Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 9

Who cares for a cobble-stone? It is a kind of nuisance anywhere—so most people think. The farmer would be glad to have every one of them carted from his fields. I have seen land so thickly covered by them as to be almost impossible to cultivate. In some regions near the coast, in New England, the loose rounded stones lie so close over hundreds of acres that I have traveled by simply stepping from stone to stone.

You will notice that cobble-stones are of various sizes. In fact, it is difficult to state where a cobble-stone is small enough to be called a “pebble,” and just where it is too large to be a cobble-stone. Pebbles differ from them only in size. Pebbles are hard and rounded, and seem formed of the same kinds of rocks; and the large, rounded, loose stones, which lie scattered over the earth’s surface, are in every respect only a larger style of cobble-stones. It is plain that these are all one class of rocks. So it has been decided; and geologists call them bowlders. This is an old name used by common people before the science of geology existed, because these stones are rounded like balls or bolls; and, being loose on the surface, are apt to be bowled about. Even grains of gravel and sand appear to be of the same nature as bowlders. You will also notice, especially, that these rocks are all separate and detached, as well as rounded, and they are of various colors and mixtures of colors. They are apparently different kinds of rocks, which by some means have been brought promiscuously together. Ledges of rock, which you must have noticed many times, are generally all one kind of rock. They extend long distances, and continue under the earth. Should a ledge of rocks become broken up, and the fragments, large and small, have their angles rounded off, and the whole then be scattered over a township, far from the ledge, the result would be much like what we see in our actual bowlders. From all we know of rocks we are constrained to believe that our bowlders are rounded fragments of broken up ledges. But where are the ledges? Not in the next township or county. Perhaps not in the next state or province. They have strayed far away from their native ledges. They are “lost rocks.” Now, it would be very interesting to know where the parent ledges are; and it is curious how these fragments have been transported so far, and how they became so rounded, instead of remaining angular, like the stones blasted from a quarry.

Indeed, the more we think about this, the more astonishing the facts appear; for we call to mind that just such bowlders are scattered all over our northern states, and they lie buried beneath the surface in countless numbers. And the very sand and gravel, to the depth of many feet, is only the same kind of material in a finer state. What an incalculable amount of work has been accomplished in transporting all these materials so far that the places from which they came have been lost, and can not be found. Suppose it were necessary to cart all the loose stuff on a township to a distance only one mile further, on what terms do you think the contract would be taken? But all that stuff has been moved—not one mile alone, but many miles, to a certainty. And not alone the stuff on a township, but the stuff on ten thousand townships. The work was not done, you say, by the slow process of hauling in carts. No, indeed; but it was done somehow, and it is the same job whether performed by Nature’s method or by human muscle. Think of that.

Now, what do you imagine was Nature’s method? Would it not be a grand discovery if we could find out? It was Agassiz that ascertained this, and the discovery gained him great fame. But there were others who came very near to the same discovery. Suppose we could stand by and see Nature in the midst of the job—carting and dumping on the bare surface of the rocks, the gravel and sand and clay so indispensable to render the surface of the earth habitable for man or beast or plant. I think we should consider it a grand revelation of the method and mind of the Author of nature. I am happy to assure you that we have found out pretty precisely how this immense and beneficent work was done; and this knowledge is a part of geology, and we are intending to talk these matters over until you grasp the knowledge. That is, we shall put you where you will be as a bystander gazing on the progress of the great work as Nature herself carries it on.

Many bowlders attain to dimensions which are truly enormous. The largest are found in northern New England and Canada. As we proceed southward, the average size diminishes, and south of the parallel of Cincinnati, bowlders are entirely wanting, except along the Appalachians. In New Hampshire are many immense bowlders, which have excited the wonder of all who have seen them. Several of these have been described and figured by Professor C. H. Hitchcock in his Report on the Geology of New Hampshire. The Churchill Rock of Nottingham is 62 feet long, 40 feet wide and 40 feet high. It contains 75,000 cubic feet, and weighs 6,000 tons. Close by is Chase Rock, 40 feet long, 40 feet high and 30 feet wide. Vessel Rock, in Gilsum, now split by frost, weighed 2,286 tons. The Green Mountain Giant, in Whittingham, Vermont, weighs 3,000 tons; and a bowlder formerly existing at Fall River, Massachusetts, weighed 5,400 tons. At St. Ignace, in the Upper Peninsula of Michigan, lies a porphyry bowlder 25 feet in height. Mr. G. M. Dawson, in his report on the geology of the North-west Territory, describes a quartzite bowlder 42 feet long, 40 feet wide and 20 feet high, and another nearly as large. It appears that the greater part of North America, down to the latitude of Cincinnati, is overstrewn by incoherent materials containing bowlders. The situation is similar in Europe; and there, also, certain “lost rocks” or “erratics” attain vast dimensions. The “Pierre à bot” (or Toad-stone), on the Jura Mountains, about two miles west of Neufchâtel, contains 40,000 cubic feet, and weighs 3,000 tons. As far south as the Lake of Como, bowlders of large size are very frequently encountered.

