J. Brit. Astron. Assoc., 105, 6, 1995
(Note: The Association is not responsible for individual opinions expressed in articles, reviews, letters or reports of any kind.)
From Mr R. L. Stratford
Although the United Kingdom has been blessed with an extraordinary variety of geological features, there is one thing that we lack: at the present time there are no known British impact structures. Not only does this deficiency seem rather unfair; it is decidedly unexpected when one considers that estimated impact rates imply that a country with the area of the UK (2.44 × 10^5 km^2) should have about one impact structure with D > 20 km. This exception is confirmed by the fact that other European nations of similar size to the UK (e.g. France, Germany, Spain, Sweden, Finland) do have impact structures. In particular, the Ukraine, with an area of 6 × 10^5 k^2, has six impact structures, although only one has D > 20 km.
It therefore seems worthwhile to explore the geology of the United Kingdom with an eye to the possible existence of impact structures. The distinguishing feature of complex impact structures is the presence of a circular inlier (i.e. an outcrop of old rock surrounded by younger rock) associated with the central uplift of the structure. A search of the '10-mile' maps published by the Geological Survey of Great Britain has yielded not one but two structures that show this distinguishing feature, both in the Midlands of England.
The first is the Ashby Anticline of southern Derbyshire and western Leicestershire, which brings up coal measures (Upper Carboniferous) through the Triassic cover to form the South Derbyshire Coalfield. This structure is about 16 km in diameter; its centre is located between Swadlincote and Ashby-de-la-Zouch, at about SK 335190 on the Ordnance Survey Grid, or about 52° 46'.5 N, 01° 30'.2 W. Parts of the northwestern and northern boundaries of the structure are defined by the course of the River Trent between Burton-upon-Trent and Ingleby. Besides the anticlinal uplift itself, the structure is cut up by faults, with throws of up to 300 metres.
The second circular structure lies south-east of the Ashby Anticline, and is a well-known geological feature. It is in fact Charnwood Forest, in Leicestershire. The Precambrian outcrops of this area define an arc of a circle, about 14 km in diameter, centred about 4 km northwest of Markfield; this centre is at about SK 470110, or 52° 41'.7 N, 01° 18'.2 W. One odd feature of this structure is that the course of the River Soar through Leicester and Loughborough, to the east of Charnwood, forms an arc of a circle with a radius of 13 km centred on the Charnwood structure. The circular arc of Precambrian outcrops that forms Charnwood Forest itself might then be interpreted as the central uplift of an impact structure whose outer wall is defined by this outer arc formed by the River Soar. If so, the Charnwood structure and the Ashby Anticline would overlap. It is an odd coincidence that Barwell, where Britain's largest recorded meteorite fell, is only a few kilometres outside the circle defined by this 13 kilometre arc.
So far as I know, there is no positive evidence other than the circular shape and the structural uplift that either the Ashby Anticline or Charnwood Forest are impact structures. There have been no reports of the discovery of shatter cones, of high-pressure polymorphs of silica, or of the other stigmata of impact-induced shock waves. However, it may be that geologists have not looked for evidence of impact in these two structures; and, moreover, there is reason to believe that stratigraphic uplift and other 'tectonic' deformation may be found in impact structures at depths beyond the reach of shock metamorphism. If so, very deeply eroded impact structures may show no evidence of shock and may be very difficult to distinguish from ordinary tectonic domes and anticlines.
The close proximity of the Ashby Anticline and Charnwood Forest implies that, if they are impact structures, they are probably the result of a double impact. There are other examples of double impact structures, for example the Clearwater lakes of Quebec, and the Ries and Steinheim Basins of South Germany. The presence of a double impact structure is, of course, compatible with the existence of binary asteroids among the Earth-grazers, such as 433 Eros, 4179 Toutatis, and 4769 Castalia.
If the Ashby Anticline and Charnwood Forest are impact structures, and if they are the result of a double impact, they are probably of Permian age (about 250 to 300 Myr old). This follows from the fact that the Ashby Anticline brings up Upper Carboniferous rocks, and that both structures are overlain by Triassic sandstones and mudstones. The two structures were probably severely eroded during the Permian period before being buried by younger rocks during Triassic time.
Although the evidence that the Ashby Anticline and Charnwood Forest are impact structures is rather weak, I think that it is worth bringing this possiblity to the attention of any of your readers who are better placed and more competent than I am to examine the structures at first hand.
R. L. Stratford
51 Westbury Close, Hitchin, Hertfordshire SG5 2NF
From Mr Philip Hurst
'The starry sky and one's favourite books! And what, indeed, is the starry sky itself but a book? A book whose letters are of gold and silver; a book whose pages turn with the gliding years. How can such true companions ever pall?'
– The Night Skies of a Year, Joseph M. Elgie (Chorley & Pickersgill, 1910)
For several years one of my main delights in the pursuit of astronomy has been found in the collection and reading of old books on the subject. I would like to define 'old' as anything written pre–1930s, going back to the earliest publication one is able to obtain. (In my instance about 1850). This period naturally includes the great writers John Herschel, R. A. Proctor, Robert Ball, Hector MacPherson and many others, equally important, but less well known.
Old astronomy books have a character entirely of their own, engendered by their quality, content and style, quite unlike what is produced today. In addition to giving us an insight into astronomy of an age passed, they are appealing items in their own right, worthy of study and attention. There are many titles and authors which would repay research done on them.
I am sure I am not the only person who keeps and enjoys their own library of dusty old books on the stars, and for this and the above reasons I would like to try to set up an independent society dedicated to this specific field of interest. At present, I obviously need to discover how many people there are who would be interested in joining such a group.
What could a society like this do? First, it could circulate among its membership essays and circulars, written by members on particular titles and authors. All sorts of research could be undertaken, such as simply investigating how many editions a book ran into, and how many copies were sold. Deeper work could be done also, like carefully studing one author's entire output and analysing its content to assess what contribution and influence it made in its day. (Two especially suitable writers for this treatment would be Proctor and Ball). There are many other possibilities.
I would like to invite anyone who feels they may be interested in this sort of thing – whether they simply enjoy browsing through old books or looking at them more closely – to contact me at my address below.
Philip Hurst
9 Beverley Hills Mobile Home Park, Porton Road, Amesbury, Wiltshire SP4 7LH
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