The walls of Howe Caverns consist of two types of limestone (Coeymans and Manlius) from different periods in the Earth's early history, as well as a rock known as Rondout waterred. The Manlius limestone is seen most clearly and is the most abundant, while Coeymans limestone can be seen in the upper portion of the cave near the entrance. Coeymans limestone is more difficult to dissolve than the Manlius variety, so the water naturally chose a lower path through the Manlius layer. As a result, almost perfectly flat ceilings can be seen in parts of the cavern, which are actually the underside of the Coeymans limestone layer. Rondout waterred is the cream-colored rock that runs along the underground stream.

Scientists believe all of these rock layers were laid down by the ancient, extinct sea during the Silurian and Devonian periods of our Earth's formation. They are all sedimentary rock, formed by layers of deposits which settle out of a body of water and are then compressed into solid rock. To give you an idea of the age of these rocks, scientists estimate the Silurian Age began about 435 million years ago and ended when the Devonian Age began around 395 million years ago.

While there are a few fossils visible in the cavern walls, the main fossil beds lie in the layers of limestone above the cavern ceiling. This means the rock from which Howe Caverns is carved is older than most fossils. But the building process in Howe Caverns is never done! Nature is still hard at work in the great cave - as proven by the fact that we still hear, see and feel the droplets of water falling - and the cavern's face is always changing. It changes so slowly that the smudges left by smoking torches nearly a century ago can still be seen on the glowing flowstone walls today.

Stalactites, Stalagmites and Flowstone

Once the ancient subterranean stream cut its path through the limestone layers, marvelous formations named stalactites, stalagmites and flowstone began to form. These unique stone formations grow at an unbelievably slow rate: only about one cubic inch (about the size of a small ring box) will form in 100 years.

When rainwater seeps down through the soil above, it picks up a very, very small amount of limestone as it travels. In fact, there is only about a teaspoon of limestone dissolved in every gallon of rainwater that filters through the surface above the caverns. As this rainwater drips slowly through the cave's roof, the droplets of water evaporate, leaving behind tiny amounts of limestone on the cavern ceiling. In this way, stalactites grow downward from the vaulted cavern roof, particle by particle, over the course of millions of years.

The stone formations which grow up from the cavern floor are created in the same manner. Sometimes large droplets of water filtering through the cave roof don't have time to evaporate before they roll down a stalactite and drip off to the cave floor below. That's why stalagmites usually form directly below stalactites and continue to grow as more droplets fall from above. In some cases (such as the grand Pipe Organ formation at Howe Caverns), stalactites and stalagmites will actually grow together to form columns. (By the way, it's easy to remember the difference between stalactites and stalagmites. The word "stalactites" contains the letter "c" - like the word "ceiling." "Stalagmite" uses the letter "g" and so does the word "ground." Stalactites grow from cavern ceilings, while stalagmites grow upward from the ground!)

Flowstone is formed in much the same way as stalactites and stalagmites, except the water flows down the cavern walls. This wonderful rock formation resembles sheets of frozen, rippling ice. Other minerals in the water that carry the limestone give flowstone, stalactites and stalagmites their lovely colors. With the exception of the colors created by lights in the cavern, all of the colors you see at Howe Caverns are completely natural. Rust-colored formations are caused by the presence of iron in the dripping water... green is from waterborne copper... grey indicates the presence of aluminum oxide... yellow comes from sulphur... and pure calcite makes lovely, milky-white formations which are translucent.

The Growth of Speleothems
The following is an excerpt from the Cave and Karst Curriculum and Resource Guide

Features such as stalagmites are technically called Speleothems. The word "Speleothem" is derived from the Greek words "spelaion" (cave) and "thema" (deposit). The process by which Speleothems are formed is the reverse of that by which limestone is dissolved to produce caves. Speleothems consist mainly of calcite, the same mineral that makes up limestone, in its crystallize form.

Conditions are right for the process to begin when the water table lowers and air enters the cave. Calcite is dissolved from the limestone above the cave by slightly acidic water as it percolates downward through the soil.

In the soil, where plant and animal remains are decaying, the carbon dioxide content is about 300 times that of the outside atmosphere. The carbon dioxide combines with the water and produces carbonic acid, which in turn dissolves some of the limestone it passes through as it moves downward toward the cave. When the acidic water reaches the cave, the carbon dioxide is released and calcite is precipitated (redeposited) on cave walls, ceilings and floors.

Speleothems form at varying rates as calcite crystals build up, one upon the other. Several factors can determine the rate of growth. The temperature outside, which affects the rate of decay of plants and animals (amount of carbon dioxide in the soil), and the amount of rainfall are two important factors. The shape of Speleothems is determined by how the acidic water enters the cave (by dripping, seeping or splashing) and how the water stands or flows after entering the cave. Stalactites are the most common Speleothems.