Within a radius of 50 miles of Washington's Zero Milestone on the Ellipse there are sufficient wild woods and waters to last a nature lover a lifetime. A dozen of these natural areas, such as Rock Creek Park and Great Falls, are usually as crowded as a carnival midway, while the rest are largely unknown because until now there never has been a single good guidebook to the whole region.

Publication of that good guidebook, "Natural Washington," is a mixed blessing. From now on, many of us are going to find many more vistors in what have been our secret places, such as the Anacostia waterfront and Zekiah Swamp, courtesy of the diligent and informative Bill and Phyllis Thomas.

The authors missed very few of my favorite spots and may have left them out for the same reason I never mention certain areas in print: They are too small and too fragile to withstand crowds of even conscientious vistors.

More surprising was the number of places I have never heard of. In the shadow of RFK stadium, for instance, a National Children's island was being developed. And how could I have lived here 40 years without stumbling across the 40-acre Franciscan Monastery grounds, open daily, free and with parking at 1400 Quincy St. NE? Or Kensingting Orchids Inc. at 3301 Plyers Mill Rd., the outgrowth of the fabulous collection of Dr. Edgar McPeak? Or the great Ladew Topiary Garden just north of Towson? Or Red Rock Overlook Park near Leesburg, or Wildcat Mountain Natural Area near Warrenton?

Random checks of their facts and phone numbers showed "Natural Washington" to be considerably more thorough and accurate than your average quickie guidester, which is the more unusual because the Thomases don't even live here.

Their guide is long overdue. It brings together information previously available only from individual agencies and park managers, and the authors clearly either went over much of the ground themselves or found the right people to talk to, because many of the details they give are not the sorts of things to be found in park brochures.

Washington and its environs are blessed with an extraordinay variety of natural attractions, from respectable mountains and fabulous swamps and marshes to oceanlike stretches of Chesapeake Bay, but even a nature-loving native is likely to be astonished at the number of nearby outdoors opportunities.

"Natural Washington" is especially valuabel in these days of wine-priced gasoline, for although the Thomases have stretched their 50-mile circle in a couple of places, most of the areas they describe are not more than a few gallons away.