Susan Sontag defines photography as "the inventory of our mortality." Like an ancient handprint on a cave wall, a photograph records an individual's momentary presence while serving as a reminder that that moment has passed.

In his ambitious ninth collection, "World Tree," David Wojahn draws on photos, cave paintings and sound recordings to create his own inventory of mortality. He pays tribute to dead musicians, memorializes favorite poets and remembers friends who have passed.

Wojahn, who was born in St. Paul in 1953 and educated at the University of Minnesota, is known for expansive and socially engaged poems. Many poems in "World Tree" read like essays; their long lines track Wojahn thinking as he grapples with the big subjects of life and death. Like an essayist, he brings together a breadth of scholarly knowledge and contemporary observation.

In one, a man napping with his wife remembers a quote from English poet John Clare: "The place/ we occupy/ seems all the world." Indeed, in this book Wojahn attempts -- and often succeeds at -- taking on "all the world."

The series "Ochre" spans some 20,000 years of human history and is accompanied by illustrations of cave paintings, anonymous turn-of-the-century snapshots and digital photos. Wojahn writes, "We are but charcoal, ink & pixel before Thy/ Manifold unspeakability."

A Paleolithic cave drawing of a man repeatedly stabbed appears with a poem asking: "How to depict/ The human figure mangled, the whole reduced/ To the gutted sum of its parts, a brilliant ooze." Turn the page to a photo of an American soldier standing over a dead prisoner at Abu Ghraib.

Throughout the book, contemporary politics collide with ancient history. In one poem, Wojahn imagines the president of the National Rifle Association in Dante's hell.

In other poems the political and the personal are woven together with more subtlety. In "Scribal: My Mother in the Voting Booth," he writes: "Stylus through Nixon, stylus through Agnew. Two hours/ she's waited in the wet/ November snow of Minnesota & her cold next week will worsen/ to pneumonia. Over/ the churning columns she'll cough & pass out & waken in County General,/ shrouded in an oxygen tent."

The country teeters on the cusp of political change as the speaker's mother lies motionless in the hospital.

At times the poems can sag under the weight of the references, and there is a group of poems marred by the unfortunate choice to print a dingbat between each line. But overall, Wojahn's acrobatic logic, intellectual ambition and well-crafted sentences make for an exhilarating read.

Elizabeth Hoover is the associate editor of Sampsonia Way magazine, a publication of City of Asylum/Pittsburgh.