  Twisted bilayer graphene (TwBLG) is one of the simplest van der Waals
heterostructures, yet it yields a complex electronic system with intricate
interplay between moir\'{e} physics and interlayer hybridization effects. We
report on electronic transport measurements of high mobility small angle TwBLG
devices showing clear evidence for insulating states at the superlattice band
edges, with thermal activation gaps several times larger than theoretically
predicted. Moreover, Shubnikov-de Haas oscillations and tight binding
calculations reveal that the band structure consists of two intersecting Fermi
contours whose crossing points are effectively unhybridized. We attribute this
to exponentially suppressed interlayer hopping amplitudes for momentum
transfers larger than the moir\'{e} wavevector.
