Deltas form where a river meets the ocean and deposits its sediment. As sediment builds up above sea level a non-marine coal swamp develops, while at the same time sandstone is being deposited offshore. At some point the river will switch course and begin depositing a new delta lobe elsewhere along the coast line. The abandoned delta lobe then begins to subside due to compaction from the weight of sediment and the coal swamp eventually becomes flooded by seawater. In the delta switching model the apparent fluctuations in sea level is due to build up of sediment followed by subsidence.
The coal deposits in the Appalachian Basin are associated with delta deposits. This outcrop from NE Kentucky shows alternating layers of sandstone, siltstone, shale and coal which are characteristic of deltas.
Another model for cyclothems is glacial eustatic sea level fluctuations. In this model the land surface remains stationary and sea level rises and falls. By glacial eustatic, geologist mean world wide fluctuation in sea level caused by the waxing and waning of glaciers. During an interglacial period, such as the one we are in today, the polar ice caps are small and most of the earths water is in the oceans. Because there is more water in the oceans, sea level is relatively high. During a glacial period the ice caps become larger tying up more water. Sea level is lowered because there is less water in the oceans. Repeated cycling between glacial and interglacial periods results in repeated fluctuations in sea level and alternating marine and non-marine deposits. Coal deposits of the Appalachian Basin were formed during a geologic period characterized by glacial-interglacial cycles. It is most likely that these deposits formed by a combination of delta switching and glacial eustatic sea level fluctuations. As an added note, one concern over global warming is that the polar ice caps could melt resulting in a rise in sea level and flooding of coastal cities.