Real Caves and Caverns

A Tie-In Article for Ptolus: Beneath the Streets (Ptolus: Part IV)

Here's a look at a "classic" DM's Only article from a couple years ago at montecook.com. It discusses one of my favorite game settings: caves and caverns. Those of you who bought the recent Beneath the Streets eBook (or who are thinking about running Part IV of Ptolus might find some of these observations handy for your games.

On our trip to Ireland a couple years back, Sue and I visited Marble Arch Caves, a vast series of solution, or limestone, caves. As we explored this place, I was struck by how these real caves seemed so unlike the caves found in D&D adventures. Here are a few thoughts on that topic, thanks in part to research I did first hand and from sources like the National Speleological Society, Kentucky Caverns, the Cave Exploration Group of South Australia, and others. I took the shots you see here (click on each image to see a larger version). For more great pictures, go to the Virtual Cave. There's even a map there you could use almost directly for your next adventure.

Cave Types
When putting caves in your adventures, give some thought to how they were formed. How a cave formed dictates its features, appearance, size, and more.

1. Eolian Caves: When desert winds blow fine sand or silt against a rock face, the eventual result or this erosion is an Eolian cave. These caves can get pretty big, though a typical desert might have more sandstone caves than caves of this type. Eolian caves do not have features like stalactites or flowstone (see below) and are usually very dry.

2. Glacier Caves: Over the course of centuries, water carves drainage tunnels through ice to form glacier caves. These small and narrow caves do not have many features of solution caves, like stalactites or flowstone.

3. Lava Caves: When a lava flow begins to cool, it hardens on the outside, while the lava keeps moving on the inside, forming a tunnel of sorts. After all the molten rock has drained out of this tube, you're left with a lava cave. These caves are best described as smooth, round, featureless channels. Although usually very small by adventuring standards, these tubes can become quite mazelike.

4. Sea Caves: Crashing waves constantly pound away at oceanfront cliffsides, the water combining with the sand and gravel it carries to erode caves out of the rock. Like Eolian caves, the forces that carve sea caves do not produce features like stalactites and stalagmites.

Large flowstone cavern at Marble Arch

5. Solution Caves: The classic stalactite-and-stalagmite caves are called solution caves. They form when, over thousands of years, water seeping down through mineral-rich rock such as limestone, marble, and gypsum, dissolves some of the rock. This action creates interesting features like large caverns, tunnels, and irregular passages. Most caves-and almost all of the large ones-are of this type. They always feel damp, usually thanks to the pools and streams running through them.

6. Talus Caves: Small and unstable talus caves form when water washes through piles of rocks at the base of a cliff, leaving narrow tunnels within as it clears out dirt and smaller stones.

7. Tectonic Caves: When plate activity along a fault line causes rock to shift or cleave off from a hillside, tectonic caves can form. These small caverns contain none of the features of solution caves, lying underground between rock layers.

And there are two other cave distinctions as well:

* Wind Caves: Unlike other cave types, wind caves are not named for how they are formed. Instead, their name comes from the fact that atmospheric pressure changes cause strong air currents to blow in or out of them.

* Ice Caves: When a cave, usually a solution or lava cave, contains ice for most of the year, we call it an ice cave.

Cave Layout
If you're going for realism, quickly realize that caves are more than just dungeons with squiggly lines rather than straight ones for the walls.
First of all, think three-dimensionally. Natural caves rarely form nice, flat floors. Ledges, steplike formations, slopes, and uneven floors are far more common. Make this a feature, rather than a drawback. A typical dungeon fight suddenly becomes much more interesting when everyone's on a different level and constantly having to make balance checks to avoid falling down; this can get old, but for one adventure, or part of an adventure, it's an interesting diversion. A large cavern that requires some climbing to cross is also more interesting than a simple room you walk through. Don't neglect to have the cave's inhabitants take advantage of tiny, high-placed ledges from which to fire their crossbows, or trick intruders into sliding down a steep slope of loose rocks.

An enemy could drop down unseen in the violet light	Handy tunnel ledge for a dark elf archer

Second, don't forget the water. Many caves, particularly limestone caves, are formed by water, and that water is always present. That means many surfaces are slick (more balance checks), and streams and pools pop up all over. Streams in caves are cold and often move very quickly. Moreover, they disappear into the rock and then come out again elsewhere. You could easily design a cave system where the characters could reach some portions only by going down into the cold, rapid-moving stream and swimming underwater.

Features of a Solution Cave
These are just a few of the features found in caves. Use them liberally when describing the environment to your players to give verisimilitude to the environment. If the dark elf ambush is set up behind some wavy violet draperies and deep red flowstone rather than just a big rock, all the cooler.

Cave Corals: These bizarre stone masses are formed by slowly seeping water. Cave corals look like small clusters of their namesake. Sometimes colored lighter or darker than the stone around them (if they contain minerals like onyx), they can give a cavern an alien appearance.

Columns: When a stalactite and the stalagmite right below it grow together, the result is called a column. These can become massive, and the inside of one of these makes an interesting place for a burrowing monster to lair.

Drapery: When water begins to trickle down a slope within a cave, the calcite it deposits on the walls can form a drapery. The resulting curtain flows in a stone wave from ceiling down the cavern wall. These can be very thin (like actual drapes) or they might eventually become thick enough to serve as strange, wavy walls in a large cave.

Altarlike flowstone	Cascading drapery
Wall flowstone	Drapery from below

Fill, Talus, or Scree: This material varies from sand and clay to stratified gravel and was long ago washed in by running water. Many of the stones deposited this way glisten like jewels, but they are not worth anything to adventurers. When a cavern's ceiling collapses, the fallen rock, from small chips to large blocks, collects in piles called breakdown.

Flowstone: The most common cave feature, flowstone forms when a very slow flowing action deposits a mound of calcite on the cave floor. Flowstone can take on the eerie appearance of a large brain or even an ooze, like a black pudding or ochre jelly. Flowstone is usually wet and glistening, often with dark colors.

Spar: This feature looks like a mass of exposed crystals jaggedly protruding from the wall. Spar is usually deposits of calcite or gypsum, but sometimes of barite, halite, or quartz. Their facets and colors make these jagged formations appear dangerous yet interesting. They frequently "grow" in pools, sometimes jutting up and out of the pool.

Stalactites and Stalagmites: Although flowstone is the most common cave feature, these are the formations most familiar to us. Stalactites hang down from the ceiling, formed with drops of water seeping through the rock into the cave deposit a small bit of calcite before dripping to the floor. With each new drop, more calcite is added, till a dangling cylinder is visible on the cave's ceiling. Most stalactites are very small, no wider than a drop of water -- their fanglike appearance gives a cave a frightening feel. Larger, cone-shaped stalactites are older, showing growth of deposits around these narrow tubes. Stalagmites form as calcite collects from the water that drips off stalactites. They're found only in old, drier caves, where water does not run through the cave system regularly -- such action washes away the deposits before they can form anything.

A mouth of stalactite teeth	Marble Arch Caves' longest stalactite

Part 1 of this article deals with real castles and dungeons.