Often these lost rocks lie perched on the summits of sharp cliffs; and sometimes we find them so nicely poised that the strength of a man suffices to give them a tilt. They are then called “rocking stones.” In Hanover, New Hampshire, half a mile east of Dartmouth College, is a rocking stone 12 feet long, 10 feet wide, 5½ feet thick, containing 480 cubic feet. In Goffstown is one 8 feet high and 42 feet in circumference. In Barre, Massachusetts, is one having a smaller bowlder on its back, which, when in motion, suggests the idea of a child’s rocking horse. One in Fall River, poised on granite, weighs 160 tons.

We find bowlders at various altitudes, from the level of the sea, to the height of perhaps six thousand feet; but above this, though rock fragments are extremely numerous, they are mostly angular, and appear to be derived from rocky ledges close by. They are not "lost rocks." The summit of Mt. Washington is covered by a bed of angular fragments, and such fragments are common for two thousand feet below the summit. Lower than this, rounded bowlders are abundant. Professor C. H. Hitchcock, however, thinks he finds real transported rocks to the very summit. The great quartzite bowlder in the North-west Territory, Canada, is 3,250 feet above sea-level. Many others in that part of the continent are up to 4,400 feet in elevation; and, in one region, attain 5,280 feet. Some erratics on the flanks of the Sweet Grass Hills lie at an elevation of 4,660 feet. The Pierre à bot, in Switzerland, is 800 feet above Lake Neufchâtel, which lies itself 1,427 feet above sea-level.

We observe, in passing over the country, that the larger bowlders are northward; while toward the south, their average size diminishes to cobble-stones, and finally, all indications of transported rocks disappear. Since we have concluded that all these lost rocks have been removed from extensive ledges somewhere, it seems probable that the direction of these ledges is to the north. We notice also, that bowlders of any particular kind become more numerous, as well as larger, as we proceed northward. In fact, in some cases, by following up a train of bowlders of a particular kind, we trace them to their origin. That origin is often sixty or one hundred miles, or even two hundred miles away. Such are the distances to which the forces of Nature have moved much of these incoherent materials.

It is not always possible to trace bowlders to their source by following back a train. But we can always consider where is the nearest locality of bed-rocks of the same kind as any particular bowlders. For instance, in Connecticut, we can find bed-rocks sometimes, in the near vicinity, but at other times, not farther away than Massachusetts. In Ontario, the nearest sources of the bowlders are in the regions east and north of Georgian Bay. At Chautauqua, the nearest bed-rock for the hard bowlders is beyond Lake Ontario and Lake Simcoe. In Michigan, the nearest source is north of Lake Huron and south of Lake Superior. So in Indiana, Illinois, and the north-west generally, we must go northward to find rocks in place which are of the same sorts as the bowlders. This is plainly demonstrated in the case of bowlders of native copper, which are frequently found in Wisconsin, Illinois, Indiana, Michigan, and Ohio. There is no other credible source than the native copper region south of Lake Superior. So, in the case of the Pierre à bot, near Neufchâtel, the nearest credible source is the Mont Blanc chain of Alps, seventy miles distant, and separated by the valley of Switzerland and the Lake of Geneva.

We seem authorized to conclude, therefore, that the bowlders have been transported generally from the north; that many of them have been moved one or two hundred miles; that they have sometimes been borne over regions which are now lake-basins; that they have been carried, at times, to higher levels than their origin, and much higher than valleys over which they passed; that a vast mass of sand, gravel, and clay was moved with them, since they lie imbedded in these accumulations, to the depth, sometimes, of one or two hundred feet.

These conclusions will be borne in mind when we come to seek for the nature of that tremendous agency which could have performed so vast a work over all the northern half of at least two continents. (Talks IV and XLVII.)