Valley Section is 30 to 50 miles wide and the folds are much more open, and the surface generally lower than the Ouachita Mountain section to the south. The Ouachita Mountains are actually part of the same fold belt as the Appalachian range and separated from it by the Mississippi Embayment. The topography of the Ouachita Mountains is solely the result of weathering, mass wasting, and stream erosion. The ridges were formed because of the superior resistance of certain formations, particularly the quartzites and novaculite stratas. Novaculite is a metamorphosed chert native to the Ouachita Mountains.

The regional highlights within the Ouachita Physiographic Province are concerned with the Hot Springs National Park in Arkansas and the Ouachita National Forest in Arkansas.

The Hot Springs National Park, at Hot Springs, Arkansas, is unique in that it was established to preserve hot springs that were believed to have extraordinary healing qualities. While the surface waters in the form of streams have played an important role in shaping the valley and ridge topography of the area, the subsurface waters are of greater significance in the park. They descend along faults and fractures on the southwest flank of the Hot Springs Mountain and surface as springs. Numerous cold and warm springs occur in the area, but the hot springs are the main attraction. There are 47 hot springs in the park and 45 of them are covered to prevent pollution.

The main hot springs are located in downtown Hot Springs where the Park Service maintains the historic bath buildings and a short trail to the two open springs where one can see the natural springs and deposits of calcareous tufa (a porous form of limestone). The remainder of the park occupies the Hot Springs Anticline and other nearby ridges where there are scenic drives and 26 miles of trails.

The Ouachita National forest is the south's oldest national forest. It occupies 1.6 million acres in the rugged Ouachita Mountains. It stretches from the center of Arkansas to southeast Oklahoma. Rivers range from flat water to whitewater. More than 65,000 acres are designated as wilderness in six areas. It has more than 600 miles of trails for hiking, biking and horseback riding. The longest trail, the Ouachita National Trail, is 192 miles in length.

The following descriptions and images are presented to illustrate the primary features of the physiographic province. For a more complete description of the physiography refer to Fenneman's 'Physiography of Eastern United States.'

Along the northern boundary of the Arkansas Valley Section the Boston Mountain uplands merge with the hills of the valley. At places the topographic break is abrupt, mainly where the boundary is against a local south-dipping monocline or where the break in topography follows a fault. The eastern boundary against the Mississippi Embayment is distinct.

All of the rocks in the Arkansas Valley Section are sedimentary of Pennsylvanian age. Resistance to erosion increases slightly to the south as closer folding has induced weak metamorphism. The folding is gentle in the section compared to the Ouachita Mountains, but it is distinct and everywhere present.

The most prominent residual masses in the Valley Section are synclinal mountains or mesas. These features range in height from 1,000 to 2,800 ft and are all isolated and synclinal. The general appearance of the larger mountains is that of steep sided buttes. Some mountains are broad enough to have flat summits, and the slopes of all are more or less distinctly terraced by structural benches. In the center of former synclines the resistant layers may have yielded nearly horizontal capped mesas when the surrounds were eroded away.

Below a certain level there is more consistency in the heights of the ridges and mesas. Cuestas in the section necessarily have the same direction as the strike, thus east-west trends are more prominent than any other.

A large part of the Arkansas Valley is a rolling lowland, 500 to 600 feet high, underlain mainly by shale and lying between sandstone ridges. The streams that controlled the terrain levels in making the valley now wind through an alluvial plain that is sunk 100 ft below the general terrain level near the Arkansas River. The alluvial plain of the Arkansas River is 2 to 6 miles wide. On the tributaries it is much narrower and less deeply entrenched.

Figure 5.1.4-2 is an annotated shaded relief image produced from the Russellville-W, Arkansas, 1:250,000-scale DEM in the Arkansas Valley along the Arkansas River. Though this is a rather low-resolution shaded relief image, the terrain pattern of the Ozark Plateaus at 'A' is easily seen, as is the escarpment boundary with the Arkansas Valley Section. The upturned ridges that conform to the escarpment boundary are pointed to by 'B.' The channel of the Arkansas River is indicated at 'C' while a large relatively flat-topped area is seen at 'D'. Not seen at 'E', but will be apparent in the following illustration, is a low anticlinal structure. At 'F' is one of the higher mountain masses in the Arkansas Valley Section.

Figure 5.1.4-2. Russellville, AR 1:250,000-scale USGS DEM indicating selected terrain features in the Arkansas Valley Section of the Ouachita Mountains Physiographic Province.

Figure 5.1.4-3 is a 3x3 shade relief mosaic of 1:24,000-scale DEMs from an area within Figure 5.1.4-2. Note the Ozark Plateau escarpment at the top of the mosaic and the low upturned ridges that conform to the escarpment boundary. The Arkansas River channel is clearly visible in the Clarksville quadrant, but not well-defined elsewhere. The anticline clearly visible in the Scranton quadrangle was not seen in Figure 5.1.4-2 at 'E.' The east- west ridges along the bottom of Figure 5.1.4-3, and south of the anticline are clearly visible, but only show as one ridge in Figure 5.1.4-2.

