Earth and Sky Every Child Should Know Part 7

Slowly they travel, and finally stop where the nose of the glacier melts and leaves all debris that the mountain stream, fed by the melting of the ice, cannot carry away.

The bedrock under a glacier is sc.r.a.ped and ground and scored by the glacier's tools--the rock fragments frozen into the bottom of the ice.

These rocks are worn away by constant grinding, just as a steel knife becomes thin and narrow by use. Scratches and scorings and polished surfaces are found in all rocks that pa.s.s one another in close contact.

Its worn-out tools the glacier drops at the point where its ice melts.

This great, unsorted ma.s.s of rock meal and coa.r.s.er debris the stream is gradually scattering down the valley.

The name "moraine" has been given to the earth rubbish a glacier collects and finally dumps. The _top moraine_ is at the surface of the ice. The _lateral moraines_, one at each side, are the debris gathered from the sides of the valley. The _ground moraine_ is what debris the ice pushes and drags along on the bottom. The _terminal moraine_ is the dumping-ground of this ma.s.s of material, where the ice river melts.

Glaciers, like other rivers, often have tributary streams. A _median moraine_, seen as a dark streak running lengthwise on the surface of a glacier, means that two branch glaciers have united to form this one. Go back far enough and you will reach the place where the two streams come together. The two lateral moraines that join form the middle line of debris, the median moraine. Three ice-streams joined produce two top moraines. They locate the lateral moraines of the middle glacier.

The surface of a glacier is often a ma.s.s of broken and rough ice, forming a series of pits and pinnacles that make crossing impossible.

The sun melts the surface, forming pools and percolating streams of water, that honeycomb the ma.s.s. Underneath, the ice is tunnelled, and a rus.h.i.+ng stream flows out under the end of the glacier. It is not clear, but black with mud, called _boulder clay_, or _till_, made of ground rock, and mixed with fragments of all shapes and sizes. This is the meal from the glacier's mill, dumped where the water can sift it.

"Balanced rocks" are boulders, one upon another, that once lay on a glacier, and were left in this strange, unstable position when the supporting ice walls melted away from them. In Bronx Park in New York the "rocking stone" always attracts attention. The glacier that lodged it there, also rounded the granite dome in Central Park and scattered the rock-strewn boulder clay on Long Island. Doubtless in an earlier day the edges of this glacier were thrust out into the Atlantic, not far from the Great South Bay, and icebergs broke off and floated away.

[Ill.u.s.tration: Potsdam sandstone showing ripple marks]

[Ill.u.s.tration: _By permission of the American Museum of Natural History_

Glacial striae on Lower Helderberg limestone]

[Ill.u.s.tration: Glacial grooves in the South Meadow, Central Park, New York]

[Ill.u.s.tration: _By permission of the American Museum of Natural History_

Mt. Tom, West 83d St., New York]

Glaciers are small to-day compared with what they were long ago, in Europe and in America. The climate became warmer, and the ice-cap retreated. Old moraines show that the ice rivers of the Alps once came much farther down the valleys than they do now. Smooth, deeply scored domes of rock, the one in Central Park and the bald head of Mount Tom, are just like those that lie in Alpine valleys from which the glaciers have long ago retreated. There are old moraines far up the sides of valleys, showing that once the glaciers were far deeper than now. No other power could have brought rocks from strata higher up the mountains, and lodged them thus.

Nearer home, Mt. Shasta and Mt. Rainier still have glaciers that have dwindled in size, until they bear little comparison to the gigantic ice-streams that once filled the smooth beds their puny successors flow into. Remnants of glaciers lie in the hollows of the Sierras. We must go north to find the snow-fields of Alaska and glaciers worthy to be compared with those ancient ice rivers whose work is plainly to be seen, though they are gone.

