The sea level was low. The rivers therefore flowed swiftly, scrubbing out the sediment. The record of river deposits is valuable but incomplete. The sediments represent only a small fraction of the time in which glacial epochs have occurred. Furthermore, due to erosion, little or nothing remains of the oldest deposits.
Scientists examine a deposit for pollen and remnants of plants and animals.
The depths at which specimens are found indicate when the specimens were deposited. In general, deeper material was deposited earlier. The comparison with present growth patterns indicates what the climatic conditions were when the specimens were deposited. For example, researchers count the shells of different kinds of tiny ocean-dwelling animals called foraminifera found at various depths in the cores. Some of these species grow well in warm water, while others grow well in cold water.