The sandwich horizon is the (presumably) time-transgressive surface along which upper border series roof rocks and layered series floor rocks finally met as the last Skaergaard pluton liquid solidified. The horizon occurs by definition between UZc and UBZγ. The McBirney (1989) geologic map shows the location of the sandwich horizon surrounding Basistoppen and on Nunatak 1. However, the three field trip leaders concurred that the sandwich horizon is very difficult to actually find in the field. As a residual liquid, the sandwich horizon should be extremely iron-rich, should have Na-rich feldspar, and probably should have abundant minerals rich in otherwise incompatible elements including, perhaps, zircon, K-feldspar, and biotite. We found two candidates.
Basistoppen summit (885 m), photographed from a minor summit at 504 m just west of Basistoppen. Our traverse took us around the right side of the summit cone where the first sandwich horizon candidate was found. The sandwich horizon is supposed to skirt around the lower slopes of this peak, but it is apparently difficult to identify.
Outcrop of the possible sandwich horizon candidate 1. Trip leader Kent Brooks concluded that this was about as close as we could get to the sandwich horizon, which is very difficult to find in the field. Trip leader Neil Irvine said that this was probably the most likely sandwich horizon candidate he had (ever?) seen.
Photograph of the sandwich horizon candidate 1 rock. This rock appeared to be outcrop, but I could not be certain that it was not a large fallen block; note that the Basistoppen sheet is above this point and some blocks from there were part of the local talus. Thin section photos here and here.
The east flank of Nunatak 1, showing the sandwich horizon candidate 2 rock. This was certainly bona fide outcrop. Trip leader Jens Andersen thought this was the best candidate for the sandwich horizon. Gabbrofjeld is the snowy peak in the distance.
Closeup of the isotropic sandwich horizon candidate 2 rock on the eastern flank of Nunatak 1, shown in the photo above. It is rather fine-grained and has little in the way of preferred orientation of crystals. Thin section photo here.