THE GREAT ICE-SHEET

Greenland is green only along its southern edge, and only in summer, so its name is misleading. It is a frozen continent lying under a great ice-cap, which covers 500,000 square miles and is several thousand feet in thickness. The top of this icy table-land rises from five thousand to ten thousand feet above the sea-level. The long, cold winters are marked by great snowfall, and the drifts do not have time to melt during the short summer; and so they keep getting deeper and deeper. Streams of ice flow down the steeps into the sea, and break off by their weight when they are pushed out into the water. These are the icebergs which float off into the North Atlantic, and are often seen by pa.s.sengers on transatlantic steamers.

Long ago Greenland better deserved its name. Explorers who have climbed the mountain steeps that guard the unknown ice-fields of the interior have discovered, a thousand feet above the sea-level, an ancient beach, strewn with sh.e.l.ls of molluscs like those which now inhabit salt water, and skeletons of fishes lie buried in the sand. It is impossible to think that the ocean has subsided. The only explanation that accounts for the ancient beach, high and dry on the side of Greenland's icy mountain is that the continent has been lifted a thousand feet above its former level. This is an accepted fact.

We know that climate changes with changed alt.i.tude as well as lat.i.tude.

Going up the side of a mountain, even in tropical regions, we may reach the snow-line in the middle of summer. Magnolia trees and tree ferns once grew luxuriantly in Greenland forests. Their fossil remains have been found in the rocks. This was long before the continent was lifted into the alt.i.tude of ice and snow. And it is believed that the climate of northern lat.i.tudes has become more severe than formerly from other causes. It is possible that the earth's...o...b..t has gradually changed in form and position.

If Greenland should ever subside until the ancient beach rests again at sea-level, the secrets of that unknown land would be revealed by the melting of the glacial sheet that overspreads it. Possibly it would turn out to be a mere flock of islands. We can only guess. North America had, not so long ago, two-thirds of its area covered with an ice-sheet like that of Greenland, and a climate as cold as Greenland's. At this time the land was lifted two to three thousand feet higher than its present level. All of the rain fell as snow, and the ice acc.u.mulated and became thicker year by year. Instead of glaciers filling the gorges, a great ice flood covered all the land, and pushed southward as far as the Ohio River on the east and Yellowstone Park in the west. The Rocky Mountains and some parts of the Appalachian system acc.u.mulated snow and formed local glaciers, separated from the vast ice-sheet.

The unstable crust of the earth began to sink at length, and gradually the ice-sheet's progress southward was checked, and it began to recede by melting. All along the borders of this great fan-shaped ice-field water acc.u.mulated from the melting, and flooded the streams which drained it to the Atlantic and the Gulf. Icebergs broken off of the edge of the retiring ice-sheet floated in a great inland sea. The land sank lower and lower until the general level was five hundred to one thousand feet lower than it now is. The climate became correspondingly warm, and the icebergs melted away. Then the land rose again, and in time the inland sea was drained away into the ocean, except for the waters that remained in thousands of lakes great and small that now occupy the region covered by the ice.

Ancient sea beaches mark the level of high water at the time that the flood followed the melting ice. On the sh.o.r.es of Lake Champlain, but nearly five hundred feet higher than the present level of the lake, curious geologists have found many kinds of marine sh.e.l.ls on a well-marked old sea beach. The members of one exploring party in the same region were surprised and delighted to come by digging upon the skeleton of a whale that had drifted ash.o.r.e in the ancient days when the inland sea joined the Atlantic.

Lake Ontario's ancient beach is five hundred feet above the present water-level; Lake Erie's is two hundred fifty feet above it; Lake Superior's three hundred thirty feet higher than the present beach. No doubt when the water stood at the highest level, the Great Lakes formed one single sheet of water which settled to a lower level as the rivers flowing south cut their channels deep enough to draw off the water toward the Gulf. Lake Winnipeg is now the small remnant of a vast lake the sh.o.r.es of which have been traced. The Minnesota River finally made its way into the Mississippi and drained this great area the stranded beaches of which still remain. The name of Aga.s.siz has been given to the ancient lake formed by the glacial flood and drained away thousands of years ago but not until it had built the terraced beach which locates it on the geological map of the region.

