HOT SPRINGS Analyses of the Waters of The Hot Springs of Arkansas Geological Sketch of Hot Springs, Arkansas

GEOLOGICAL SKETCH OF THE HOT SPRINGS DISTRICT, ARKANSAS.

As already noted, the hot-spring area is characterized by a deposit of calcareous tufa, or travertine, formed by the hot waters, and covering not only a large part of the mountain slope about the existing hot springs, but also extending westward to the Happy Hollow Ravine and occurring far above any existing springs in the slope above the band stand. Tufa deposits are common about both hot and cold water springs whose waters carry alkaline-earth bicarbonates in solution. Such materials are precipitated when the carbon dioxide of the waters escapes upon exposure of the water to the atmosphere. At the Arkansas Hot Springs only moderate quantities of the alkaline earths are in solution in the waters, yet they are sufficient to coat the hot-water pipes and to fill wooden troughs used to conduct the waters. In the Cave Spring and at the Dripping Spring the tufa may be seen now forming. It is therefore not certain that the waters which formed the great tufa deposits of the place were any richer in calcium than those of to-day. This tufa is seen in its natural state at many places about the springs, but is particularly well seen a the Cave Spring back of the Arlington Hotel. It is of a gray color, and porous texture on the surface, but when quarried is pure white, compact, and crystalline.

This tufa consists almost wholly of carbonate of lime, carrying very small and varying amounts of manganese (oxide) and iron oxide. The manganese is frequently prominent as a black powder, or occurs in blackish layers through the rock. The analysis made for Owen in 1859 of the material deposited in the pipe accords so exactly with that of the deposit now forming that it is reproduced.

Analysis of hot-spring tufa formed in pipes carrying hot water to bath houses.

In the Cave Spring the freshly deposited tufa is tinted orange by the algae that live in hot water, and green by the species that flourish at slightly lower temperatures. These colors are purely vegetable and disappear if the deposit be heated.

This tufa deposit covers an area of approximately 20 acres, and varies from a few inches to 6 or 8 feet in thickness. Its occurrence shows that some of the springs formerly flowed to the west, and that the waters covered a larger area than at present.

The broad area covered by the tufa does not mean that the hot waters covered this entire area at any one time, for the algous growth described as filing the hot-water streams causes a filling up of the channel and a diversion of the water to a different place. In two instances the waters built up mounds about the springs. The most noticeable of these is that of the Cave Spring, which has been artificially breached in the development of a larger water supply from the spring. Above the music pavilion another area of tufa indicates the former presence of springs at a level higher than any now existing.

The thickness of the tufa deposit is likely to be overestimated, as it covers steep slopes and even cliff faces.

The earliest description of the place tells of its forming overhanging masses alongside the creek, whose flood waters swept away its support. The natural exposures of conglomerate and sandstone outcropping near the pavilion show that the tufa is there underlain by hard rock. Farther west, however, the tufa overlies soft, shaly rocks, which have been digested by the hot waters and vapors for so long a time that the material is as soft as ashes, and in the development of new water supplies near Spring No. 1 a pipe was driven 38 feet down into this material. Immediately beneath the tufa there is a breccia of novaculite sandstone or shale fragments cemented by iron oxide, manganese oxide and carbonate of lime. This is seen under the tufa at the Cave Spring and at the Dripping Spring. It merely represents the old hillside débris cemented by the hot-water deposit and material deposited later beneath the tufa mantle.

Vegetation of tufa area.—The tufa area is described by all earlier writers as being distinguished from the adjacent slope by its peculiar vegetation. In the improvement of the reservation this distinction has been largely obliterated, as flowers and shrubs have been freely planted. The tufa cliffs and rougher exposures show, however, the limestone-loving ferns Cheilanthes alabamensis Kunze and Adiantum capillus-veneris L., which occur nowhere else in this region. Owen mentions these ferns especially, besides numerous peculiar mosses and algae, and the stonecrop, sage, lobelia, and senna as characteristic of the tufa area.