The rise in sea level against the Maine coast during recent millennia is marked by upward growth and landward migration of salt marshes. Salt marshes are large and widespread in many coastal areas of Maine and Maritime Canada. The three major species of marsh grasses in coastal Maine have a consistent distribution governed by the salinity of the water. Past and present high-tide sea levels are marked by the species of grass called Spartina patens (saltmeadow cordgrass).

Manmade colonial structures, such as wharves, tidal mills and shipyards, yield further indications of sea level rise in historic time. Many such structures that were originally at (or slightly above) sea level are now partially or wholly submerged during high tide. We can study historical records to find out when these structures were built. Then it is possible to measure the thickness of salt marsh peat that has accumulated on them. This, in turn, provides a direct measure of how much sea level rise has taken place since construction. For example, the old wharves on the waterfront in Machias were in constant use for shipping lumber from about 1825 until after World War I. Today, these wharves are completely submerged at mean high tide, which indicates a marked rise in sea level since they were built.

A fascinating series of dikes and drainage systems were built on the salt marshes in colonial times. The agricultural diking of marshes was practiced in this region for about 375 years. Dikes were designed to lower the salinity of the marshes so the hay could be used for animal feed. The mechanisms and tools used to build these structures were specially designed and workers kept the dikes in good order. The dikes themselves were made by the laborious task of excavating and stacking blocks of salt marsh peat. The continuing rise in sea level has caused the marshes to build up against the dikes, reducing their apparent height. Subdued remnants of these long linear features can still be seen in places, including some of the stops on the Ice Age Trail. Their history is similar from the Maine coast to the Tantramar Marshes in New Brunswick and Nova Scotia, Canada. Marsh hay was a major crop until the 1920s, when automobiles began to replace horses as a mode of transportation. Hay agriculture is remembered now only by the “hay market squares” in coastal cities, such as in Boston, Portland and Bangor. Salt marsh hay is still harvested in small quantities for mulch, bedding and packing.

To read more about Maine in the Ice Age, check out these more indepth explanations: Eskers and Deltas, Marine Clay, Glacial Retreat, Moraines, Chronology, Abrupt Climate Change