The other day, I took some time off from building my deck to lend a hand with the Indian longhouse that is taking shape at the Chancellor's Point Natural History Center.

It was quite a sudden shift -- from the bolted grid work of joist and beam to the arching skeleton of lashed saplings. One moment, I was beginning to master the demands of the skill saw, the lag screw, and the 16-penny nail. The next, I found myself learning how to bend and tuck handfolds of dried marsh grass to make a wall of thatching.

How irrelevant such tools as a carpenter's level seemed here at the longhouse, where structures were woven from the irregular shapes and textures of nature. You live by different tools, and a different perspective, as a child of the forest.

The State Commission on Indian Affairs conceived the longhouse project and obtained a $9,000 grant from the Maryland Humanities Council to pursue it. The groups see the longhouse as an important contribution to the state's 350th anniversary celebration -- a reminder of the people who were living here when the European colonists arrived.

They couldn't have picked a better location for the project than Chancellor's Point. The nature center, run by the St. Mary's City Commission, always has been a kind of refuge and alternative, devoted to ecology and wildlife and also to organic gardening, the growing of edible wild plants, and star gazing.

The center gives attention to the region's lost Indian culture, too. Every October, Chancellor's Point stages what arguably is Southern Maryland's most unusual festival -- Aboriginal Life Day. I have watched craftsmen making stone tools, archeologists analyzing backyard artifacts and Boy Scouts purging themselves in the nature center's homemade sweat lodge.

I've also always made a point of tasting the annual edition of aboriginal potpourri, usually a grayish stew containing beans, corn, assorted tubers, and, depending on availability, venison, raccoon, rabbit and/or squirrel.

The longhouse project has aimed for the same authenticity, but on a much more complex scale. The structure's design is based very precisely on the post-hole pattern, or floor plan, of an actual longhouse excavated in 1981 near Norfolk. The materials are based on descriptions written by the first colonists.

These materials include dozens of carefully chosen saplings, for the framework; about 60 deer (and, if necessary, cow) hides for lashing; and about two tons of marsh grass for the thatch. The volunteers doing the work have used replicas of Indian tools, to the extent possible.

Enforcing this authenticity -- as longhouse designer, trainer of the volunteers, periodic visitor and consultant, and firm but friendly taskmaster -- is Errett Callahan of Lynchburg, Va. An expert in the intriguing field of "experimental archeology," Callahan has made a career of building authentic dwellings from the vanished past.

The idea is not just to produce something that is truly educational rather than simply evocative. By actually constructing a relic, a specialist like Callahan can explore a variety of questions about the people he is studying.

How long would it have taken the Indians of the Late Woodland period to build a longhouse? What tools would they have used? What kinds of debris would have been left behind, first by the construction, then by the house's use?

How did the Indians use their hearths? Their storage pits? The various doorways in a longhouse? What did they keep in their houses? And what archeological traces would this material have left?

How well could a longhouse stand up to the weather? What maintenance was necessary? How long did the structure last?

By monitoring and recording every element of construction, and then doing the same for various activities conducted in the completed longhouse, Callahan hopes to test possible answers.

He and the volunteers are creating a living artifact and observing it as it deposits its evidence. At the very least, their effort should help conventional archeologists recognize and interpret the genuine prehistoric evidence they unearth in digs.

As I helped tuck thatching into place on the longhouse, I wondered what traces my backyard deck would leave, and what people in the future might conjecture about me. Would they appreciate the reassurance of the carpenter's level, as I was beginning to appreciate the safisfactions of marsh grass thatching -- its weight in the hand, its smell, its way of bending and tucking into place?