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Popular Science Monthly/Volume 46/December 1894/The Geology of Natural Scenery

THE GEOLOGY OF NATURAL SCENERY.
By FREDERICK J. H. MERRILL, Ph. D.

IN this age of scientific progress, poets and prose writers may-no longer tell us of "eternal hills" and streams which "flow on forever." The science of earth-knowledge shows that the mountains are but creatures of yesterday in geologic time, and to-morrow may be cast into the sea through the agency of that wondrous source of power, the sun. Moreover, as the study of Nature advances, the veil of superstition is torn aside and the mystery is dispelled in which ignorance involves the causes of natural phenomena; so that to the mind a broader field of view is opened, while the knowledge of the changes which have occurred and are taking place about us on the earth adds much to the inspiration which beautiful scenery gives, without detracting from the poetic quality of the feelings it excites.

Foremost among the minds which have felt this inspiration and expressed their pleasure in the English tongue are those of Byron and Sir Walter Scott. But, in their time, truth in description of natural phenomena was not expected. The poet interpreted at will the scenes which impressed him, and the reader, charmed with the rhythmic cadence of the lines, was content to admire the beautiful clothing of the writer's thought, little caring whether the phenomena were truthfully described or properly explained. But from the writers of the future we must expect a reconciliation between scientific truth and poetic fancy.

It is fallacious to claim that natural science bridles too closely the poetic mind. Science is but truth, and what is not true has no part in science. Surely, in the presence of the most famous works of man the knowledge of their history in no way detracts from the interest which they inspire. Mountain and valley, hill and plain, river, lake, and sea, have each their history for the observer to read; and-instead of imagining convulsions of Nature and picturing speculative catastrophes, the mind may dwell on the action of simple and familiar agents working through long ages and bringing about by slow degrees that beauty and grandeur of terrestrial form which is never caused by cataclysms.

To instance this hypothesis we need not journey far; our own land is rich in scenery unsurpassed, and whether we seek our illustrations in Rhineland or the valley of the Hudson, among the Alps or the Sierras, we are only turning the pages of one great book. But though the turning of those pages might go on forever, and though the most earnest student can never know their number, the history which they contain is intelligible to all who would know it, infinite in the variety of its expression and charming in the simplicity of its style.

Among the varied scenes which charm the eye there are none so pleasing as those which combine both land and water; and the vicinity of New York city, with its varied and rich combination of water and landscape, will serve as a fitting example to introduce the subject.

In order to consider the origin and history of this scenery, it is necessary to look backward a short distance in geologic time. Ten thousand years or more ago the continental glacier extended from the highlands of British America southward to Long Island, and at its margin, where it may have been a thousand feet or more in thickness, accumulated a mass of rock débris brought in part from northern New York and New England. When the advent of a warmer climate caused the ice sheet to retreat, its terminal mass of debris, called by geologists the moraine, formed a range of lofty hills, locally known on Long Island as the "backbone," and which crosses Staten Island, northern New Jersey, Pennsylvania, and Ohio, continuing northwestward far beyond the Mississippi.

The land was then much depressed to the northward, and the waters of the ocean communicated with New York Bay through the valleys of the St. Lawrence, Lake Champlain, and the Hudson. In place of Lake Champlain was a great estuary, its surface four hundred feet above the present tide level, stretching from the foothills of the Adirondacks on the west to the Green Mountains on the east, and of which the beaches with their sea shells may still be found. Into the estuary which occupied the valley of the Hudson the streams from the surrounding country brought their sediment, gradually forming deposits of clay and sand, while on the outer shores of Staten and Long Islands the ocean waves broke sixty feet or more above their present level. Gradually the continent rose from its submergence, the elevation of the land caused the ocean to retreat, and the Hudson, the Mohawk, and their tributaries sought an outlet southward to the sea. The rivers thus revived cut a channel in their seaward journey deeper and deeper through the sand and clay deposited in the Hudson Valley, and near the end of their united course carved through the moraine a passage which we call the Narrows.

When Long Island Sound is viewed from the Westchester shore the lofty hills of the moraine form a most picturesque background to the blue waters of that beautiful estuary which but for the advent of the ice sheet would probably have been separated from the ocean only by a low, sandy plain. From the summit of the moraine in Prospect Park, Brooklyn; from Harbor Hill, or from any of the numerous eminences on Long Island, the widespread view of sea and land impresses itself upon the observer, and in every way in which this range of hills is brought to our notice we are unconsciously led to appreciate the scenic importance of this legacy of the continental ice sheet.

From the scenery of Long Island Sound and the moraine one may turn to that of the Hudson River.

On the east shore of this noble stream is a terrace, picturesquely dotted with handsome country seats, which extends almost continuously from New York to Peekskill, and, after a brief interruption where the steep slopes of Anthony's Nose and the Dunderberg form the lower gate of the Highlands, reappears at Garrison's and Cold Spring and forms the plain upon which the Military Academy is located, at West Point, This terrace is about seventy-five feet above tide at the Riverside Park in New York city, and increases in height northward to about one hundred and twenty feet at the State Camp near Peekskill and one hundred and eighty feet at West Point. North of the Highlands the remnants of terraces may be seen on both shores of the river as far as Troy and throughout the valley of Lake Champlain, increasing in height northward to the St. Lawrence River, where, in the vicinity of Montreal, they have an altitude of over five hundred feet.

In these terraces is recorded ineffaceably the history of the continental submergence previously mentioned. They are the remnants of the deposit of sand and clay formed in the Hudson and Champlain Valleys when submerged, and the surface of the highest terrace indicates approximately the old sea level.