When the ice-sheet came down from the north it dragged along all of the soil and loose rock material that lay in its path. With the boulders frozen into its lower surface it scratched and grooved the firm bedrock over which it slid, and rounded it to a smooth and billowy surface. The progress of the ice-sheet was southward, but it spread like a fan so that its widening border turned to east and west.

When it reached its southernmost limit and began to melt, it laid down a great ridge of unsorted rock material, remnants of which remain to this day,--the terminal moraine of the ancient ice-sheet. The line of this ancient deposit starts on Long Island, crosses New Jersey and Pennsylvania, then dips southward, following the general course of the Ohio River to its mouth, forming bluffs in southern Ohio, Indiana, and Illinois. The line bends upward as it crosses central Missouri, a corner of Kansas, and eastern Nebraska, parallel with the course of the Missouri.

As the ice-sheet melted, boulders were dropped all over the Northern States and Canada. These were both angular and rounded. In some places they are scattered thickly over the surface and are so numerous as to be a great hindrance to agriculture. In many places great boulders of thousands of tons weight are perched on very slight foundations, just where they lodged when the ice went off and left them, after carrying them hundreds of miles. Around them are scattered quant.i.ties of loose rock material, not scored or ground as are those which were carried on the under-surface of the glacial ice. These unscarred fragments rode on the top of the ice. They were a part of the top moraine of the glacial sheet.

The finest material deposited is rock meal, ground by the great glacial mill, and called "boulder clay." It is a stiff, dense, stony paste in which boulders of all sizes, gravel, pebbles, and cobblestones are cemented.

The "drift" of the ice-sheet is the rubbish, coa.r.s.e and fine, it left behind as it retreated. Below the Ohio River there is a deep soil produced by the decay of rocks that lie under it. North of Ohio is spread that peculiar mixture of earth and rock fragments which was transported from the north and spread over the land which the ice-sheet swept bare and ground smooth and polished.

The drift has been washed away in places by the floods that followed the ice. Granite domes are thus exposed, the grooves and scratches of which tell in what direction the ice flood was travelling. Miles away from that scored granite, but in the same direction as the scratches, scattered fragments of the same foundation rock cover fields and meadows. Thus, much of the drift material can be traced to its original home, and the course of the ice-sheet can be determined. Many immense boulders the home of which was in the northern highlands of Canada rode southward, frozen into icebergs that floated in the great inland sea.

Great quant.i.ties of debris were added to the original glacial drift through the agency of these floating ice ma.s.ses, which melted by slow degrees.

FOLLOWING SOME LOST RIVERS

What would you think if the boat in which you were floating down a pleasant river should suddenly grate upon sand, and you should look over the gunwale and find that here the waters sank out of sight, the river ended? I believe you would rub your eyes, and feel sure that you were dreaming. Do not all rivers flow along their beds, growing larger with every mile, and finally empty their waters into a sea, or bay, or lake, or flow into some larger stream? This is the way of most rivers, but there are exceptions. In the Far West there are some great rivers that absolutely disappear before they reach a larger body of water. They simply sink away into the sand, and sometimes reappear to finish their courses after flowing underground for miles. Do you know the name of one great western river of which I am thinking? Is there any stream in your neighbourhood which has such peculiar ways?

Down in Kentucky there is a region where, it is said, one may walk fifty miles without crossing running water. In the middle of our country, in the region of plentiful rainfall, and in a state covered with beautiful woodlands and famous for blue gra.s.s and other grain crops, it is amazing that, over a large area, brooks and larger streams are lacking.

In most of the state there is plenty of water flowing in streams like those in other parts of the eastern half of the United States. In the near neighbourhood of this peculiar section of the state the streams come to an end suddenly, pouring their water into funnel-shaped depressions of the ground called sink-holes. After a heavy rain the surface water, acc.u.mulating in rivulets, may also be traced to small depressions which seem like leaks in the earth's crust, into which the water trickles and disappears.

