The relationship of the river net–work in the Central Highlands of Scotland to structural and climatic conditions (with thirteen maps)

Abstract

Finding the percentages of all rivers or reaches of rivers of the same orientation in the Central Highlands, a general river -star (fig. 1), on the same scale as that used in individual stars, was plotted for the Central Highlands as a whole.Before discussing the pattern of the general river star the classification of the individual stars into groups will co-ordinate the foregoing description of the fluvial provinces and will throw much light on the main orientation of drainage in the Central Highlands. Broadly two main groups are apparent. (A). Those provinces with drainage largely dependent on structure (strike and fracture) have prevalent "Caledonian trends" closely related to the NE-SW and NNE-SSW strike (map 1). Areas so characterised are coloured yellow on map 10. (B). Those provinces with drainage largely independent of outcrop and trend of folds have prevalent "Grampian trends." The directions involved in this trend lie between NW to SE-S. There are two probable reasons for the predominance of "Grampian trends" in certain regions, first the master rock, jointing perpendicular to the strike and secondly the general NW-SE slope of the country which has controlled certain large rivers like the Garry-Tay and the rivers of Angus. The chief exception the Southerly Drainage of Western Angus (III c, Map 11) most clearly shows the influence of regional slope. But ths "Grampian" trend is not confined to the cis-Grampian provinces; but is also prevalent over a wide area in the Central Highlands where the core of Grampian Mountains is located. Hence the choice of the term Grampian to designate this trend. In contrast, to the long wide "Caledonian" valleys and "Latitudinal" valleys, drainage with "Grampian trend" is confined mostly to short tributaries draining into the major systems of "Caledonian" orientation or into the "Latitudinal" Valleys. That master jointing and the general slope of the land accounts for the Predominance of the "Grampian trends" in certain areas is most probable; and it will be recalled that Hauek found most of the small rivers to coincide with joints (see Chapter One).The three provinces - the Spey Basin, the Southerly Lochs and the Ericht Highlands show .leanings to both "Caledonian" and "Grampian river-stare". in the fluvial Provinces Ericht Highlands were placed in the Grampian Mountain Provinces, because the longest single direction is NW-SE, but its river-star falls under the "Caledonian" group, because the longest trend is SW-NE. The Spey Basin and Southerly Lochs shows the same contradiction: they should be grouped according to trend in the Grampian Mou.ntaïn. Provinces, but their longest drainage directions favour the "Caledonian." croup and "Grampian" respectively.In the Caledonian provinces the river-stars an usually better developed in the NE or SW- quadrants than in the others. As just mentioned, the Spey Basin is an exception, for while the longest trend is "Grampian" (NW-SE), the two longest directions are SSW-NNE and SW-NE, i.e. " Caledonian", and the province was therefore placed in the "Caledonian" group.The prevalence of "Caledonian" trend lines throughout the basin as compared with the smaller number of NW-SE lineations gives added justification for its inclusion into ,that group. The most developed q drants in the "Grampian" stars are NW and SE, except for those stare of the cis-Grampian Provinces in which the SE-quadrant is almost undeveloped.Thera are two provinces with WSW-ENE and SSW-NNE trends respectively, which cannot be included in either of the first two categories. The third group is shown by light blue in map 10. The following terms therefore have been suggested for the classification of river-stars of the Central Highlands.1. Caledonian River-stars.2. Grampian River-stars.3. Neutral River-stars.The distribution of fluvial provinces with "Caledonian' river-stars is shown in map 10. The distribution approximately divides the Central Highlands into two elongated parts, almost equal in size and more or less parallel with the Glen More on the one hand and the Highland Boundary Fault on the other. The dividing line between throat - shown by violet dashes runs from lower Loch Fyne. to Rattray Head, Within the western half which may be called the Caledonian Drainage District, there is a central zone of exceptional character with "Grampian tread" stretching from Etive massif in southwest to Lossie Basin in northeast.

Although the belt of country constituting the largest region with Caledonian, tread and extending between the Spey Basin and the Southwestern lochs appears from the map to be discontinuous, it must be pointed out that the breaks are due to the limitations of the present methods of investigation. The connection does in fact, exist, as is seen from the trend lines east of Loch Leiden and naehsast of Loch Ericht.Turning now to the seater' half of the Central Highlands, there is an area i n which the "Caledonian" influence upon drainage is little apparent. This is called Grampian Drainage District.In the general river-star, the four quardrants are almost equally developed. The oblique "Grampian-Caledonian" crossed axes at first sight does not resemble any pattern of the individual stars, save perhaps, for figure 20 (Badenoch and Lower Spean). The general outline in the form of a cross however, is more akin to figure 7 (South-western Lochs) and figure 13 (Ericht Highlands); though the cross of the general star is not so distinct as those of the two individual provinces named.The explanations submitted in case of those provincial stars also account for the pattern of the general star. The first factor of importance is the presumed .master jointing of the Grampian Mountain perpendicular to "Caledonian" strike. The second is that of slope either general or local. These two combine to make the "Grampian" drainage trend the longest. The third influence is the ''Caledonian" strike and fractures. The resemblance of figures 7, 13 and 20 to the general river -star is impressive evidence of the far reaching influence of the factors particularly operative in these areas.The total length of rivers and lochs as measured from the one-inch maps of the Central Highlands is 15410 miles, of which the longest single direction NNW-SSE takes 1430 miles or 9.2. Next in importance is the SSW-NNW (8.7%), which is closely followed by the SSE-NNW (8.6%). The fourth direction is SW-NE (8.4 %) and is again closely followed by the NW-SE (8.3 %). The least developed direction is E-W 460 miles or 3 %.The longest drainage trend is shown in the general river star is the NNW-SSE (17.8%); the "Caledonian" trend SSW-NNE (15.9%) falls almost short of that trend, in contrast to the order of single directions, the NW-SE trend (15.4%) is higher than SW-NE (15.1%), owing to the greater influence of the "Grampian trend" over that of the

"Caledonian". The W-E trend (7.6%) is least developed in spite of the large "latitudinal" valleys of the original WE drainages The length would be greater if the centre lines of the valleys had been measured. This is due largely to the measurement of the length of rivers which wander in detail on their valley floors; consequently though the valleys lie from west to east their river. contribute considerable mileage to various other directions. This is notably true of the N-S trend which has a quite high percentage (10.8%) in which both the main rivers and especially their tributaries play a large part. In fact it emerges that the influence of a large valley upon the river-star is chiefly to increase the length of the rays at right angles to the valleys axis, for from these directions the main river receives most of its minor tributaries.Perhaps the moat important fact about the Central Highlands drainage now revealed is the predominance of the "Grampian" trend over the "Caledonian!" trend; in the aggregate the transverse drainage is of greater importance than the longitudinal. Furthermore it would seen to boo established that in this region the influence of joints on drainage is greater than that of fault. Moreover the influence of joints combined with that of slope, general and local exceeds that of strike, a point which the writer believes has not received previous recognition.