Figure 5.1.4-4 shows a large-scale shaded relief image of the Clarksville 1:24,000-scale DEM with 25 foot contours superimposed. Note the low river bars and islands in the Arkansas River at 'A' and the relatively steep river banks. An old north-south channel is seen at 'B' entering the north side of the Arkansas River channel near the left edge of the image. The rounded relief feature at 'C' appears indicative of a river erosional remnant. The steep slopes of the northwest-southeast trending ridges at 'D' have down-slope erosional drainage gullies as seen from the contour crenulations and the shaded relief shadows. Ridges in the vicinity of 'E' are east-west trending indicating the possible location of a fault between the ridges of 'D' and 'E.' the low irregular surface in the vicinity of "F' is undefined.

Figure 5.1.4-5 is a shaded relief image prepared from the Scranton, AR, USGS 1:24,000-scale DEM with superimposed 50-foot contours. This area is seen at smaller scales in Figures 5.1.4-2 and 5.1.4-3. The scale in Figure 5.1.4-5 is in kilometers.

In Figure 5.1.4-5 'A' is the irregular surface of the anticlinal center. The high ridge at 'B,' while not having a level crest, it is smooth and relatively sharp. The large ridge to the south and west has an irregular crest. Both ridges have steep slopes to the northeast and eroded slopes on the southwest. The discontinuous ridges on the north side of the anticline are low with smooth and generally level crest surfaces. The ridge at 'C' is related to the anticline, but this is not apparent from Figure 5.1.4-5 alone, but is easily seen in Figure 5.1.4-3. There is a wide drainage channel at 'D' and 'E' appears to be an old alluvial surface that is much smoother than any other surface in the image other than the drainage channel.

The Ouachita Mountain Section is a lens-shaped area having a width of 60 miles and a length of 225 miles. It is essentially coextensive with a closely compressed and faulted anticlinorium so far as that structure is exposed to view. The Coastal Plain delimits it on the east and south. The entire area consists of mountains, intermountain valleys, or piedmont from which mountains have been carried away by erosion.

The trends of many of the mountain ridges are in the general direction of east-west, but deviations and intersections are frequent. Many of the ridges are short and overlapping, curved, or hooked, following the outcrops of truncated pitching folds. Descriptions of these mountains as being even-topped, or having straight horizontal crests, must not be taken in too strict a sense. At places this is literally true, and when viewed in the light of their long and eventful history the accordance of the highest summit levels throughout the province is noteworthy. In detail, however, there is much irregularity.

The Ouachita Mountain Section of the Ouachita Physiographic Province has three districts or subsections: the Fourche-Kiamichi Belt District (indicated at 'Aa' in Figure 5.1.4-1), the Novaculite Uplift District ('Ab' in Figure 5.1.4-1) and the Athens Piedmont Plateau District ('Ac' in Figure 5.1.4-1).

This belt occupies two thirds of the entire Ouachita Mountain Section, including the highest and most massive mountains. This district extends across the entire length of the Ouachita Mountains Section and north of the other two districts of the section. West of the Oklahoma-Arkansas north-south boundary this district extends to the south and is the only district represented as seen in Figure 5.1.4-1.

About half the area is an uneven valley floor, and is cut by valleys 100 to 200 feet deep and generally old enough to have flood plains. About half the area of the district is an uneven valley floor. The floors of the larger valleys or basins are highest near the center of the area.

Figure 5.1.4-6 is a section of the McAlester, OK, 1:250,000-scale USGS DEM and is annotated with rectangles indicating spatial coverage of illustrations to follow. In rectangle 'A' the dominant features are the long, unbroken, east-west mountain ridge to the north that appears to be the highest feature in the total image because of its long shadow and the system of east-west trending ridges in the southern sector. Between these areas there are two low curvilinear mountain ridges separated by a low irregular ridge running east-west in the center of the 'A' frame. There are also large featureless low areas.

Frame 'B' is dominated by high east-west ridges to the north and lower, generally east-west trending, ridges to in the middle. These ridge areas are separated by a uniform low area. The bottom third of the frame is generally featureless at this scale.

Frame 'C' in Figure 5.1.4-6 has a generally featureless low area in the top quarter with the quarter to the south being a continuation of the mid-frame mountains in frame 'B.' The lower half of frame 'C' appears to have low ridges in the form of a large anticline or syncline in the western two-thirds wile the eastern third is rather featureless, like the southern third of frame 'B.'

