Page Two
The level of contact between the solid crust of the Earth and the semi-liquid rocks beneath is marked by an interface known as the "ductile-friable transition", at present 2.5 - 4 kilometres beneath the surface (in the past this may not have been so deep). The explosive impact of a body of rock ranging in size from a "battleship" scaled meteorite to a medium "mountain" sized asteroid, moving at a speed of several Km/Sec would send radial fractures out from the central impact point as well as creating rings of "shock-wave" tangential fractures as the outward impetus of the explosion reacts with the inertia of the surrounding crust in concentric circle formation centred on the initial point of vapourisation. Just such "ring fractures" are clearly marked on several of the Voyager pictures of the moons of Jupiter. One in particular, of Callisto, shows a dramatic crater with concentric rings reaching nearly halfway round a world one third bigger than the Earth's Moon. On Earth these fractures would reach down to the ductile-friable transition, and thereafter become self perpetuating as the soft rocks of the lower ductile area through time solidify to become friable (brittle) rocks in their turn, and the fracture imprint is propagated downwards.

The Earth is distinguished from its companion worlds Mars, Venus and Mercury by the presence of large masses of water on its surface, as well as in its atmosphere. Though the earlier most intensive bombardments took place when terrestrial volcanic action was beginning to create both water and atmosphere, the later more sporadic events must have triggered off chain reactions composed of rapid shifts in sea floor level, volcanic eruptions, and land movements, as the Earth had acquired both its seas and its land (and also an actively mobile continental crust). Both on land and in the sea, life in its early stages must have seen rapid changes in environment as syncline changed to anticline and land rose and fell due to intermittent but massive shocks to the crust.

The sudden excavation of even a medium sized 3OKm diameter crater anywhere in the vicinity of the shallow inland seas, such as at a later date - "lake Orcadie", would cause extensive tidal waves with resultant deposits of mud over the surrounding area covering hundreds of square kilometres. Dust slowly falling back through the air to the deserts and water basins, would form blanket-like but finer bands of sediment, interspersed with mud deposits and volcanic ash layers in the banded strata that are so characteristic of ancient rocks. The many thousands of feet of these early sediments could have thus been created by a combination of sporadic but overwhelming impact forces in conjunction with the routine sandstorms, flash floods, and massive tides of the youthful planet itself.

The case of the "Iridium" layer which has recently been noted by independent observers both in sea-bed sediments and in rocks on land in many parts of the world, and universally dated to 65 million years ago, is increasingly being attributed to a single massive meteorite on the scale of the Vredevoort Ring impact. This global 1 cm layer of clay is found at a level corresponding to the Cretaceous-Tertiary boundary and exactly contemporary with the widespread extinction of the Dinosaurs and many other species.

The calculations involved in the prediction of the size of meteorite required to produce the energy for such an event involve fitting the mass and speed of the meteorite or asteroid into the "simple" kinetic energy equation E = ¹/₂MV² (V=velocity) so that with no newfangled nuclear energy involved, an asteroid 10Km in diameter (for instance one of the Apollo group whose orbits cross that of the Earth) moving at between 30 and 40 Km/sec would create an impact crater 175Km across and spread pulverised rock dust into the stratosphere to settle as a thin layer across the surface of the Earth (New Scientist 10 Jan. and 6 March 1980).

The kinetic energy having been converted into heat at the moment of impact, the explosive power created is thus estimated at 100 millions megatonnes of TNT which if the event happened at sea, would be sufficient to vapourise 16000 cu. km of water which is enough to provide 2cm of rainfall over the entire Earth. Iridium is considered to be an extra-terrestrial mineral and is found both in meteorites and on the Moon, so that the lcm clay layer noted above must have come in considerable quantities from outside the Earth at that time.

