Geology of the Bluffs and Ravines
Ten thousand years ago the last glacier covered most of Lake County with ice. When the ice moved here from the north, it carried with it rocks, boulders, clay and sand that it had scoured from the soil as it made its way south. When the atmosphere warmed and the glaciers melted, it left this material, called glacial till, on top of the previous surface. As rainwater ran off this new surface, over time it gouged out large gullies that we call ravines. The clay in the glacial till is very tough and can support heavy structures such as buildings and swimming pools, until it gets wet. Underneath the clay, there are layers of sand and gravel scattered at various depths. Water tends to travel along the seams of these different layers making the layers more prone to slippage. When severe slippage occurs, the entire top layer can start to slide away, carrying with it any structures or trees that were on it. As a result, the point where these seams intersect the bluff or ravine slope are subject to periodic movement.

The glacial till tends to be stable if the slope is about 35 or fewer degrees from the horizontal. If the slope is steeper, it is inherently unstable and tends to collapse. Unfortunately, much of the lakefront is on a bluff whose slope is steeper than 35 degrees, and many ravines have sides that are steeper than 35 degrees. Since this is an unstable condition, every precaution must be taken to eliminate factors which aggravate this instability, and to provide reasonable means to enhance stability. How to do this will be discussed later in this brochure.