L50 Cayley Plains
Smooth, light-toned plains surrounding the small crater Cayley, their origin argued over for years between volcanic flow and basin ejecta — the same question that sent Apollo 16 to sample this plains material (the “Cayley Formation”) at its type locality in the Descartes Highlands, some 500 km to the south.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L50 Cayley Plains
Sinus Medii · Light Plains Deposit📉 Vital Statistics
🔭 Field Notes
The Cayley Plains take their name from the small crater Cayley — the crater itself is just a landmark, unrelated to the plains beyond lending them a name. What you’re actually looking for is a smooth, light-toned unit filling the gaps between the crater Cayley and its neighbors Ariadaeus, Hyginus, and Dionysius: younger than the rugged Fra Mauro ejecta from the Imbrium impact, but older than the dark mare lavas that lap up against its edges further east.
- ▶ A Volcanic Theory, Overturned: When USGS geologists first mapped these plains in 1965, the smooth surface and moderate crater density looked like classic ash-flow or ashfall volcanic deposits — a reasonable interpretation at the time. Apollo 16 was sent to a southern exposure of this same unit expecting to sample lunar volcanic rock. Instead, astronauts John Young and Charles Duke found the ground covered almost entirely in breccias — fractured, cemented rock fragments — and the volcanic interpretation collapsed within days of the landing.
- ▶ Impact Ejecta, But Whose?: The current leading interpretation is that Cayley-type plains are fluidized ejecta from one or more large basin-forming impacts, most likely Imbrium and/or Orientale. Studies of the Apollo 16 samples specifically point toward Orientale as a major contributor there, since the returned rocks are more feldspathic than the Imbrium ejecta sampled at the Apollo 14 site.
- ▶ Still Not Fully Settled: Crater counts on different patches mapped as Cayley Formation have turned up a range of ages, some younger than either the Imbrium or Orientale impacts — meaning the plains aren’t necessarily one single deposit from one single event. That lingering ambiguity is exactly why Wood’s Lunar 100 still lists this as a target “of uncertain origin,” even decades after Apollo 16 largely settled the volcanic-versus-impact debate for the Cayley Formation at its own landing site.
📍 Nearby L100 Targets
- L24 Hyginus Rille: A 220 km rille bent around the crater Hyginus, roughly 285 km northwest of Cayley. Long treated as evidence of shallow volcanic dike intrusion beneath Sinus Medii, it’s a useful counterpoint to the Cayley Plains — an actual candidate for lunar volcanism sitting right beside a deposit once mistaken for one.
- L29 Ariadaeus Rille: This long, straight graben begins at the crater Ariadaeus barely 70 km northeast of Cayley — close enough that Rima Ariadaeus is often visible in the same field of view as the Cayley Plains type locality. It cuts directly across Cayley-type material on its way toward Mare Vaporum.
- L93 Dionysius Rays: This small, bright ray crater sits about 75 km southeast of Cayley, on the western shore of Mare Tranquillitatis. Its unusual mix of both bright and dark rays makes it a good next stop after tracing the smooth, featureless Cayley Plains just to its northwest.
🚀 Mission Log
Target Acquisition — L50 Cayley Plains
Forget centering a crater — anchor on a neighborhood
Cayley isn’t a landform you zoom in on; it’s a texture spread across a discontinuous patch of ground. Start at the small crater Cayley itself, in the central lunar highlands between Mare Vaporum, Sinus Medii, and Mare Tranquillitatis, and look for the smooth, light-toned plains filling the gaps between Cayley and its neighbors — Rima Ariadaeus and Hyginus to the northwest, Dionysius to the southeast. The plains are comparatively smooth and lack the strong lineated sculpture seen across much of the surrounding highlands, but the mare lavas beside them clearly lap over — and are therefore younger than — the Cayley material at this locality.
Pick your lighting for the question you’re asking
At the type locality, you don’t need dramatic shadow — you’re comparing crater density by eye between the Cayley plains and the adjacent mare, part of how geologists originally dated this unit. But if you’re hunting the buried, ghostly crater outlines on the floor of Albategnius, you’ll want low, grazing light — the ghosts only read as subdued rounded shadows near the terminator.
Work three sites, three different stories
The type locality near Cayley itself is the namesake exposure — flat, featureless, and deceptively ordinary-looking. Albategnius’s floor is the rare spot where you can actually catch the plains mantling something older beneath them. And the Descartes highlands, south of Ptolemaeus and Albategnius, is where Apollo 16 put down to sample both the Cayley and Descartes formations — visually unremarkable ground that triggered a real course-correction in lunar geology.
Pair it with the Sinus Medii neighborhood
Hyginus Rille (L24) sits about 285 km northwest — a genuine candidate for lunar volcanism right beside a deposit once widely interpreted as volcanic itself. Ariadaeus Rille (L29) begins barely 70 km northeast, close enough to share a field of view with the type locality, and cuts straight across Cayley material on its way to Mare Vaporum. Dionysius Rays (L93), about 75 km southeast, makes a good next stop after tracing the smooth plains to its northwest.
📝 Observation Log — L50 Cayley Plains
0/4 CompleteIs the Cayley Plains visible tonight?
