This was originally deposited in a shallow sub-tropical sea and contains the fossils of many marine animals such as bivalves, gastropods and sharks; as well as the remains of many species of plants, such as the seeds of palms, climbers, magnolias and mangroves. These can also be found in the surrounding London Clay and were probably washed out to sea by large rivers. Much of the Kennet Valley is defined as a functional flood plain, and is as a consequence subject to frequent flooding. There are areas in the flow of the river Kennet where drainage is not as good as it might be. There are also areas where the streams and rivers taking water away are nearing capacity and therefore heavy rains lead to localised flooding.
