The fossilized remains of trilobites -- an extinct marine animal -- in the exposed bedrock are the prime attraction of Craigleith. Ancient sea muds, rich in organic material, provided feeding grounds for these small sea creatures. Although they became extinct millions of years ago, their fossilized shapes were retained in the mud as the pressure of new deposits turned earlier sediments into shale and limestone. Of intense interest to paleontologists, Craigleith is one of few places in southern Ontario where the 450-million-year-old Blue Mountain shale has been exposed.