In this lyrical epic poem of the American frontier, Campbell McGrath uses a specific story as a springboard for larger ruminations. "Shannon" is based on the historical figure of George Shannon, who, as a teenager, was the youngest member of the Lewis and Clark expedition.

On Aug. 26, 1804, he became separated from the rest of the Corps of Discovery on the plains and nearly died before reuniting with the group 16 days later.

"Shannon" is McGrath's imagining of what the youth went through, and how he saw his country as it was just beginning to be.

Survivalist stories are popular in these apocalyptic days, but Shannon is no mere tale of how to stay alive in the wilderness. He was also one of the most educated members of the corps, able to converse intelligently with the two leaders, whose journals include some of the most important prose in American letters. McGrath quotes from Lewis and Clark to frame his poem.

Then his imagination takes off, using verse to both get at the deeper issues of exploration and capture the sometimes earnest, sometimes hallucinatory, sometimes profound thoughts of a young man having a transformative experience.

Alone in the wilderness, Shannon admires the budding civilization to which he hopes to return:

"... I intend to study Law/Therefore. Must be something in it/To set us so as equals." (Shannon did indeed become a politician, a colorful one at that, McGrath writes in the afterword.)

McGrath, a Miami-based poet and winner of both MacArthur and Guggenheim grants, is a clear, concise writer, who grounds lofty thoughts in natural observations of stars and animals. The language subtly reflects the somewhat halting style of 19th-century letters, but never in a pretentious way.

McGrath writes mostly in short lines of free verse, although several times, as Shannon slips into delirium, he breaks into long repetitions of words and phrases that snake around the pages like poetry experiments made of concrete -- a litany of "buffalo" as herds graze around the famished but bullet-less Shannon. It's a risky trick, but McGrath pulls it off.

The book is, of course, an allegorical coming-of-age journey of discovery, with Shannon a stand-in for America. The hero curses his vanity for heading out on a horse-finding mission full of cockiness but devoid of ammunition and supplies.

His is a restless spirit -- "awaiting them feels/False to my nature & to our grand purpose/Here, that being to keep moving/To forge if even blindly/Onward."

Ultimately, he is rescued when exhaustion and hunger force him to stay put, and the corps finally catches up to him.

Along the way, Shannon thinks about the roles of church and state in this new country, about the pretty girl back home in Ohio, about the brother who died in a river. In less than 100 pages, McGrath takes us back to a pivotal point in United States history through the curious eyes of an unsung hero. It's an unexpected story and a gem of a book.