Ashbury Pruitt walked to the edge of his tiny island Tuesday and was astonished to discover that 17 feet of the island had disppeared in the last three months.

Pruitt, one of the island's 900 residents, has been watching it erode into the Chesapeake Bay for the last 15 years, but never has he seen parts of the three-square-mile island vanish so rapidly.

In all of 1978, 31 feet of its western shoreline washed away, triple the average rate that Pruitt had noted during his first 10 years of record keeping.

Residents of this southern-most Bay island are accustomed to adversity, so there is no sense of panic. But Pruitt and his neighbors told a delegation of state and U.S. officials today that the time for action has come if they are to stay on this island.

Although numerous oceanographic experts have argued that is is futile to attempt to stop the erosion of such barrier islands, there was no such talk here today. Island residents spoke optimistically of their hopes for building an 8,200-foot, $3.5 million seawall to protect them from the Bay.

Nearly all of Tangier's residents are direct descendants of the original families that first settled here in 1686 and are accustomed to doing without.

Their island is without a doctor, dentist or a movie house. When the Bay freezes in the winter, there is no way to get in or out except by air - and the air strip stands closest to the eroding west edge of the island.

A study commissioned by the Virginia Department of Commerce and completed last Dec. 27 found that at the current rate of erosion, "within the coming decade the airport runway would be inoperable and within two decades part of the inhabited west ridge would have been consumed."

Residents of the island siad they do not need any scientific studies, however, to know what is happening and why.

Edward Smith, one-half owner of Smith and Moore, the island's general store, remembers when there was a radar station on the south end of the island. "Now it's 50 feet out in the Bay," he said with a hint of of an Elizabethan dialect that is characteristic of the speech of many natives here.

Ray Crockett, 73, a member of the town board, remembers when "people used to have houses over there," pointing to the landing strip. "But the're gone now and it's good thing because they'd be under water by now."

The study last year said the rapid erosion is "due to the relatively long fetch [an uninterrupted, open expanse of water] to northwest and southwest winds, to the absence of a significant sand supply to form and maintain a buffering beach and to the nature of the marshy soils forming the island."

Captain Edward S. Bowis, who operates a cruise ship between Tangier and Reedville, Va., pointed northward from his ship today and said "There's nothing but open water between here and Reedville, Va.," where the strongest prevailing wind comes from.

"It's also a perfectly straight stretch to Norfolk (direction of the other strong wind). It's a wonder to me there'a any island left at all," Bowis said.

Pruitt got the idea about keeping track of the erosion while looking out of the window of the tiny building from which he records the accuracy of Navy jet fighters that take target practice on two World War II liberty ships that are anchored off shore.

"The water was lapping up hard over the shore and I went out to the edge with a 100-yard tape measure and marked the distance to the shore."

Since he began his surveys, he has had to move the 1 1/2-inch galvanized pipe that he uses as a marker twice to keep it from being submerged.

Another gauge of the rapid erision was Pruitt's discovery of telephone lines in the water six feet from shore. The phone company told him they had been installed 45 feet inland just 14 months ago.

The arrival of Virginia Gov. John N. Dalton, Senators John W Warner (RVA.) and Charles McC. Mathias (RMd.) and 38 members of the news media today - created quite a stir on this tiny island.

Leah Abraham, an elderly woman, rode her bicycle (along with motorbikes, the principal mode of transportation here) to the landing strip to watch the arrival of the disappointed.

"Why didn't you bring your wife?" she asked Warner and then rode off. A number of townsfolk said they were hoping to get to meet the senator's wife, the movie star, Elizabeth Taylor.

Several waitresses at the Chesapeake House, a restaurant and inn that serves sumptuous family style seafood dinners, said they volunteered to work today in hopes of getting a glimpse of the dignitaries. They had to settle for serving reporters and photographers and lesser officials, because the big shots ate lunch aboard the Chesapeake, the official Virginia state yacht.

The horde of newspeople produced a windfall to Sylvia Bonniwell, who drives one of the five golf cart-like vihicles that show tourists around this one-mile by three-mile island that appears in many ways a part of the 19th century.

The Virginia General Assmebly earlier this year appropriated $362,000 for planning the seawall that would cost $3.5 million to build. Rep. Paul S. Trible (R-Va.) has introduced legislation in Congress calling for the Army Corps of Engineers to build the wall with U.S. funds.

Colonel Douglas Haller of the Corps said he support the ideas of a seawall.

The only alternative, Haller said, "is a nonstructional solution - take all the inhabitants and move them to another location."

Mathias, standing on th mossy edge of the island, said he fears that if action isn't taken soon Tangier and its Maryland neighbor, Smith Island, will become real life examples of the fictitious Devon island in James Michener's novel, Chesapeake, which disappeared beneath the Bay. CAPTION: Map, no caption; Picture, Dalton points out erosion to Virginia's Warner and Maryland's Mathias right. By Larry Morris - The Washington Post