The Apollo 16 mission provided direct samples of two lunar highlands geological units: the Descartes and Cayley Formations. The Descartes Formation is interpreted as hummocky Nectaris basis ejecta, reworked by secondary impacts from the Imbrium basin. The Cayley Formation was emplaced as a consequence of the Imbrium impact and incorporated large quantitites of the local substrate, which in this region consists mostly of Nectaris ejecta. The composition of the upper crust in this area is primarily anorthositic gabbro; local craters near the site would produce an impact melt of this composition. I estimate the total volume of pure melt within the Apollo 16 site deposits to be about 10% or less, significantly smaller than previous estimates. Two chemically defined melt groups found in Apollo 16 are of broadly low-K Fra Mauro (LKFM) basalt composition and could not be produced by local craters in the region. The volumetrically most important melt group at Apollo 16 is of aluminous LKFM composition and appears to have formed in one event at 3.92 aeons: those rocks probably represent Nectaris basin impact melt. Imbrium basin impact melt may also be present, in quantitites of less than 1%. The model presented here suggests that the Descartes Formation (Nectaris ejecta) is a two-component mixture of anorthositic clastic debris (>90% by volume) laden with aluminous LKFM Nectaris basin impact melt (>10% by volume). If the age of Nectaris basin is 3.92 aeons as suggested then there is probably no in situ primordial surface dating from the time of crustal formation, and the total thickness of the highlands ''megaregolith'' may be greater than a factor of ten higher than the currently quoted value (1--2 km).