The Apollo 16 landing area, termed the Descartes region, is situated in the southern highlands of the Moon (latitude 9°00'01" South, longitude 15°30'59" East). For this landing, we have selected a relatively smooth area nestled in the picturesque and rugged lunar highlands. The general location is shown in figure 1. See figure 6 for a beautiful view of the site, sketched by Jerry Elmore, and figure 7 for the geography of the Descartes region. In drawing figure 6, we have combined the precision that is available from modern-day computers with the insights that can come only from an artist. Thus the features are very accurately drawn but they are displayed in a way that the human eye will see them. Then in figure 8, I show a topographic map. This map shows in detail the elevation of each point on the landing site. It represents the basic data used to construct figure 6.
Since prehistoric times, man has known that the Moon, as seen with the unaided eye, has both light areas and dark areas. The dark areas look smooth, the light areas more rugged. The dark areas are called maria (plural of mare) from the mistaken belief, now centuries old that they were once seas. (Mare is the old Latin word for sea.) We visited such areas on Apollo 11, 12, and 14. Then on Apollo 15 we landed just at the edge of a dark area and during the exploration that followed climbed part way up the initial slopes of the Apennine Mountains, a light area.
The light areas are termed highlands, a name carried over from the days when it was believed that they stood higher than the lunar seas. That they indeed stand higher than the maria is now well established by measurements made on previous Apollo flights. On Apollo 16, we will visit the highlands and examine two different kinds of rock that together cover about 11 1/2 percent of the front side of the Moon.
In the rest of this section, I will discuss the several geologic features present at the landing site: The craters, the Cayley Plains, and the Descartes formation. All of them are clearly visible in figure 6. They are shown in the simplified geologic map of figure 9.
Craters are rare on Earth. They are present everywhere on the Moon. Even the most casual TV watcher of the previous lunar landings has now seen many craters, but what he may not know is how much can be learned about the Moon from craters. The "freshness" of a crater is a measure of its relative age. Notice how sharp the crater North Ray appears in figure 6 (North Ray is the bright rayed crater in the left foreground.) Compare it with the much smoother one about one-half mile west. This comparison suggests that North Ray is the younger of the two craters. It is easy to generalize this comparison to a regular gradation of sharpness which can then be used to obtain the relative ages of many craters.
Our understanding of the details of crater formation has been greatly improved by the study of impact craters on Earth. One such crater that is generally well-known is Meteor Crater, near Flagstaff, Arizona. Other impact craters, less well-known to the public but intensely studied by geologists, exist in Tennessee, Canada, Australia, Germany, and elsewhere. An oblique photograph of Meteor Crater is shown in figure 10.
But not all features on the Moon's surface were formed by impacting objects. Some were formed by volcanism. It is never easy on the basis of photographs or telescopic observations to distinguish between an impact and a volcanic origin for a particular feature. In fact Galileo, the first man to look at the Moon through a telescope, about 350 years ago, suggested that all the craters on the Moon were due to volcanoes. His hypothesis stood unchallenged for two centuries until someone suggested the impact hypothesis. As so often happens....
....in science, long, and sometimes bitter, arguments over which hypothesis was correct raged for about 100 years. Today, we believe that most lunar features have resulted from impacts but some have been caused by volcanic processes.
The craters provide samples that came originally from below the Moon's surface and are now sitting on the surface of the Moon. Consider South Ray (shown in figure 6). Note the rays. They are the white streaks that radiate from the crater. The material along any particular ray came from different depths in the Moon. By studying craters on Earth, we have learned that the position along a ray corresponds to a particular depth. We have even watched through slow motion photography the material exhumed from depth by a large explosion; we have traced it through the air and seen it land at a particular distance along the ray. Thus by sampling the rocks from a ray, we can obtain samples of the rocks that lie at different depths. Such samples are very important and will be collected at several craters.
The shapes of craters also yield information about the subsurface rocks. Note that North Ray and the unnamed crater about 1/2 mile southwest of it both have flat bottoms. Considerably smaller craters at the landing site, such as Flag, have coneshaped bottoms. One interpretation of these features is that a relatively solid layer occurs at a depth of about 250 feet. Samples of this layer have been excavated by the impact that formed North Ray and will surely be identified in the rocks brought back to Earth. The layer is possibly a basalt flow similar in many ways to those known on Earth.