The Cayley Plains surround the small crater Cayley in the highlands south of Mare Tranquillitatis, at 4.0°N, 15.1°E — close enough to the meridian that timing depends mostly on the kind of light you want. (This same type of plains material, the “Cayley Formation,” is what Apollo 16 sampled some 500 km south in the Descartes Highlands.) Best viewing is around First Quarter (roughly Day 6) for terminator relief on the surrounding terrain, with Full Moon offering a second look, when the plains’ smooth, light-toned surface contrasts against the rougher highlands around it. Libration plays essentially no role at this longitude.
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When to Observe the Cayley Plains
This isn’t a single feature you can center in the eyepiece — the Cayley Formation is a discontinuous unit that shows up in patches all over the lunar highlands. The observing challenge here is less about resolving fine detail and more about recognizing the texture and reading the story it tells.
- For the Type Locality: Find the crater Cayley itself, in a lava-flooded region west of Mare Tranquillitatis, near Rima Ariadaeus, D’Arrest, and Whewell. The smooth plains extending from it are the formation’s namesake exposure.
- For Buried Detail: Look for a patch of Cayley plains on the floor of Albategnius crater, where subdued, ghostly crater outlines can be picked out under grazing light — a rare case where you can actually see the plains mantling something older beneath them.
- For the Human History: The Descartes highlands, south of Ptolemaeus and Albategnius, is where Apollo 16 landed specifically to sample the Cayley Formation — worth a look even though the terrain itself is visually unremarkable.
What to Look For
Cayley plains read as flat and smooth, generally lacking ridges or rilles, with an albedo brighter than the mare but not as bright as fresh highland ejecta. Rather than hunting a specific shape, you’re learning to recognize a surface quality — deceptively featureless ground that turns out to have a complicated past.
Near the crater Cayley specifically, the smooth plains show little or no obvious Imbrium sculpture, while the neighboring Mare Tranquillitatis lavas clearly embay — and are therefore younger than — the Cayley material there. The plains at this locality also carry a noticeably higher crater density than the mare beside them. Together these relationships helped pin the Cayley Formation, at least at its type locality, to a window of time after the Imbrium basin formed but before the Tranquillitatis lavas were emplaced.
In places like the floor of Albategnius, the Cayley Formation appears to mantle older, buried craters closely enough that their outlines still show through as subdued, rounded shadows at low sun. It’s a rare direct hint that these “featureless” plains are actually a blanket draped over rougher terrain.
In the Descartes highlands, Apollo 16 astronauts John Young and Charles Duke sampled Cayley material at Flag, Spook, and North Ray craters, and from Stone Mountain overlooking the Lunar Module Orion. Nothing here looks dramatic through a telescope, but this patch of ground triggered one of the bigger course-corrections in lunar geology.
The Science: A Formation That Keeps Changing Its Story
Few named lunar units have had their origin story rewritten as many times as the Cayley Formation. Each reinterpretation was driven by real evidence — and the fact that the story still isn’t finished is exactly why it made the Lunar 100.
The Volcanic Hypothesis (Pre-1972)
When USGS geologists first mapped the Cayley Formation in 1965, its flat, mantling character looked like a natural fit for some combination of ash flows and ashfall from a volcanic eruption cloud — a reasonable reading of the photogeology available at the time.
The Apollo 16 Reversal
Apollo 16 was sent specifically to the Descartes highlands to collect Cayley volcanic rock and settle the question. Instead, Young and Duke returned almost nothing but breccias — heavily fractured impact material, not lava. The volcanic interpretation rapidly lost favor, and the Cayley was reinterpreted as fluidized ejecta from the Imbrium basin, mixed with local material churned up by Imbrium’s distant secondary impacts — though the details of exactly how it was emplaced remained an active debate for years afterward.
An Orientale Twist
A 1975 USGS study by Chao, Hodges, Boyce, and Soderblom complicated that picture further. Crater size-frequency dating led them to interpret the Cayley plains at the Apollo 16 site as contemporaneous with ejecta from the younger Orientale basin rather than the older Imbrium ejecta — and the returned samples were more feldspar-rich than typical Imbrium ejecta from the Apollo 14 site. Some investigators took this as evidence that much of the Cayley material, at and near the surface, actually originated from the Orientale impact rather than Imbrium, though this remains one interpretation rather than a settled conclusion.
What Hasn’t Been Resolved
Later work by Oberbeck and colleagues argued that Cayley-type smooth plains generally can’t be explained as basin ejecta alone, and are more likely dominated by material reworked through secondary cratering — debris thrown out again by countless smaller impacts rather than delivered directly from a single distant basin. Crater-count studies by Gerhard Neukum went further still, finding that patches mapped as “Cayley Formation” in different locations have different ages — some younger than both Imbrium and Orientale — which some take to suggest the name may cover more than one genuinely distinct deposit, though that’s an inference from the age data rather than a directly demonstrated result. The relative importance of basin ejecta, secondary cratering, and local reworking in any given patch of Cayley remains debated.
Most Lunar 100 targets get on the list because they show you something. The Cayley Formation earns its spot by showing you how much lunar geology has had to unlearn — a plain, unremarkable-looking surface that forced a complete rethink of highland volcanism, then dragged in two separate impact basins, and still doesn’t have full agreement on exactly how much of it came from where.