It's a cold, cruel world to be sure, but half a billion years ago it was a whole lot colder. With an average temperature approximately 45 degrees F below modern levels, the late Ordovician period sent the Earth into a major ice age. Climate changed wildly during the Ordovician, from tropical to frigid in a mere 60 million years — a blink of the geologic eye.

To species living at the time, however, it was more than just a climatic hiccup. The temperature change led to an overall cooling of the world's oceans and, not un-relatedly, to mass extinctions. In two pulses of glaciation over the course of a million years, brachiopods, stromalites, and condonts — tiny, jawless fish that were the first known vertebrates — all suffered terrible losses. Nearly one hundred marine invertebrate families were lost.

The sudden climate change may be partly responsible for the peculiar formation that is the Trenton-Black River limestones. Limestone forms when the pressure of many feet of sediment — marine animals' skeletons and shells — causes that sediment to build up into a single piece of rock. Since different types of sediment can build up at different time periods, limestone is often layered, and those layers are identifiable by color, by size of the individual grains making up the stone, and by specific types of fossil that have been preserved. Typical limestone formations are so tightly packed that individual layers don't break apart readily, and removing intact fossils can be difficult and frustrating work. Unlike most limestone formations, however, the Trenton-Black River rock splits easily along layers, making the extraction and study of fossils easy.

It's possible that the limestone's layers are so loosely connected because, unlike typical limestones, which form in warm water, often from pieces of coral reefs, the Trenton-Black River limestones formed in a colder environment, one more like a modern cool-water community. In a cooler-watered community, sediments build up more slowly, which some geologists say may make the layers less tightly bound to one another.

Sediments don't fall evenly, though. As pressure builds in some spots more than in others, certain areas of the limestones can turn to dolomite — a form of marble — and release nearly pure methane. Those areas are more porous than others, making ideal areas for the natural gas to accumulate and flow. Because of their great depth (10,000 to 15,000 feet) the gas reserves have remained largely untapped. Drilling each well could cost as much as $3 to $5 million.