The study of the vertical changes in rocks, termed stratigraphy, provides the basic data necessary to construct the history of the Moon. (For example, many facts about the geological history of the Earth have been read from the rocks exposed in the walls and bottom of the Grand Canyon.) Thus samples obtained at different elevations are quite important. Samples originally on the tops of the mountains, such as Stone and  Smoky (figure 6), can now be collected near the bottom of the mountains. Of course material from all heights will be mixed together. One challenge to the lunar geologist is the "unmixing" of the samples and the assignment of the proper stratigraphic height to each.
Material ejected from some giant craters extends halfway across the Moon. See figure 1 for examples: the crater Tycho near the south pole is the most prominent. Material from others extends shorter distances. Everywhere on the Moon some material has been received from distant impacts. Most of the material present in the vicinity of any particular crater is undoubtedly the material that was present before the crater was formed. The exotic material, that which came from elsewhere, is probably quite rare and the amount present at the Apollo 16 site may be less than 1 part per 1,000. Only after extensive investigation of the samples back in the laboratory on Earth will be be reasonably sure about the origin of any particular sample.
Between the two bright rayed craters (North Ray and South Ray) lies a rather smooth surface, the Cayley Plains. It is on this smooth surface that the LM will land. The rocks beneath the plains make up the Cayley formation* which is the largest single rock unit* in the highlands of the....
...front side of the Moon. It covers about 7% of the front side. Samples from several levels within this unit can be collected by the astronauts. Some will be collected near the landing point, others along the traverses. Rocks exhumed by North Ray Crater (and other craters) from depths to about 600 feet can be sampled in the rays. Several layers are exposed in the east wall of North Ray Crater. They are probably the same layers that can be seen elsewhere in the landing site and form scarps (or cliffs) to the south and east of the crater. These layers may be layers of basalt. They may be something else. No one is now sure. The detailed sampling now planned for the Cayley fm should allow us to obtain data with which to decipher this puzzle.
The Descartes fm consists of highland plateau material, the surface of which is composed of hills and valleys. To the non-geologist, this description may sound odd. But then, it's the easiest way of saying that the Descartes fm is the one that occurs on the highland plateau and that it forms hills and valleys. You see, we know that on Earth certain kinds of rocks have certain characteristics-some
Occur mostly in valleys, some "hold up" ridges, others form very rough and majestic mountains, and so on. Because we have not yet seen a sample from the Descartes fm, we can best describe it in terms of those large scale characteristics that we have seen.
This unit covers about 4 1/2% of the near-side of the Moon. Abundant samples of the Descartes fm should be available at the Apollo 16 landing site.. The expected distribution of this material at the site is shown in Figure 9. This map was drawn on the bases of telescopic studies and photographs taken from orbit.
The Descartes formation is very likely composed of the igneous rock basalt. Terrestrial basalts are very common and are most likely known to you.
The rock that flows out of most volcanoes is lava. When it cools and becomes solid, then it is called basalt. A well-known example is that of the Hawaiian volcanoes. A striking illustration of a basalt flow in Hawaii is shown in figure 11. Basalt is very wide-spread throughout the western United States and most visitors to that area see solidified basalt flows. A rock that is chemically similar to basalt but slightly coarser grained is diabase. Many examples of diabase are known. Visitors to New York City often cross a prominent scarp (the steep hill ) on the western side of the Hudson River. That scarp is the face of a gently dipping, flat, tabular body of diabase, known as the Palisades sill. The rock of the Palisades sill is similar in many ways to basalts.
* Geologists use the terms formation and rock unit interchangeably to mean a single body of rock than can be recognized as such. Thus on Earth, where we have more time, geologists very often trace the outline of a formation by physically following it over the surface. Formerly they walked or rode horses. Today they use jeeps and helicopters. On the Moon, we rely on photographs and telescopes. The abbreviation for formation is fm.