Earth and envi­ron­mental sci­ences asso­ciate pro­fessor Peter Rosen said Boston Harbor is home to the nation’s only “drowned drumlin field,” a group of elon­gated hills formed under glacial ice.

He addressed roughly 100 stu­dents, fac­ulty, staff and com­mu­nity mem­bers at the Marine Sci­ence Center (MSC) in Nahant on Wednesday as part of a monthly lec­ture series on topics ranging from the evo­lu­tion of Boston Harbor to the sea­weed habi­tats of marine animals.

Rosen — whose research exam­ines eons of geo­log­ical his­tory, which can yield insights about the earth’s future — said about 200 drum­lins pepper Metro West Mass­a­chu­setts, “but what we see on land,” he noted “is a small frac­tion of the drumlin field that once existed.”

Typ­i­cally, drum­lins are up to two kilo­me­ters in length, 300 to 600 meters wide and less than 50 meters in height, with a char­ac­ter­istic shape — one steep side with a more gradual side oppo­site. Most drum­lins, said Rosen, are now sub­merged off­shore, drowned by rising sea levels caused by global warming.

Four­teen thou­sand years ago the “Lau­ren­tide Ice Sheet” (the glacier that cov­ered this region) began melting in the Boston area, Rosen said. This caused mas­sive amounts of sediment-​​laden water to flow into the harbor basin and raise the sea level about 100 feet higher than it is today, The sed­i­ment, called “Boston Blue Clay,” set­tled like a blanket over most of the glacial deposits.

The rise in sea level, Rosen said, was fol­lowed by a rapid decrease once the glacier melted: the land beneath it began to rebound upward, no longer com­pressed by the weight of the ice.

Some 12,000 years ago, sea level had reached its lowest point; con­tinual melting of the glacier (now the Polar ice) grad­u­ally filled Boston Harbor with water, iso­lating many drum­lins as islands.

Rosen said this more recent sea-​​level rise, at about 3mm a year, actu­ally rep­re­sents the slowest ascent in his­tory. A com­pre­hen­sive satel­lite study issued yes­terday reports that the global sea-​​level increase was 12mm in the years between 2003 and 2010.

How do we know about the early higher levels of the ocean?” Rosen asked. Answering his own ques­tion, he said: “From the Boston Blue Clay.”

Studies, he said, have turned up marine micro­fos­sils in the clay as far inland as Water­town, which is about 8 miles from the shore­line. These find­ings, Rosen said, indi­cate the pres­ence of salt water — and thus a higher sea level — about 14, 000 years ago.

Rosen said the gradual rise in sea level can also be deter­mined with the help of Boston Blue Clay. His team, for example, takes cores of the earth beneath salt marshes, which tend to grow at sea level. A chem­ical analysis called carbon dating can indi­cate how long ago that clay was at the surface.

The impact of rising sea levels includes loss of land due to shore ero­sion, as well as an increase in the fre­quency and mag­ni­tude of coastal flooding,” Rosen explained. “This is espe­cially impor­tant in low-​​lying areas such as Boston, where the land is mostly arti­fi­cial fill just a few feet above sea level.”