Earth and Sky Every Child Should Know Part 6

When the earth's crust is thrown up in mountain folds, and between them valleys are formed, the level of rivers is sometimes lowered and the rapidity of their flow is checked. A stream which has torn down its walls at a rapid rate becomes a sluggish water-course, its current clogged with sediment, which it has no power to carry farther. When such a river begins to build and obstruct its own waters it bars its progress and may form a lake as the outlet of its tributary streams. Many ancient rivers have been utterly changed and some obliterated by general movements of the earth's crust.

THE STORY OF A POND

Look out of the car window as you cross a flat stretch of new prairie country, and you see a great many little ponds of water dotting the green landscape. Forty years ago Iowa was a good place to see ponds of all shapes and sizes. The copious rainfall of the early spring gathered in the hollows of the land, and the stiff clay subsoil prevented the water from soaking quickly into the ground. The ponds might dry away during the hot, dry summer, leaving a baked clay basin, checked with an intricate system of cracks. Or if rains were frequent and heavy, they might keep full to the brim throughout the season.

Tall bulrushes stood around the margins of the largest ponds, and water-lilies blossomed on the surface during the summer. The ba.s.s and the treble of the spring chorus were made by frogs and toads and little hylas, all of which resorted to the ponds to lay their eggs, in coiled ropes or spongy ma.s.ses, according to their various family traditions. On many a spring night my zoology cla.s.s and I have visited the squashy margins of these ponds, and, by the light of a lantern, seen singing toads and frogs sitting on bare hummocks of gra.s.s roots that stood above the water-line. The throat of each musician was puffed out into a bag about the size and shape of a small hen's egg; and all were singing for dear life, and making a din that was almost ear-splitting at close range. So great was the self-absorption of these singers that we could approach them, daze them with the light of the lantern, and capture any number of them with our long-handled nets before they noticed us. But it was not easy to persuade them to sing in captivity, no matter how many of the comforts of home we provided in the school aquariums. So, after some very interesting nature studies, we always carried them back and liberated them, where they could rejoin their kinsfolk and neighbours.

It was when we were sc.r.a.ping the mud from our rubber boots that we realized the character of the bottoms of our prairie ponds. The slimy black deposit was made partly of the clay bottom, but largely of decaying roots and tops of water plants of various kinds. Whenever it rained or the wind blew hard, the bottom was stirred enough to make the water muddy; and on the quietest days a pail of pond water had a tinge of brown because there were always decaying leaves and other rubbish to stain its purity.

The farmers drained the ponds as fast as they were able, carrying the water, by open ditches first, and later by underground tile drains, to lower levels. Finally these trunk drain pipes discharged the water into streams or lakes. To-day a large proportion of the pond areas of Iowa has disappeared; the hollow tile of terra-cotta has been the most efficient means of converting the waste land, covered by ponds, into fertile fields.

But the ponds that have not been drained are smaller than they used to be, and are on the straight road to extinction. This process one can see at any time by visiting a pond. Every year a crop of reeds and a dozen other species of vigorous water plants dies at the top and adds the substance of their summer growth to the dust and other refuse that gathers in the bottom of the pond. Each spring roots and seeds send up another crop, if possible more vigorous than the last, and this top growth in turn dies and lies upon the bottom. The pond level varies with the rainfall of the years, but it averages a certain depth, from which something is each year subtracted by the acc.u.mulations of rotting vegetable matter in the bottom. Evaporation lowers the water-level, especially in hot, dry summers. From year to year the water plants draw in to form a smaller circle, the gra.s.sy meadow land encroaches on all sides. The end of the story is the filling up of the pond basin with the rotting substance of its own vegetation. This is what is happening to ponds and inland marshes by slow degrees. The tile drain pipes obliterate the pond in a single season. Nature is more deliberate. She may require a hundred years to fill up a single pond which the farmer can rid himself of by a few days of work and a few rods of tiling.

THE RIDDLE OF THE LOST ROCKS

Outside of my window two robins are building a nest in the crotch of a blossoming red maple tree. And just across the hedge, men are digging a big square hole in the ground--the cellar of our neighbour's new house.

