A two-ton limestone caveman clutching a heavily gnawed bone will be hoisted into place this morning high atop the north tower of the Washington Cathedral.

The caveman -- the last of 106 distinctly different gargoyles to find a roost on the cathedral's roof -- marks the final phase of 80 years of construction of the grand edifice at Wisconsin and Massachusetts avenues in Northwest Washington.

It is a cathedral of the Episcopal Church and serves as this nation's equivalent of Westminster Abbey, Great Britain's national shrine.

So little remains to be done -- at least in the way cathedral construction is measured -- that church officials have begun making plans to celebrate its completion. "You can write it down. We are going to consecrate it Sept. 29, 1990," Provost Charles Perry said yesterday.

The building timetable calls for completion by the end of the year of the twin towers that face Wisconsin Avenue. Next year, workers will labor on the eight pinnacles, one at each corner of the two towers, and carve thousands of angels and other figures into the exterior and interior stone.

The final touches will be added in 1989, in preparation for the consecration in 1990.

For all their ornately carved exteriors, gargoyles serve a mundane purpose. "They're waterspouts," said Canon Leonard Freeman, "there to carry the rain away from the building."

The Washington Cathedral's gargoyles run the gamut from traditional dragons and griffins and other mythological creatures to figures reflecting contemporary culture. There is a Darth Vader gargoyle -- "from a contest of school kids," said Freeman -- a hippie complete with beard and protest sign, and a harried businessman gargoyle in pin-stripe suit and clutching a briefcase, reportedly modeled after a prominent banker who is a patron of the cathedral.

Some donors have commissioned gargoyles -- at $5,000 to $15,000 apiece -- to reflect special interests. An environmentalist commissioned a turtle weeping over the threat to its species' extinction. A patron from Missouri gave funds for a bear framed by a miniature St. Louis arch.

Proud grandparents immortalized their young grandson in a pair of gargoyles. In the first, he wears a beatific smile and a halo; in the second, the smile turns guilty and the halo shatters as he is portrayed with his hand in a cookie jar.

The works of art are so far above eye level -- seven or eight stories up -- that only tourists with binoculars can really appreciate them. Plaster-of-Paris models of some are available in the cathedral's gift shop.

The Washington Cathedral, properly called the Cathedral of Sts. Peter and Paul, was chartered by Congress in 1893. Construction, which began in 1907, was interrupted by two world wars and several financial crises. (The construction of Chartres Cathedral in France stretched over two centuries and the Anglican Church's Canterbury Cathedral took more than five centuries.)

Just a decade ago, facing bankruptcy and a $10.5 million debt created by efforts to finish construction in time for the nation's Bicentennial, the church halted all building. Austerity measures went into effect: The staff was pared, the heat was turned down and lights were turned off.

Extraordinary fund-raising efforts quickly proved effective and building was resumed in 1981, "when we saw the pledges would pay off" the debt, said Perry.

Under the leadership of Perry and Washington Episcopal Bishop John T. Walker, the cathedral has tried to serve a constituency far beyond the tree-lined, high-income Northwest neighborhood in which it is located.

"We decided we were going to (a) finish the cathedral and (b) grow the cathedral's outreach," Perry said. "We would not do one without the other."

Perry cited a new program, developed primarily for children in D.C. schools "who don't have money for school trips." The cathedral secured grants to bring children to the cathedral for visits and developed a resource center for programs in history, literature, art and music.

The cathedral has provided space for a variety of religious groups, ranging from Polish Catholic to Jewish congregations. It also regularly hosts funerals and memorial services for national leaders and commemrative events of national significance, such as last week's prayer vigil for the success of the summit talks.

Gargoyle is made of gray Indiana limestone.

Estimated weight: 2 tons.