Figure 5.1.4-7 is a 3x3 mosaic of 1:24,000-scale USGS DEMs in the Ouachita Mountains, AR and centered on the Peanut Mountain DEM. This mosaic occupies the area indicated by 'A' in Figure 5.1.4-6. This mosaic indicates the terrain is composed of an east-west valley containing several low synclines and this valley is bordered on the north and south by mountain chains. The mountains and the higher elevations in the valley are the only features that were apparent in the smaller 1:250,000-scale Figure 5.1.4-6.

Of the multitude of visible features of Figure 5.1.4-7, a few standout. For example the single limb of the anticline fold at 'A' overlaps the multi-strata fold at 'B' and the fold at 'A' is higher in elevation than that at 'B' because it is apparent in Figure 5.1.4-6. The anticline at 'C' is a part of a multi-strata multi-limbed Z-fold. At 'D' there is a single strata anticline limb facing the same direction as that at 'A.' The high ridge at 'E' appears to be a portion of an anticlinal limb. At 'F' a series of low parallel ridges is seen at the base of much higher mountains across the width of this illustration. The visible strata of these low ridges is an example of differential erosion with the strata having more resistant to erosion being higher than that of the weaker adjacent stratums. At 'G' another area of parallel ridges are seen. And the ridges across the bottom of the illustration are probably parts of old folds. The ridge at 'H' is seen to extend much further west in Figure 5.1.4-6.

Figure 5.1.4-8 presents the shaded relief and filled contour images with 20 meter contours for Peanut Mountain and Boles, AR 1:24,000-scale USGS DEMs. From the filled contour image it is seen that ridges 'A,' 'B,' and 'C' have the highest elevations in the images, while 'E,' 'F,' and 'G' are low areas. Also note that the ridge at 'C' divides the local drainage. The ridge at 'A' is believed to be a near-vertical erosion resistant stratum along the ridge line, because of the relatively easily eroded strata to either side. The slopes on either side of the ridge 'A' are similar. On the ridge at 'D' note that the degree and length of slopes to either side of the ridge very different.

Figure 5.1.4-8. Shaded relief and filled contour (20 meter interval) images for Peanut Mountain and Boiles, AR. USGS 1,2400 USGS DEM data.

Figure 5.1.4-9 is a merged 3x3 1:24,000-scale USGS DEMs centered on the Ludlow, OK quadrangle. While this illustration within the Fourche-Kiamichi Belt District has many of the terrain features in Figure 5.1.4-7, the features in Figure 5.1.4-9 have more relative elevation differences between the high elevations and adjacent lows and therefore are better accented topographically in shaded relief images.

There appear to be six east-west terrain belts in the illustration. The syncline at 'A' is the most obvious feature in the illustration. The center of the syncline is at 'A' and the syncline be comes obscure to the east after the fifth or sixth high ridge, although there are evidences of low ridges to the east. At 'B' the low east-west ridges of an undefined syncline to the south are seen and the transition between areas of 'A' and 'B' is so distinct so that it could only be attributed to a fault. The low trough at 'C' between the higher areas of 'A' and 'D' appears to be fault defined in the north boundary in it's west portion but becomes undefined in the east. Area 'D' is comprised of high west-east trending ridges that are broken by wind or water gaps. The northern-most ridge is continuous and appears to be level crested. The north slope of this ridge is severely eroded with gullies in piedmont or alluvial fans at the lower levels transitioning into the low valley at 'E.' This valley is extensive and the boundary between it and the mountains in area 'F' is well defined.

Novaculite District

The Novaculite District is outlined as 'Ab' in Figure 5.1.4-1. The upper portion of Figure 5.1.4-10 illustrates the Novaculite ridges and large basins in the Novaculite District of the Ouachita Mountains Physiographic Section. On or near the axis of the Ouachita anticlinorium, where the uplift was greatest, Carboniferous rocks have been entirely eroded away. Rocks that are exposed have a distinctive character in the mountains and basins. The belt has a width of only 25 miles and a length of about 100 miles. This is the area of Arkansas novaculite. It outcrops in an almost continuous band on the borders of the district, except against the Coastal Plain. The resistant novaculite rock strata come to the surface in steeply dipping strata. Most of the summits are sharp and where unbroken may appear even or generally level. Where there are weaker rocks, valleys and basins are formed. Some of the basins are very large, as seen in the upper portion of Figure 5.1.4-10.

The Athens Piedmont Plateau District of the Ouachita Mountains Physiographic Section is outlined as area 'Ac' in Figure 5.1.4-1. The lower portion of Figure 5.1.4-10 illustrates the subdued nature of the terrain in this area of the Ouachita Physiographic Province. The Athens Piedmont Plateau lies at the southeast border of the Ouachita Mountain Section and adjacent to the Coastal Plain on the south. This district has no mountains. The dominant feature of the topography is a series of east-west strike ridges and intervening valleys. The ridges increase in height from south to north and in cases reach a height of 300 feet above the valleys. Generally the horizon is nearly flat. Folding and faulting are evidenced in the illustration, but the terrain features are characterized by low relief of a peneplained area.