The characters in Lanford Wilson's "The Mound Builders" are engaged in an archeological dig in southern Illinois, where they are on the verge of unearthing significant remains of an ancient Mississippian culture. Time is not on their side; the rising water of an artificially created lake is about to inundate the site of their explorations.

They are not just digging down into the distant past, however. At the same time, they're probing their own relationships, uncovering buried emotions and letting skeletons out of the closet. Both a family drama and a meditation on the evanescence of human culture, "The Mound Builders" is a dense and brooding work.

At the Source Theatre's Mainstage, however, you'll have to do a lot of digging yourself to get to the play's essence. The production, which runs through July 11, is turgid going, mired in unfocused introspection, the significance of which comes to light only fitfully. Director Brian Hemmingsen can't seem to find the play's ultimate shape.

For every scene that coalesces dramatically, there are two that flounder aimlessly and leave you grasping for your bearings. Characters need not necessarily be cognizant of their plight or precise about the state of their emotions. But if the director doesn't possess the lucidity they lack, you're in trouble.

"TheMound Builders," although early Wilson (1975), is consistent with his later work ("Fifth of July," "Talley's Folly"), in which the narrative emerges, often obliquely, from a seeming flux of ruminations and reflections. At the outset of the drama, Prof. August Howe (Richard Mancini) is flashing slides on the wall of his study and dictating notes into a tape recorder about the ill-fated expedition. The action then flashes back to the events of the previous summer. So we know in advance that the dig is seriously jeopardized and the lives of the participants are marked for upheaval.

"How" is the question we're faced with. In this production, the little omens that crop up along the way, the telling symbols, the ironic counterpoints, are subsumed by a general moodiness. The drama never really locks in place until the final minutes of the second act.

One of the play's fiercest rivalries involves Dr. Dan Loggins (Christopher Henley), the passionate assistant archeologist fighting to preserve the site, and Chad Jasker (Christopher Wilson), a yokel whose family owns much of what will soon be valuable lakeside land. The animosity is further fueled by Jasker's unrequited love for Loggins' wife (sunnily played by Jane Beard in what is the evening's least self-conscious performance).

The subtle complexities of this triangle -- indeed, the emotions that seem to entangle all the characters sexually and intellectually -- are sacrificed either to overheated ranting or to inaudible mumbling of the sort of which Ann Todaro (Prof. Howe's wife) is particularly fond. The acting, for the most part, calls more attention to itself than to the script.

Some relief is provided by Nancy Robinette as Howe's sister -- a globe-trotting novelist who is on the edge of mental and physical collapse, but still manages sharply sardonic observations on civilizations past and incivilities present. But Hemmingsen hasn't integrated her strange presence into the proceedings and she tends to function as a surrealistic footnote.

What we end up with are shards of drama, broken bits of insight, fragments of scenes -- not all that dissimilar from what the archeologists themselves are sifting out of the imperiled earth.

The Mound Builders, by Lanford Wilson. Directed by Brian Hemmingsen; set, Paul Falcon; lighting, Jennifer Lincoln. With Richard Mancini, Ann Todaro, Morgan Dickson, Nancy Robinette, Christopher Henley, Jane Beard, Christopher Wilson. At the Source Theatre Main Stage through July 11.