These terraces, relics of Quaternary subsidence, are not, however, the only interesting geological phenomena to be seen along the valley of the Hudson. The bold, precipitous Palisades, known as widely as the river which they overlook, call our attention to a period of volcanic activity in Mesozoic time. These lofty cliff's form the margin of a sheet of trap or igneous rock, three hundred to four hundred feet in thickness, which was forced to the surface in a molten state between the beds of red Triassic sandstone which form the eastern border of northern New Jersey and of Rockland County, New York.

Throughout most of its extent the Palisade range has been leveled off by wave action at some remote period of subsidence, while immediately west and north of Nyack a portion which has escaped erosion rises in high peaks with irregular outlines.

Leaving the Palisades behind us, we enter the gorge of the Highlands. The rocks which form these rugged steeps are of the oldest and hardest in our State. Though they have yielded to the cutting of the river current in past ages, they resist the degrading influences of the atmosphere and rear their summits far above the surrounding country.

When Storm King is passed in our northward course, the scenery changes completely. The gneiss hills of Westchester County, abruptly rolling to the east, the craggy Palisades and the Highland mountains give way to a region of moderate elevation and gently rolling surface, abruptly notched by the river valley. This is a belt of slate and limestone territory extending from Vermont to Alabama, and of world-wide fame in its fertility, known in Pennsylvania as the Great Valley, and in Virginia as the Valley of the Shenandoah.

From Cornwall to Troy the scenery of the Hudson shows but little variation, as the geology does not change materially except where the hard Catskill sandstones and conglomerates rear their lofty pile, furrowed, channeled, and notched so deeply and widely that of the original plateau only the present mountain remnants may be seen.

As a geological map of the Hudson Valley would tell what variety of scenery might be expected, so one may forecast the scenic pleasures of the Rhineland without entering its territory.

From Basle to Mainz the Rhine flows through a low plain of Quaternary age. In this portion of the valley the natural scenery is monotonous, varied only by distant views of hill country. From Biebrich to Bingen the river skirts the foot of the Rheingau, which affords on the right bank the picturesque scenery of that famous vine-clad slope. From Bingen northward the river has abruptly cut its channel through the Devonian rocks which extend from Bingen nearly to Bonn, and, in meandering through the defiles of its valley, exhibits to the traveler a variety of imposing scenery which in remote centuries as well as in recent times has inspired the poet and the minstrel, and in the richness of its historic associations and its relics of the feudal system is unsurpassed and probably unequaled by any other valley in the world. From Bonn to the sea the country bordering the river is of more recent origin and proportionately deficient in relief and scenic variety.

In connection with these instances may be stated a general principle of the relation of scenery to geology: the older the formation, the higher its relief and the more striking its scenery.

To exemplify this principle in its broadest form let the reader compare the topography of some of the newest and oldest formations. For instance, compare the London basin with the Scottish Highlands, or Dover Cliffs with Mount Snowdon, or the drift hills of Long Island with the Adirondack peaks, or the sand plains of Florida, Georgia, and the Carolinas with the mountain region about Asheville. The most inexperienced observer can not fail to note the contrasts.

To this rule, as to all others, there are exceptions. Some of the highest mountains of the world are composed of Tertiary rocks, and volcanic cones, which vary greatly in age, are characterized by similarity of form.

The dependence of scenic contrast on geologic age may well be exemplified along another well-beaten line of travel.

If we journey from Paris to Lausanne we find the Tertiary plain of France comparatively monotonous; but in crossing the Jurassic rocks between Tonnerre and Dijon we find them deeply incised by valleys, adding much to the picturesqueness of their scenery by their high relief. At Dijon we are on the western margin of the valley of the Saone, a basin filled with Tertiary and Quaternary sediments and comparatively uniform in surface. Crossing the Saone Valley we ascend the slope of the Jura Mountains, pass their summit, and when we descend their eastern flank, from the southeast there bursts upon the eye a vision of serried mountain peaks, lofty, abrupt in outline, and in most cases capped with snow, looking like curling breakers in a stormy sea. This is the Bernese Oberland. Of this well-known Alpine chain the highest peaks are formed of rocks very old in geologic time. Passing southward beyond the valley of the Rhone we may cross the Pennine Alps by the Simplon Pass and descend to the Quaternary plain of northern Italy. Here the contrast is abrupt and easily observed, as is the change from the mountain region of Tyrol to the plain of Bavaria about Munich.

To multiply these instances is unnecessary; the writer's object is only to explain these scenic contrasts which have been seen by every intelligent observer. In our own country the Atlantic coast plain and the flat sedimentary plains of the Mississippi basin differentiate themselves from the Appalachian mountain region, and the plutonic masses of the Adirondack chain stand out in bold contrast to the glacial and post-glacial deposits of the western portion of this interesting wilderness. So in the Rocky Mountain region the central masses of Archæan rocks stand out in strong relief above the later formations which border them.

To elaborate this subject in detail would be to write the geology of the whole world, a task from which the writer refrains.

 


 
From the fact manifest in the Alps that glaciers rarely form till the mean annual temperature falls below 27° Fahr., Prof. T. G. Bonney estimates that a fall of 20° Fahr. would produce large glaciers in the hill districts of Britain; one of from this to about 12° Fahr. would bring them back in the various districts on the globe where traces of them have been observed, and in some of these the small size of the vanished glaciers shows that the fall can not have exceeded about 15° Fahr.