It must have been noticed by the early settlers who came over the mountains from the eastern colonies, and settled in the new, wild, hilly country, which they called Kentucky. The first settlers built their log cabins along the streams they found, and shot deer and wild turkey and other game that was plentiful in the woods. The deer showed them where salt was to be found in earthy deposits near the streams; for salt is necessary to every creature. Deer trails led from many directions to the "salt licks" which the wild animals visited frequently.

Perhaps the same pioneers who dug the salt out of the earth found likewise deposits of _nitre_, called also _saltpetre_, a very precious mineral, for it is one of the elements necessary in the manufacture of gunpowder. With the Indians all about him, and often showing themselves unfriendly, the pioneer counted gunpowder a necessity of life. He relied on his gun to defend and to feed his family. There were men among those first settlers who knew how to make gunpowder, and saltpetre was one of the things that had to be carried across the mountains into Kentucky, until they found it in the hills. No wonder that prospectors went about looking for nitre beds in the overhanging ledges of rocks along stream-beds. In such situations the deposits of nitre were found. The earth was washed in troughs of running water to remove the clayey impurity. After a filtering through wood-ashes, the water which held the nitre in solution was boiled down, and left to evaporate, after which the crystals of saltpetre remained.

Solid ma.s.ses of saltpetre weighing hundreds of pounds were sometimes found in protected corners under shelving rocks. It was no doubt in the fascinating hunt for lumps of this pure nitre that the early prospectors discovered that the streams which disappeared into the sink-holes made their way into caverns underground. Digging in the sides of ravines often made the earthy wall cave in, and the surprised prospector stood at the door of a cavern. The discoverer of a cave had hopes that by entering he might find nitre beds richer than those he could uncover on the surface, and this often turned out to be true. The hope of finding precious metals and beds of iron ore also encouraged the exploration of these caves. By the time the war of 1812 was declared, the mining of saltpetre was a good-sized industry in Kentucky. Most of the mineral was taken out of small caves, and s.h.i.+pped, when purified, over the mountains, on mule-back by trails, and in carts over good roads that were built on purpose to bring this mineral product to market. As long as war threatened the country, the Government was ready to buy all the saltpetre the Kentucky frontiersmen could produce. And the miners were constantly in search of richer beds that promised better returns for their labour.

It was this search that led to the exploration of the caves discovered, although the explorer took his life in his hands when he left the daylight behind him and plunged into the under-world.

Not all lost rivers tell as interesting stories or reveal as valuable secrets as did those the neighbours of Daniel Boone traced along their dark pa.s.sages underground, and finally saw emerge as hillside springs, in many cases, to feed Kentucky rivers. But it is plain that no river sinks from sight unless it finds porous or honeycombed rocks that let it through. The water seeks the nearest and easiest route to the sea. Its weight presses toward the lowest level, always. The more water absorbs of acid, the more powerfully does it attack and carry away the substance of lime rocks through which it pa.s.ses.

THE MAMMOTH CAVE OF KENTUCKY

There is no more fertile soil in the country than that of the famous blue gra.s.s region of Kentucky. The surface soil rests upon a deep foundation of limestone rocks, and very gradually the plant food locked up in these underlying strata is pulled up to the surface by the soil water, and greedily appropriated by the roots of the plants.

Part of the water of the abundant rainfall of this region soaks into the layers of the lime rock, carrying various acids in solution which give it power to dissolve the limestone particles, and thus to make its way easily through comparatively porous rock to the very depths of the earth. So it has come about that the surface of the earth is undermined.

Vast empty chambers have been carved by the patient work of trickling water, which has carried away the lime that once formed solid and continuous layers of the earth's crust. We must believe that the work has taken thousands of years, at least, for no perceptible change has come to these wonderful caves since the discovery and exploration of them a century and more ago.

The streams that flow into the region of these caves disappear suddenly into sink-holes and flow through caverns. After wearing away their subterranean channels, leaping down from one level to another, forming waterfalls and lakes, some emerge finally through hillsides in the form of springs.