« AnteriorContinuar »
MAPPING OF SUPERFICIAL ACCUMULATIONS.
EVERY one is aware of the value of a good reliable geological map, exhibiting by distinctive colours the position and extent of the various rock-formations, and by appropriate marks and signs the outcrops, inclinations, and dislocations of their respective strata. With such a map, and a few well-chosen sections, the geological reader is in possession of a fund of information which volumes might fail to convey; and where the contour-lines are marked, or a graduated system of hill-shading is adopted, he has at command all, or nearly all, that is necessary to complete a conception of the physical framework of a country. But this framework of rocks, of heights and hollows, however accurately delineated, can afford him but little idea of the superficial aspects of the land—its soils and subsoils, its mosses, moorlands, sand dunes, and carse-clays, upon which its amenity and agricultural value are so intimately dependent. What is needed for this purpose is the laying down of these surface-formations on a separate sheet in distinctive colours, and with the same precision as the older rock-systems. We have long advocated the necessity for two sets of geological maps—one exhibiting the underlying rock-formations as is ordinarily done, and another the superficial accumulations which mask these formations, and frequently have no lithological connection with nor characteristic dependence upon them. In the present paper we renew this advocacy, and give some of the reasons upon which it is founded.
Were soils and subsoils always derived by disintegration from the subjacent rocks, there might be no great necessity for a set of superficial maps ; but in nine cases out of ten these soils and subsoils have no connection with the rocks on which they rest, but have been brought from a distance, and in many instances of an altogether opposite character. The sands and gravels, for example, which occupy so much of Strathmore, Strathearn, and Stratheden, in Scotland, have little or no connection with the underlying Old Red Sandstone ; and the geological map that indicates these places merely as Old Red Sandstone, conveys no idea whatever of the immense masses of glacial drift which constitute their surface. Again, a vast extent of estuary silts constitutes the Carse of Gowrie and the Carses of Falkirk and Stirling ; but no one looking at an ordinary geological map could learn anything of this fact; or if these carseclays were indicated by distinctive colours, these would obscure the Middle Old Red that lies beneath the former, and the Coal-formation on which the latter reposes. Further, no one could know from our ordinary geological maps that the sand-dunes which extend so largely between the Tay and the Eden reposed on boulder-clay, and this again on the Old Red Sandstone ; or that the recent marine silts of the Lincolnshire fens were underlaid by drifts and clays, and these again by the Chalk and Greensand. Or, still further, who could know from a map of the old rock-formation where peat-mosses, lake-silts, or boulder-clays occurred; could understand where the old rocks came hard and bare to the surface, or where masked by superficial accumulations many fathoms in thickness ?
It is true that on the maps of the Geological Survey an attempt is made to indicate "alluvium, peat-moss, &c.,” by a distinctive colour, but this attempt more frequently misleads than instructs. Lithologically speaking, peat-moss has no connection with alluvium; they are formed by totally different agencies; and the former may be thousands of years old, while the latter is still in course of formation. Not only so, but the same pale tint that indicates “alluvium" includes not only recent river and lake silts, but old lake-silts, glacial sands, and gravels and clays, thus confounding things not only of totally different composition and character, but of totally different chronology. What is wanted is a separate map, indicating by different colours the position and extent of recent river alluvia, of lake-silts, marls, peat-mosses, ancient estuary deposits or carse-clays, boulder-clays, glacial sands, and gravels—of all those accumulations, in fine, which overlie the old rock formations, and which has each its own geological history in time as well as in mode of formation. Such a mapping could not fail to be of use to the farmer, to the land-agent, landscapegardener, civil engineer, road-maker, and all those whose business leads them to deal with the soils, subsoils, and general superficies of a country.
Many years ago we constructed such a map of Fifeshire, and the late Professor Johnston, while chemist to the Highland Society, invariably exhibited it along with the ordinary geological map when illustrating to the farmers of that county the intimate connection between geology and agriculture. Had he exhibited the mere rock-formations, the connection would have been apparent only in the case of the trap-soils, which constitute but a small fraction of the county, the greater portion being composed of old glacial clays and gravels, of lake-silts, peat-mosses, and sand-drifts, which have no lithological relationship whatever with the underlying Old Red Sandstone and Carboniferous formations. Let any one consult a geological map of the district or county with which he is most familiar, and he will find how little information it conveys respecting the superficial accumulations; and then let him reflect on the connection between these accumulations and the aspect and fertility of the country, and he will perceive at a glance the importance of such delineations as we are now contending for. As well, indeed, might the artist attempt to present the aspect and outlines of the human form by its skeleton, as the geologist the physical geography or geology of a country without its superficial formations.
But altogether independent of their practical or industrial value, such mappings are absolutely necessary in a purely scientific point of view. The geology of a country by no means closes with even the latest of the so-called “Stratified systems.” The old boulder-clays and glacial drifts, the carse-clays, the lake-silts, and peat-mosses, have all their tales to tell of change and time-of changes as important in the history of the globe as those that accompanied the older rock-systems, and of time so vast that we vainly endeavour to reckon it by any standard of years and centuries. What changes have taken place in the physical aspects of these islands of ours since the glacier filled their mountainglens and the iceberg floated on their estuaries; what mutations in their vital aspects since the whale and the seal frequented their sea-lochs—the mammoth, the reindeer, and Irish elk their plains and valleys—and the beaver, bear, and wild boar their rivers and forests; and what revolutions in human history since the pre-Celtic savage paddled his log-canoe across their lakes, made his feast of shells by their sea-shores, or sheltered himself in their caverns ! And yet of all these changes we have no other record save that preserved in these old carse-clays, lake-silts, peat-mosses, and cave-earths—those Superficial Accumulations, in fine, which, though often described in words, are seldom or ever attempted to be delineated on our geological maps, or traced in our geological sections.