  We present a study of the formation of clustered, massive galaxies at large
look--back times, via high resolution spectroscopic imaging of CO in the unique
GN20 proto-cluster. The data reveal evidence for rich structure and gas
dynamics in unprecedented detail, allowing us to image the molecular gas with a
resolution of only 1 kpc just 1.5 Gyr after the Big Bang. These
state-of-the-art data give new insight into the detailed physical processes
involved in early massive galaxy formation, and they provide a first glimpse of
the morphological studies that will become feasible on a regular basis with
ALMA.
