Off the coasts of Maryland and Virginia, a buffer between the Atlantic and the mainland, lies a precious natural treasure of this region, with a special history behind its unspoiled splendor. Thanks to the long and vigorous efforts of far- sighted public officials and citizens' groups, Assateague Island's pristine beaches, grasses, dunes and marshes have survived the threats of civilization and remained a sanctuary for both wildlife and any humans content to be at peace with nature. But let down the defenses for a moment and some new threat quickly materializes.

The latest came from the Interior Department in the form of a proposal to open another two-mile section of the island to a species classified as the ORV--the off-road vehicle, which is identified by its four-wheel drive and its capacity to disturb the natural peace. But Interior Secretary James Watt, with a little help from Assateague's friends, thought better of it. He has just reversed this bad plan.

As some 8,000 letter-writers argued when the plan first came to light, life on the island already includes seasonal infestations of the North American dune buggy, the spotted jeepster and other mechanical predators. Nearly 16 miles of beach are already accessible to ORVs, and any increase in this zone merely would invite more beach erosion and more damage to plant and animal growth.

In deciding against expanding the ORV zone, Mr. Watt respected the wishes not only of those who wrote to object and of 13 members of Congress from Maryland, Virginia and Delaware. He respected also the legacy of another interior secretary--the late Rogers C.B. Morton of Maryland, who was instrumental in inducing Congress and the executive branch to establish Assateague as a national seashore. Mr. Watt's latest decision is an important affirmation of that hard-won protection.