Cayley Formation

Lore & Background

The Cayley Formation was first recognized in 1965 by the Astrogeology group of the United States Geological Survey, having previously been mapped as part of the Fra Mauro Formation. Its type area lies east of the unrelated crater Cayley, for which it is named. The formation is described as having a generally flat and smooth surface, lacking ridges and rilles, with an intermediate albedo similar to the Fra Mauro Formation and higher crater density than the Procellarum Group. It appears to mantle subjacent terrain, with buried ghost craters evident in some areas.

Reader's Guide

The Cayley Formation was initially interpreted as volcanic materials, possibly ash-flow or ash-fall tuff with intercalated ballistically deposited layers. This interpretation was based on its plains-forming character and stratigraphic position: it overlies Imbrium sculpture but is embayed by mare material, indicating it postdates the Imbrian sculpturing event but predates the mare. One of the main objectives of the Apollo 16 mission in 1972 was to obtain samples of the Cayley Formation, alongside the adjacent Descartes Formation. Most scientists predicted volcanic rocks would be found, but astronauts Charles Duke and John Young returned almost nothing but breccias. The lack of volcanic rocks forced a reinterpretation: the Cayley Formation is now considered a fluidized ejecta deposit resulting from the formation of the Imbrium basin or the Orientale basin, which occurred after the Imbrium basin-forming event. This shift in understanding highlights the importance of direct sampling in lunar geology.

Did You Know?

• Apollo 16 astronauts Charles Duke and John Young found almost nothing but breccias in the Cayley Formation, contrary to predictions of volcanic rocks.
• The formation was reinterpreted as a fluidized ejecta deposit from the Imbrium or Orientale basin-forming events.

Discovery and Cartographic History

The Cayley Formation is a patchy, plains-forming deposit that blankets portions of the lunar surface. It was not always recognized as a distinct geological unit. For years, the material that would later bear the Cayley name was simply folded into the broader Fra Mauro formation on early lunar maps. It was not until 1965 that the Astrogeology group at the United States Geological Survey identified it as a separate entity in the central near side of the Moon. The designation "Cayley" was borrowed from the nearby crater Cayley, though the type area sits just to the east of that crater, roughly between 3.5 and 4.5 degrees north latitude and 15.5 to 16.25 degrees east longitude. The formation's discontinuous nature made it a cartographic challenge. During the Apollo era, geologists extended their mapping well beyond the near side, tracing the Cayley into regions such as the Mendeleev basin on the far side. This broader recognition confirmed that the unit was not a local curiosity but a widespread, if fragmented, component of the Moon's geological record.

Surface Expression and Physical Character

When the USGS first described the Cayley in the 1967 edition of the Geologic Map of the Julius Caesar Quadrangle (sheet LAC-60), they painted a picture of a remarkably uniform landscape. The surface is generally flat and smooth, conspicuously lacking the ridges and rilles that characterize many other lunar terrains. Rather than building up topography, the material appears to drape over and mantle the terrain beneath it. In certain areas, particularly around 3°30' N and 14°40' E, the formation partially buries older craters, leaving what geologists call ghost craters—faint circular shadows where the original features remain discernible beneath the deposit. The albedo of the Cayley sits at an intermediate level, roughly matching the Fra Mauro Formation and exceeding that of the Procellarum Group. Its crater density is notably higher than the Procellarum Group, suggesting it is older than those mare basalts. Near the Cayley crater specifically, the formation sits atop Imbrium-age sculpturing yet is partially embayed by younger mare material, placing it in a precise stratigraphic window between the Imbrian basin event and subsequent mare flooding.

Apollo 16 and the Breccia Surprise

In 1972, the Apollo 16 mission set out with a clear geological mandate: collect samples from the Cayley Formation and its neighbor, the Descartes Formation. The scientific community had a strong expectation. Given the formation's smooth, plains-forming appearance and its intermediate albedo, most researchers predicted that the astronauts would uncover volcanic rocks—perhaps basaltic flows or tuff layers indicative of ancient lunar volcanism. Instead, astronauts Charles Duke and John Young encountered a landscape dominated almost entirely by breccias. The team conducted intensive sampling at three small craters—Flag, Spook, and North Ray—and also gathered material at the landing site of the Lunar Module Orion and at geology stations positioned to the southwest. The absence of the expected volcanic lithologies was a genuine shock to the lunar geology community. It meant that the entire working model for what the Cayley Formation represented was fundamentally wrong, and a major reinterpretation of its origin was about to follow.

From Volcanism to Impact Ejecta: A Revised Origin

Before Apollo 16, the prevailing interpretation of the Cayley Formation was volcanic. Geologists envisioned it as a deposit of ash-flow or ash-fall tuff, possibly intercalated with layers of ballistically ejected material—essentially the products of ancient lunar eruptions spread across the surface. The smooth, flat character of the terrain and its intermediate albedo seemed to support this reading. The breccia-rich sample suite returned by Duke and Young shattered that model. With no volcanic rocks to anchor the hypothesis, researchers were forced to reconsider. The new consensus identified the Cayley as a fluidized ejecta deposit, the product of a massive impact event rather than volcanic activity. Specifically, it was linked to the formation of either the Imbrium basin or the Orientale basin, the latter having formed after the Imbrian basin-forming impact. This reclassification transformed the Cayley from a record of lunar volcanism into a record of one of the Moon's most violent episodes, preserving the chaotic aftermath of a cataclysmic collision in its flat, ghost-cratered plains.

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