  We have used the Hubble Space Telescope's Advanced Camera for Surveys to
measure the mass density function of morphologically selected early-type
galaxies in the Gemini Deep Deep Survey fields, over the redshift range 0.9 < z
< 1.6. Our imaging data set covers four well-separated sight-lines, and is
roughly intermediate (in terms of both depth and area) between the GOODS/GEMS
imaging data, and the images obtained in the Hubble Deep Field campaigns. Our
images contain 144 galaxies with ultra-deep spectroscopy, and they have been
analyzed using a new purpose-written morphological analysis code which improves
the reliability of morphological classifications by adopting a
'quasi-petrosian' image thresholding technique. We find that at z = 1
approximately 70% of the stars in massive galaxies reside in early-type
systems. This fraction is remarkably similar to that seen in the local
Universe. However, we detect very rapid evolution in this fraction over the
range 1.0 < z < 1.6, suggesting that in this epoch the strong color-morphology
relationship seen in the nearby Universe is beginning to fall into place.