It looks now as if the robins would get their house built first, for they need but one room, and they do not trouble about a cellar. I shall watch both houses as they grow through the breezy March days.

The brown sod was first torn up by a plough, which uncovered the red New Jersey soil. Two men, with a team hitched to a sc.r.a.per, have carried load after load of the loose earth to a heap on the back of the lot, while two other men with pickaxes dug into the hard subsoil, loosening it, so that the sc.r.a.per could scoop it up.

This subsoil is heavy, like clay, and it breaks apart into hard clods.

At the surface the men found a network of tree roots, about which the soil easily crumbled. Often I hear a sharp, metallic stroke, unlike the dull sound of the picks striking into the earth. The digger has struck a stone, and he must work around it, pry it up and lift it out of the way.

A row of these stones is seen at one side of the cellar hole, ranged along the bank. They are all different in size and shape, and red with clay, so I can't tell what they are made of. But from this distance I see plainly that they are irregular in form and have no sharp corners.

The soil strewn along the lot by the sc.r.a.per is full of stones, mostly irregular, but some rounded; some are as big as your head, others grade down to the sizes of marbles.

When I went down and examined this red earth, I found pebbles of all shapes and sizes, gravel in with the clay, and grains of sand. This rock-sprinkled soil in New Jersey is very much like soil which I know very well in Iowa; it looks different in colour, but those pebbles and rock fragments must be explained in the same way here as there.

These are not native stones, the outcrop of near-by hillsides, but strangers in this region. The stones in Iowa soil are also imported.

The prairie land of Iowa has not many big rocks on the surface, yet enough of them to make trouble. The man who was ploughing kept a sharp lookout, and swung his plough point away from a buried rock that showed above ground, lest it should break the steel blade. One of the farmer's jobs for the less busy season was to go out with sledge and dynamite sticks, and blast into fragments the buried boulders too large to move.

Sometimes building a hot fire on the top of it, and throwing on water, would crack the stubborn "dornick" into pieces small enough to be loaded on stone-boats.

I remember when the last giant boulder whose buried bulk scarcely showed at the surface, was fractured by dynamite. Its total weight proved to be many tons. We hauled the pieces to the great stone pile which furnished materials for walling the sides of a deep well and for laying the foundation of the new house. Yet for years stones have been acc.u.mulating, all of them turned out of the same farm, when pastures and swampy land came under the plough.

Draw a line on the map from New York to St. Louis, and then turn northward a little and extend it to the Yellowstone Park. The boulder-strewn states lie north of this line, and are not found south of it, anywhere. Canada has boulders just like those of our Northern States. The same power scattered them over all of the vast northern half of North America and a large part of Europe.

What explanation is there for this extensive distribution of unsorted debris?

THE QUESTION ANSWERED

The rocks tell their own story, partly, but not wholly. They told just enough to keep the early geologists guessing; and only very recently has the guessing come upon the truth.

These things the rocks told:

1. We have come from a distance.

2. We have had our sharp corners worn off.

3. Many of us have deep scratches on our sides.

4. At various places we have been dumped in long ridges, mixed with much earth.

5. A big boulder is often balanced on another one.

The first thing the geologist noted was the fact that these boulders are strangers--that is, they are not the native rocks that outcrop on hillsides and on mountain slopes near where they are found. Far to the north are beds of rock from which this debris undoubtedly came. Could a flood have scattered them as they are found? No, for water sorts the rock debris it deposits, and it rounds and polishes rock fragments, instead of scratching and grooving them and leaving them angular, as these are.

Professor Aga.s.siz went to Switzerland and studied the glaciers. He found unsorted rock fragments where the glacier's nose melted, and let them fall. They were worn and scratched and grooved, by being frozen into the ice, and dragged over the rocky bed of the stream. The rocky walls of the valley were scored by the glacier's tools. Rounded domes of rock jutted out of the ground, in the paths of the ice streams, just like the granite outcrop in Central Park in New York, and many others in the region of scattered boulders.

After long studies in Europe and in North America, Professor Aga.s.siz declared his belief that a great ice-sheet once covered the northern half of both countries, rounding the hills, scooping out the valleys and lake basins, and scattering the boulders, gravel, and clay, as it gradually melted away.

