Oakland's Turtle Island String Quartet is a chamber group that can play like a tight, drummerless jazz combo. Though the instrumentation is that of a classical string quartet -- Darol Anger and David Balakrishnan on violins, Katrina Wreede on viola and Mark Summer on cello -- the musicians' backgrounds in new-grass and jazz bands enable them to establish a groove and improvise on chord progressions.

The quartet proved its lively, playful spontaneity at the Wolf Trap Barns Saturday night. For some passages, the four musicians referred to the notated sheets on their music stands, but for other passages, Summer established a walking jazz bass line on his cello, while Anger, Balakrishnan or Wreede took off on a freewheeling solo. Whichever players weren't soloing reinforced the rhythm with short bow scrapes, bow slaps, staccato string plucking or even flat-picking.

Turtle Island played compositions by such jazz masters as Miles Davis, Dizzy Gillespie, Chick Corea, Pat Metheny and John Coltrane (Coltrane's tender ballad "Naima" was somberly dedicated "to all the people in the gulf"), but the set was dominated by original pieces with catchy melodies from the quartet's recent Windham Hill album, "Skylife." When the group reappeared for the encore, Anger said, "You've probably gathered that the four of us are really frustrated rock-and-roll guitar players," and he proved it by leading the group through a riff-stomping version of Robert Johnson's "Crossroads."