The Plains of Passage - Jean M. Auel [245]
But the cave had not been used recently. All the signs were old, including the charcoal in a shallow pit from the fire of some other human visitor. Ayla and Jondalar went into the cave for some distance, but it seemed to go on forever, and beyond the dry front opening there were no signs of use. Stone columns, seeming to grow up from the floor or down from the ceiling and sometimes meeting in the middle, were the only inhabitants of the cool damp interior.
When they came to a bend, they thought they heard running water from deep within, and they decided to turn back. They knew the makeshift torch would not last long, and neither of them wanted to go beyond sight of the fading light from the entrance. They walked back touching the limestone walls and were glad to see the drab gold of dry grass and brilliant golden light outlining clouds in the west.
As they rode deeper into the highlands north of the great central plain, Ayla and Jondalar noticed more changes. The terrain was becoming pocked with caves, caverns, and sinkholes that ranged from bowl-shaped dips covered with grass, to inaccessible drop-offs that fell to great depths. It was a peculiar landscape that made them feel vaguely uneasy. While surface streams and lakes were rare, they sometimes heard the eerie sound of rivers running underground.
Unknown creatures of warm ancient seas were the cause of that strange and unpredictable land. Over untold millennia, the seafloors grew thick with their settling shells and skeletons. After even longer eons, the sediment of calcium hardened, was lifted high by conflicting movements of the earth, and became rocks of calcium carbonate, limestone. Underlying great stretches of land, most of the earth’s caves were formed out of limestone because, given the right conditions, the hard sedimentary rock will dissolve.
In pure water, it is hardly soluble at all, but water that is even slightly acid attacks limestone. During warmer seasons and when climates were humid, circulating ground water, bearing carbonic acid from plants and charged with carbon dioxide, dissolved vast quantities of the carbonate rock.
Flowing along flat bedding planes and down minute cracks at the vertical joints in the thick layers of the calcareous stone, the ground water gradually widened and deepened the fissures. It carved jagged pavements and intricate grooves as it carried the dissolved limestone away, to escape in seepages and springs. Forced to lower levels by gravity, the acidic water enlarged underground cracks into caves. Caves became caverns and stream channels, with narrow vertical shafts opening into them, and eventually joined with others to become entire subterranean water systems.
The dissolving rock below the ground had a profound effect on the land above it, and the landscape, called karst, displayed unusual and distinctive features. As caves became larger, and their tops extended closer to the surface, they collapsed, creating steep-walled sinkholes. Occasional remnants of the cavern roofs left natural bridges. Streams and rivers running along the surface would suddenly disappear into the sinkholes and flow underground, sometimes leaving valleys that had been formed earlier by rivers, high and dry.
Water was becoming harder to find. Running water quickly sank into cavities and potholes in the rocks. Even after a heavy rainfall, the water disappeared almost instantly, with no rivulets or streams running across the ground. Once the travelers had to go to a small pool at the bottom of a sinkhole for the precious fluid. Another time, water suddenly appeared in a large spring, flowed across the surface for a while, then disappeared underground again.
The ground was barren and rocky, with thin surface soil that exposed underlying rock. Animal life was scarce as well. Except for some mouflon, with their tightly curled wool coats thickened for winter, and heavy curling horns, the only