Letters In the Deep makes clear that Cadillac Sky’s ambitions run far beyond those of a conventional bluegrass band. The sheer ferocity of the combo’s musical attack sets them apart from their ‘grassy peers — but it’s their tradition-bending songwriting that truly makes them distinctive. A modern note of angst runs through semi-crazed numbers like “Trapped Under The Ice,” “Bathsheeba” and “3rd Degree,” brought to life by band leader Bryan Simpson’s feverish vocal delivery and manic mandolin strums. As a unit, Cadillac Sky combines delicate virtuosity with eruptions of energy worthy of the Pogues in their prime. Nods towards the Band and American Beauty-era Grateful Dead are evident in the mixture of melancholy and delight heard in “Hangman,” “Pitiful Waltz” and “Trash Bag.” The desolate “Hypocrite” takes the bluegrass penchant for lonesome sounds to shiver-inducing heights. Balancing these moody interludes are heartfelt numbers like “Human Cannonball” and “Tired Old Phrases.”