Astronomical Society of Coonabarabran

Bullialdus W: a strange valley, and even stranger "bridge"

by Harry Roberts

Can natural bridges occur on the Moon? Percy Wilkins in the fifties was sure they did! He recorded a huge natural arch at the W edge of Mare Crisium (spanning 2 km or so!)[1]. And Nininger in the nineteenth C. was sure that the Messier pair of craters was in fact the two ends of a tunnel drilled through a lunar ridge by a glancing meteorite![2] A tunnel 6 km in diameter! Both observers had the natural bridges of western USA in mind. No lunar arches exist, of course. But readers may not be aware that a nice causeway DOES (Fig 1)!

I learnt of this intriguing feature recently while checking Rukl's "Atlas"[3] for details of a "valley" near crater Wolf, in Mare Nubium. In the notes to map 53 Rukl says of large crater Bullialdus that "Other interesting objects (nearby) include an elevated "causeway" across the valley Bullialdus W". Fascinated by this reference I planned to observe the area when next possible.

My references did not list a valley, much less a causeway, near crater Bullialdus. Neither North[4] nor Wood's "Modern Moon"[5] mention any Vallis Bullialdus; nor did Rukl include the valley in his feature lists on page 224. A few nights later I had my first look at this strange area. The terminator was well placed and showed both the valley and the causeway, together with short ridges crossing the valley floor, and casting long shadows westwards. As I had not recorded Bullialdus before I chose to draw it in detail, together with the valley and the very obvious causeway. The sky was clear, but "seeing" variable. I used the 8" Celestron (tracking) to map all the major features at 160 mag, and a 10" Dob. at 250 mag. for finer detail. This took two hours and several coffees to complete; but when the session ended I had a very strong sense of both wonder and satisfaction.

Bullialdus itself is a mighty crater, 61 km in diameter, with very distinct bright terraces, surrounded by a wide wreath of radial ejecta. A small cluster of central peaks glowed brightly. My main interest though, was the valley Bullialdus W. It runs NW from Bullialdus' rim, past large flooded Lubiniezky, thence to small crater Agatharchides O (marked O in Fig 1), where the causeway crosses at right angles. From here the valley seems to continue between raised banks, while growing steadily narrower, to terminate at large "ghost" ring Lubiniezky E. The valley seems to have been partly "dammed" by a short bright ridge, just east of the causeway. The valley looks like a flow formation, perhaps formed by thin lavas; maybe the same lavas that flooded Lubiniezky E?

I have no idea what formed the strange causeway! Wood discusses valley formation processes, as does Murray[6], but neither mentions such formations within lava fields like valley Bullialdus W or its strange "bridge." Wood does say: "you'll see secondaries (chains of small craters) extend about one Bullialdus diameter away from the crater's rim except to the northwest. There the secondaries are covered by the same lavas that filled Lubiniezky and embayed the ejecta ridges from Bullialdus. These lavas are daubed by the much younger rays from Tycho."[7] (My italics.) Wood is indirectly referring to valley Bullialdus W, as being formed by lavas that flowed during the period between the formation of Bullialdus (~3.0 billion y. ago) and the impact of Tycho ( only ~110 million y. ago!). I think he means that the valley resulted when two ridges of ejecta material from the big crater held back mare lavas, leaving an unflooded region between the ridges, much lower than the flooded plains surrounding. This is a workable explanation. Subsequent flooding may then have coated the valley floor, wiping away traces of old craters, resulting in a smooth-floored "valley" with flow-like features. These fresher lavas may have been channelled NW to flood Lubiniezky E.

Lunar Orbiter and Consolidated images show Tycho ray material deposited over both the valley AND the causeway. In fact the Tycho ray that passes through the centre of Kies 250 km SE of the causeway, also drapes the causeway itself. Orbiter images show both the valley and bridge to have only few very small craters, confirming they are young formations; but they must be older than Tycho (110 million years).

Fig 2 is a detail from my drawing, showing the causeway (or bridge) where it crosses the valley. Note the two craters on the valley's south side. One is Agatharchides O, the other is unnamed, labelled "X" in my sketch. It is seen to lie at an incline on the bank of the valley, and its lower rim has been covered by lava that formed the bridge; proof that the bridge is a lava formation. Perhaps a fracture in the valley bed, concentric with the huge Bullialdus impact site, became a fissure for lava flow from below. The lava then spread from both sides of the fissure to build up the very regular causeway, which is about 15 km. wide.

Strange formations indeed! You will need good seeing and, say, 150 magnification. I'd be happy to hear your reports.
Enjoy the "Modern" Moon in winter skies.

[1] Dobbins and Baum "O'Neill's Bridge.." Sky & Telescope. Jan 1998. p105. Percy used the 60" Mt Wilson reflector to confirm O'Neill's report made with a 4"!
[2] Wood, C. "The Modern Moon - a Personal View." Sky Pub. 2004. p 94.
[3] Rukl, A. "Atlas of the Moon". Sky Pub. 2004. p 132.
[4] North, G. "Observing the Moon". Cambridge Uni. Press. 2000. p 181.
[5] Wood, Ibid. p 148
[6] Murray, B. Malin, M et al "Earthlike Planets - surfaces of Mercury etc." W. H. Freeman & Co. 1981. p 180 et seq.
[7] Wood, Ibid. p 149.

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