Geological time mostly runs incredibly slowly, in measures of hundreds of thousands, if not millions or billions of years. No wonder Bob Woodruff was excited about what happened over the weekend.

On Saturday he and three other experts, in association with the Vineyard Conservation Society, led about 120 people on a geology tour at Lucy Vincent Beach, educating them about what short-term land forms the Wequobsque cliffs — indeed the whole of Martha’s Vineyard — were.

On Sunday the storm came in, and spectacularly underlined their point.

“You’ve got to see it,” Mr. Woodruff said over the telephone on Tuesday. “The beach has dropped I’d say three feet — the whole length of the beach. Rocks that were chest high are way over your head.

“A huge section of the cliff has come down. All the softer yellow stuff behind the cemented gravel has fallen.”

And indeed the change to the area was remarkable. Where there had been mostly sand on Saturday, there was mostly stony cobble. All along the back of the beach were fresh piles of fallen soil and rock, the beach grasses and other plants still attached.

Of course, the beach will come back; Mr. Woodruff said it had come and gone two or three times over the winter. But the cliffs will not. They are in the process of being leveled. Quite rapidly.

As another of the tour’s guides, William Wilcox, said on Saturday: “Our great grandchildren won’t have much to look at.”

He pointed at several rocks sticking up out of the sea several hundred feet offshore, which had been part of the Island less than 100 years ago. Since 1915, the shoreline has receded some 400 feet. The cliffs, which slope back from the shore, are about half the height they were.

This is geology on a human time scale. Each year, the southern shore of the Vineyard retreats: the rate varies from about 11 feet at the Wasque end of Katama to about two or three feet at the Squibnocket end. At Wequobsque, it is about seven feet.

Mr. Woodruff told the story of a local landowner who planned a new house about 30 feet in from the shoreline nearby until he heard about the rate of erosion and realized he would have about four years in residence before it tumbled into the Atlantic Ocean.

The Island is shrinking, too, from the northwest, although at a slower rate; about two feet a year at Lambert’s Cove.

Only in the Lobsterville area is there any buildup. But it is sand and it is flat and it will provide no protection from the other threat to the Island ߝ rising sea levels due to global warming.

In geological time, Martha’s Vineyard is an ephemeral piece of Earth. The whole place is probably only a million or so years old — although some of its component materials are much, much older — the product of the great ice sheets which scraped their way across much of North America.

The ice only finished forming the Island about 15,000 years ago, when it last retreated from this part of New England, after a succession of advances and retreats.

The Island was not even an island until relatively recent times. Ten thousand years ago all of Georges Bank was above sea level.

The sea has been reclaiming the Vineyard ever since the ice began to melt and recede. Ocean levels have been rising about 11 feet every thousand years, and continue to come up about eight inches a century, a rate likely accelerating due to global warming.

And that global warming, almost all scientists now agree, is in large part due to human activity which pumps out huge quantities of the principal greenhouse gas, carbon dioxide, by burning fossil fuels.

At the start of the industrial revolution, Mr. Wilcox said, atmospheric concentrations of CO2 were about 200 parts per million. Now they are about 350 parts per million.

All of this paints a grim long-term picture for those who own coastal or low-lying real estate. But it also makes for fascinating study for those interested in geology.

The glaciers, when they retreated, left behind an extraordinary jumble of materials, which the sea continues to expose and which have puzzled and excited earth scientists for hundreds of years.

“You go back to the late 1700s,” Mr. Woodruff said, “and they thought the Gay Head cliffs were caused by a volcanic eruption, because they found what they thought was charred wood. It was the lignite [a type of low grade coal], in the Cretaceous clay.

“Those Cretaceous clays, 60 to 100 million years old, were probably scraped from the bottom of Buzzards Bay.”

The action of the glaciers juxtaposed such ancient sediments with far more recent ones.

“In the Gay Head cliffs they found all sorts of fossils ߝ whales, horses, rhinoceroses, camels even laid down Pleistocene sediments, in the past couple of million years,” he said.

(Digging for fossils is now strictly prohibited by the town of Chilmark and the Wampanoag tribe, he stressed.)

It’s the same sort of hodgepodge in the Wequobsque cliffs. You’ll find ancient clays, recent fossils, iron-cemented conglomerates and granite boulders just feet from one another, embedded in one another.

Only repeated glaciation could do it — scraping things up, turning them over, mixing and carrying them for hundreds of miles before dropping them again in their terminal moraines.

All of this was explained on Saturday’s geology discovery walk. It was engrossing stuff, as 120 participants would no doubt agree, particularly if they subsequently went back to the scene to see the changes the weekend storm wrought.

There will be other such walks, although Mr. Woodruff said he is not sure when he will be able to get his volunteer guides, Mr. Wilcox, Charles Ratte and Craig Saunders together again.

See it, before it’s gone.