The belief of Professor Aga.s.siz was not accepted at once, but further studies prove that he guessed the riddle of the boulders. The rich soil of the Northern States is the glacial drift--the mixture of rock fragments of all sizes with fine boulder clay, left by the gradual melting of the great ice-sheet as it retreated northward at the end of the "Glacial Epoch."

GLACIERS AMONG THE ALPS

Switzerland is a little country without any seacoast, mountainous, with steep, lofty peaks, and narrow valleys. The climate is cool and moist, and snow falls the year round on the mountain slopes. A snow-cap covers the lower peaks and ridges. Above the level of nine thousand feet the bare peaks rise into a dry atmosphere; but below this alt.i.tude, and above the six thousand-foot mark, lies the belt of greatest snowfall.

Peaks between six and nine thousand feet high are buried under the Alpine snow-field, which adds thickness with each storm, and is drained away to feed the rushing mountain streams in the lower valleys.

The snow that falls on the steep, smooth slope clings at first; but as the thickness and the weight of these snow banks increase, their hold on the slope weakens. They may slip off, at any moment. The village at the foot of the slope is in danger of being buried under a snow-slide, which people call an avalanche. "Challanche" is another name for it. The hunter on the snow-clad mountains dares not shout for fear that his voice, reechoing among the silent mountains, may start an avalanche on its deadly plunge into the valley.

On the surface of the snow-field, light snow-flakes rest. Under them the snow is packed closer. Deeper down, the snow is granular, like pellets of ice; and still under this is ice, made of snow under pressure. The weight of the acc.u.mulated snow presses the underlying ice out into the valleys. These streams are the glaciers--rivers of ice.

The glaciers of the Alps vary in length from five to fifteen miles, from one to three miles in width, and from two hundred to six hundred feet in thickness. They flow at the rate of from one to three feet a day, going faster on the steeper slopes.

It is hard to believe that any substance as solid and brittle as ice can flow. Its movement is like that of stiff mola.s.ses, or wax, or pitch. The tremendous pressure of the snow-field pushes the ma.s.s of ice out into the valleys, and its own weight, combined with the constant pressure from behind, keeps it moving.

The glacier's progress is hindered by the uneven walls and bed of the valley, and by any decrease in the slope of the bed. When a flat, broad area is reached, a lake of ice may be formed. These are not frequent in the Alps. The water near the banks and at the bottom of a river does not flow as swiftly as in the middle and at the surface of the stream. The flow of ice in a glacier is just so. Friction with the banks and bottom r.e.t.a.r.ds the ice while the middle parts go forward, melting under the strain, and freezing again. There is a constant readjusting of particles, which does not affect the solidity of the ma.s.s.

The ice moulds itself over any unevenness in its bed if it cannot remove the obstruction. The drop which would cause a small waterfall in a river, makes a bend in the thick body of the ice river. Great cracks, called _creva.s.ses_, are made at the surface, along the line of the bend.

The width of the V-shaped openings depends upon the depth of the glacier and the sharpness of the bend that causes the breaks.

Rocky ridges in the bed of the ice-stream may cause creva.s.ses that run lengthwise of the glacier. Snow may fill these chasms or bridge them over. The hunter or the tourist who ventures on the glacier is in constant danger, unless he sees solid ice under him. Men rope themselves together in climbing over perilous places, so that if one slips into a creva.s.se his mates can save him.

A glacier tears away and carries away quant.i.ties of rock and earth that form the walls of its bed. As the valley narrows, tremendous pressure crowds the ice against the sides, tearing trees out by the roots and causing rock ma.s.ses to fall on the top of the glacier, or to be dragged along frozen solidly into its sides. The weight of the ice bears on the bed of the glacier, and its progress crowds irresistibly against all loose rock material. The glacier's tools are the rocks it carries frozen into its icy walls and bottom. These rocks rub against the walls, grinding off debris which is pushed or carried along. No matter how heavy the boulders are that fall in the way of the ice river, the ice carries them along. It cannot drop them as a river of water would do.