Lack of oxygen slowed bacterial decomposition and kept scavengers away . Leaves of palms , ferns and sycamores , some showing the insect damage got during their growth , were covered with fine-grained sediment and preserved . Lack of oxygen slowed bacterial decomposition and kept scavengers away , so leaves of palms , ferns and sycamores , some showing the insect damage they had sustained during their growth , were covered with fine-grained sediment and preserved .