The upper parts of each marine sequence form shoreline-attached sheet sandstones. Each coarsening-upward sequence is bounded by marine shale laid down during a transgression when coarser sediment was trapped in nonmarine or marginal marine areas. Some sequences break up seaward into two coarsening-upward units because of minor transgressions. (SEPM Society for Sedimentary Geology)

Seaward migrations of the shoreline are known as regressions. Landward migrations of the shoreline are known as transgressions. The causes of such shoreline migrations are manifold. Essentially they are the result of the interaction between accommodation space, that is the space available for potential sediment to accumulate, and sediment supply. When sediment supply exceeds the space available for it to accumulate, then regression occurs. When the space available for sediment accumulation exceeds sediment supply, then transgressions occur. (Net Industries and its Licensors)

Uplift of the North Bay outlet between 10,000 and 5,000 years BP raised lake level to above the present (the Nipissing transgression), submerging the forest and valley system. Submerged stumps from those forests have often been encountered on the present lake floor; some stumps have been dated. (Springer)