The limestone cedar glades of the Central Basin of Tennessee are a unique habitat characterized by a lack of soil, an extreme micro-climate with wetland conditions in the winter and desert conditions in the summer. Trees cannot grow in its central zones, but, it is home to quite a number of species of plants found nowhere else in the world, so called endemic species.

Because it is so extreme the cedar glades are a good laboratory to study geologic and ecological processes. It is the exception that proves the rule.

Where are the Glades?

The map below shows where glades have been preserved by the Tennessee Department of Environment and Conservation. Most of the unprotected glades are found in the same area? Can you explain why that is so?

Why are the glades in the inner Central Basin?

If the central feature of glades is the absence of soil, why would that be a characteristic of the inner basin? Almost all of the solid material in soil comes from rock, so let's look at the rocks.

Each of the zones shown on the map above has its own type of rock exposed.

Can you formulate a hypothesis on why the glades are in the inner basin?

Soil is made up mostly of minerals in the surrounding rock or minerals wash down from upstream.

• When the limestone, shale and chert of the highland rim breaks down into sediment, the indestructable chert is concentrated, leaving sediment which is largely silicon dioxide sand and silt.
• When the Chattaooga Shale breaks down, it leaves clay.
• Limestone, however is solluble, even in weak acid, and this is especially true of limestone sediment. So in areas such as the inner basin where there is little else, there is very little mineral to make up soil.

There are several sites which give outlines of soil formation: The Globe Project has a very accessible one.

The Rule: soil forms primarily from broken up rock. The Glades Exception: where the rock is all but pure limestone, there is very little soil.

Why are some parts of the glades bare rock while others have some soil?

However, while almost all glades are in the Inner Basin, not all of the Inner Basin is covered with glades. Take, for example, Couchville Glade (# 12 on the map above). The satelite photograph below shows the vegetation in the area. Light colored areas are treeless glades. Trees can be seen in the darker areas