The surface of the Moon outside its mountain areas has been verified as a 50m dust layer overlaying a deeply cracked and faulted hard rock sub- surface with the faults often showing up as straight line features criss- crossing the dusty surface. These features are known as "rills" when found singly, "horsts and graben" when parallel, and as "sinuous rills" when linked in zig-zag combinations. Frequently the rills are lined with craters and often end in craters or domes of solidified lava. These small craters are not thought to be impact originated but to be caused by volcanic explosions released from within the surface as the fault or crack spread. Other linear features are rows of minor craters (or craterlets) centred on major impact craters and thought to be caused by fragments ejected from the primary impact or, once again, to be of volcanic origin and caused by explosions along radial cracks spreading out from the impact centre itself. The lack of water and air on the Moon has preserved many of these fragile sandy forms since their creation. On the Earth however they were buried under many thousands of feet of sediments which covered the hard cracked rock surface with its counterparts of the rills, lava domes, craters and crater chains of the lunar surface. Later the whole mass turned to stone, was metamorphosed and then eroded, but the impact forms were not destroyed, in fact they re-emerged altered into repetitive landscape features found everywhere in Highland scenery and may well underlie topographic elements all over the world.

Accordingly the quickest way to verify whether there are traces of concentric or radial linear patterning (such as should be detectable as the hallmark of an ancient impacted area), always supposing that any such patterns have not been blurred out of existence by later geological events, would be to fit the co-ordinates of all accurately mapped geological features in selected areas of the N. W. Highlands into a computer programme, and after a suitably specific set of characteristics have been isolated, print out those sets of co-ordinates which conform to interpretation as linear features and also to determine graphically what patterns emerge. The only features plotted accurately enough by the (H.M.) Ordnance Survey theoretically to provide an instant check if such a simple programme were to be run through a computer, are the spot heights of hill summits end road high points, so that if these clearly defined positions are analysed together with other points such as the tips of sharp promontories and the positions of fault generated features such as waterfalls, edges of steep cliffs etc. then it might be possible to pin-point any recurrent "tell-tale" left-over imprints of shock patterning that may have survived.

Short of having a computer to "join up the dots" it is just as much fun to use the human eye and a ruler to try to determine what if any regular features can be determined in any section of a O.S. map of the northern Highlands. That such features do quite clearly display themselves if such a systematic search is carried out is the subject of the last part of this article. It is hoped that the previous paragraphs will form a rough guide to a new and quickly expanding subject that will no doubt continue to produce interesting and unexpected results. The presence of straight line hill summit systems with great numbers of examples in the Moine schist and Helmsdale granite hills of Northern Sutherland was noted by the writer in 1976 (Caithness Field Club Bulletin Vol. 1 No. 8) though the systems came to light through an independent investigation of the relationship of iron-age brochs to the structure of the surrounding terrain (if the brochs were to function as signal stations as recorded in local tradition).

Finally here are some specific terms to define the forms under discussion.

1      "Imprint Chain"

An Imprint Chain is a linear series of geological and geomorphological features such as, hill summits, high points in valleys, cliffs, river courses, meltwater channels, and ridges, sharing many features with fault lines but having a deeper origin in ancient fractures in the Earth's crust, and sharing the vertical nature of rifts.

2.      "Mayonach Centre" (from Gaelic "Meadhonach" = "Middle")

A Mayonach Centre is a central point, often a hill summit, from which several Imprint Chains radiate outwards.

An "Acute" Mayonach - these are found in rings concentrically round some Mayonach Centres and seem to have their origin in tangential or chord formations produced by such waves and perhaps underlying ancient crater rims.

3.      "Petrification Relief Core"

This is an attempt to explain the phenomenon noted by Godard (Recherches De Geomorphology En Ecosse Due Nord-Ouest) and commented on by Sissons (The Geomorphology Of The British Isles - Scotland - Methuen & Co. P.24 and 25) whereby basic hill contours heights and forms remain morphologically constant despite the actual rock type of which the hills are formed. Thus the factor determining the outline of any hill or mountain is dependent on the attitude of the minute crystals in the sediments as they petrify (or in the case of metamorphic rocks resolidify) in that crystals as they form tend to align on minute electric fields in a similar manner to iron filings round a magnet. Whether or not the rock is granite or limestone, this has affected the relative resistance to weathering of the whole rock mass irrespective of the individual sedimentary or metamorphic layers. Viewed from this point of view the high and low points in the silhouette or a range of hills are the same as the peaks and troughs of a graph showing the resistance to weathering of the rocks of which the hills are composed, and the peaks coincide with the intersections of ancient cracks formed during periods of crater formation shared by the Earth and the Moon.