The mouth of a tunnel-like cave overlooks a large river. The entrance is elevated only a few yards/meters above the river's banks. Not far into the cliff, the cave ends in a somewhat larger rounded chamber with an arched roof and sandy floor. River water reaches the back room of the cave only in rare floods.
Photo by David Wilson on Flickr (Creative Commons)
Dimensions The river bluff is 100 feet (30 m) high, and the floor of the cave entrance is about 15 feet (5 meters) above usual river level. The entry passage is 30 feet (10 meters) wide with 20 feet (6 meters) of vertical clearance. Furthest from the river, the interior chamber is more spacious, about 30 feet (10 meters) wide and 40 feet (13 meters) high (partly due to a floor level that is about 10 feet below than that of the entry passage). The river is very large, about 700 yards/meters across.
Key Details
The limestone bluffs here are quite steep, near-vertical cliffs in many places where the river's erosion has trimmed them back over time. Loose rubble makes up a sloping, narrow strip between the cliff base and the water. Bluff height varies quite a lot along the area's waterfront, diminishing to nothing nearby, allowing better docking places for boats. Most of the shoreline is open, free of trees, offering no concealment for vessels docked there.
The cave is a single straight passage with arched ceiling. The bedrock floor is flat along most of its length, but then falls away in a few irregular steps to the lower sand-mud floor at the interior end. Geologists speculate that this cave once extended much further inland, angling downward through the bedrock, but that most of that rock cavity has been filled in by sediments from the river and the land above.
Walls and ceiling of the cave are rough, and lack any stalactites or other travertine formations. In one part of the passage there is a small pile of rock slabs that have fallen from the ceiling in recent decades. Collapses aren't typical for limestone caves, which are typically deeper in the ground and have very stable temperature conditions. Here, freezing winter air enters through the cave's big opening, intruding even to the back of the cave, and ice wedges open cracks in the rock to break up its surface.
There is a fissure in the arched ceiling of the rounded back room, through which daylight or stars can peek down. It is wide enough for a person to squeeze through and descend by rope.
At the cave entrance there is a narrow trench, probably artificially cut into the limestone by prior inhabitants. It allows an easier angle of ascent to the natural floor of the cave for those entering.
The limestone cliff walls contain many chert nodules, blobs of a harder silica mineral. (Some of these are shown on the left photo below.) Broken edges of chert are fairly sharp; it is sometimes collected for knapping into stone knives and axe heads.
A bit further down the riverfront, a much smaller opening in the cliff base exposes a chimney-like alcove that leads upward to another fissure breaching the ground at the cliff top. Two logs lean against the chimney wall. It's unclear whether these fell down from the forested ground above, or were intentionally placed there to aid persons climbing between the cliff top and the river's bank.
Story Elements
This is a natural sentry post or permanent dwelling for groups that wish to continually observe/control river traffic.
Characters might encounter a large, potentially hostile group controlling the river from the cave. This could be a place for a social challenge to keep gatekeepers' attention riveted on an invented distraction in the most interior chamber, while boats or rafts surreptitiously float by on the river outside.
The faces of the limestone cliffs are quite rough, making them easy for experienced climbers to scale without ropes. While the cave walls are also rough, climbing height is limited by their downward-angled, arching forms. The smaller rock chimney a bit further down the riverfront might be valued as a preferred, partly concealed ascent/descent route.
There are several areas of difficult ground here, both inside and outside the cave. Someone might easily slip or turn an ankle.
The artificially cut trench in the floor of the cave entrance could be omitted in storytelling, to depict a fully natural setting. Alternatively, it could be included for a more complex close-combat environment at the cave entrance. At any rate, it is only a few yards/meters long, so it doesn't increase flooding in the back of the cave.
When a major river flood peaks, water will cascade down the rough bedrock steps into the back room, adding drama and making escape more challenging. Once level with the river outside, the water in the cave becomes more calm -- making escape easier. As the river flood declines slowly, a large pool of water will remain in the back, typically for weeks after the flood has abated.
Flooding of the back area of the cave, while rare, might thwart retrieval of buried treasure.
Some valuable material (mushrooms/algae/mineral crusts) might grow on the cave's sand floor as flood water trapped in the back dries. Locals (and strangers!) might compete to harvest this material, with awareness there might be interlopers outside the cave waiting to steal gathered material.
A community's benign use of this cave might be threatened by news of river pirates expanding into the region. How could the locals manage to avoid being displaced from this prime hideout location? Is there a way to conceal the cave from sight, or make it appear inhospitable?
Walls of the cave have a higher concentration of chert nodules than is apparent along the base of the cliff outside. They could be mined by chipping away at the limestone, or by dissolving it with acid.
People might quarry out additional space in the cave to meet their needs over the years. Eventual collapse of the naturally stable ceiling might be an unintended consequence. Careful tunneling into the sand in the back room might open up a subterranean network of alternate passages to the surface further inland (perfect for smuggling).
Ghosts could inhabit this cave from a long history of misdeeds, massacres, etc. here. Beyond humanoid apparitions, there might be hoards of ghost fish, frogs, or bugs etc. from prior mass deaths.
The fissure in the ceiling provides added light to the room at the back of the cave, making it possible to carefully enter during the day. While its east-west angle does not allow sunlight to reach the cave floor, the room's more diffuse glow is brightest in the central area. That spot could be the favored position of an orator, or a best-lighted work area for a community dwelling there.
The fissure allows some rain to fall directly into the room below, and could function as a chimney for cooking fires there. Someone on the bluff above could spy/eavesdrop through this opening.
What occupant(s) of the further extent of this rock tunnel might be waiting under the sand to threaten inhabitants/explorers?
Reference Location
Cave In Rock State Park, on the Ohio River banks in southern Illinois.