Then, at the locations where contamination had permeated the underlying sediment, the EPA would cap the dredges with multiple layers of clean material. The multi-layer caps consist of an "active" layer made of a specific type of clay that would remove contamination that could well up from below. Atop the clay layer is an "isolation" layer of sand and gravel that will ensure that the contaminants are not exposed. Next, an "armor" layer of heavier gravel and stone prevents boat traffic and canal currents from eroding the underlying layers. The topmost layer comprises sufficient clean sand atop the "armor" layer, filling the gaps in the layer of stones and establishing sufficient depth to restore the canal bottom as a habitat. In the middle and upper segments of the canal, where liquid coal tar has seeped into the natural sediment, the EPA would stabilize that sediment by mixing it with concrete or similar materials. The stabilized areas would then be covered with the multiple-layer caps.