The Wearing Away of the Land (1939, sound)

foreign you have seen how rocks are made and mountains formed and also how and worn away the crumbling of rocks is largely a mechanical process there is also a chemical process that destroys the solid rocks this is called decomposition decomp rocks by the chemical action of acids in water here is a piece of rock over which strong acid is Running part of the rock is dissolved and carried away in solution this is just what happens in nature only more slowly rain water soon becomes a weak acid as it trickles through decaying vegetable matter in the soil see how rain water flowing over these rocks has disiled grooves in them the queer shape of this rock is caused by chemical decomposition together with mechanical crumbling in fact these two processes always help each other certain materials dissolve more quickly than others thus the parts which were decomposed first have left the holes in this rock rain water is nearly everywhere present in pores and cracks a few feet below the surface and at some places it flows out from underground in Springs you can find Springs trickling out from crevices in solid rocks or in sandy soil see the water bubbling up under the surface of a pool rain water often seeps far underground in this diagram you see how it can work its way down along cracks and joints in Limestone the water dissolves the rock until at last it has formed a great cave there are innumerable such caves here you are looking up from inside at the entrance to the world's largest cave down below are miles of enormous rooms and passageways from the ceilings of these rooms like icicles hang points of Rock water seeps down from far above carrying dissolved rot in solution as it dips into the cave it evaporates leaving deposits on ceiling and floor and in thousands of years it gradually builds up these columns occasionally underground water dissolves so much rock out of a cave that the roof Falls in making big holes in the ground these are called sinkholes and usually filled with water this sinkhole was probably one second it may be hundreds of feet to the bottom another process that wears away and also builds up the land is the work of glaciers Glaciers are large masses of ice that move over the Earth thirty thousand years ago much of the North American continent was buried beneath ice one ice sheet formed in the Northwest this was joined by a second and later by a third a fourth spread over Greenland thousands of years were required for the ice to creep over all this country then taking more thousands of years it receded until only the great continental glacier on Greenland was left this Still Remains and today you can see parts of its Edge advancing into the ocean as it moves forward into deeper water huge pieces break off with a terrific noise often the pieces are bigger than an ocean liner each year many blocks of this glacial ice float far out to sea and are called icebergs as well as Continental ice sheets there are smaller glaciers in high mountains the snow does not all melt away if enough snow collects at the head of a valley it changes into ice and the whole Mass creeps down the valley as it moves it picks up loose rocks tears away parts of the Hillside and carries the fragments along some parts of the ice move faster than others in this model the action is speeded up so that you can see how the uneven movement causes cracks to develop this makes the surface very rough sometimes great open chasms like this appear where the glacier moves over uneven ground the loose rocks and material that are frozen into a glacier are deposited where the ice melts notice all the stones and dirt once Frozen in the ice the glacier that was once in this Valley has melted away leaving these piles of stones and boulders they are called moraines grass and trees may have grown over the moraines but underneath you can see the boulders and Pebbles and dirt a glacier carried here you can imagine how these Boulders scraped away the land and the eyes pushed them alone at places the solid rock of the Earth's crust has been gouged out in long straight grooves by material carried in a large glacier the appearance of all country sides has been changed in this manner moraines and Stony soil have been deposited Hills in high places have been rounded off in the dwelling country thus you have seen how the power of moving ICE destroys and also builds up the land another important Earth wearing horse is the wind and mountain tops strong winds frequently Drive the snow and clouds about with great power little vegetation can grow on these high places trees become twisted and bent this tree is still alive but the winds kept it from growing upright and have blown away all the bark the force of wind is best shown in the Sand Hills of deserts and on shores of lakes and oceans as rock is decomposed and fumbles away to sand the wind picks up the sand and sweeps it into great piles called Dunes these sand dunes constantly shift about their surface is marked with small ridges called Ripple marks as the sand blows it may cover shrubs and grasses even whole forests have been completely covered by an advancing Sand Dune only to be uncovered again when the sand moves on the trunks of these trees are all that is left of a forest that once grew here glowing sand when it beats against solid objects scratches and polishes them steadily it gnaws away the hard rocks yet what the wind destroys in one place it builds up elsewhere there is still another great force that builds up and wears away the land this can be seen along the shore of the ocean where waves eat away parts of the coast and change the shorelines Rocky Ledges and cliffs are worn away under the terrific impact of the pounding waves the joints gradually weaken the swirling Waters carry away the broken rocks rounding them into Boulders splitting and grinding them into pebbles finally powdering them into sand the hardest rock cannot forever withstand the force of waves like these and whole coastlines are changed as the land is worn away a good example of this is an island in the North Sea which has almost disappeared in the memory of man as the waves attacked it in 800 A.D the distance around the island was 120 miles by 1300 it measured only 45 miles by 1649 it was eight miles and by 1900 it was a small Cliff of rock only three miles around where the land has recently risen or the level of the sea has been lowered you can sometimes see the rocky floor of the ocean where the coast has sunk the sea will reach farther up into the low parts of the land forming long narrow bays and inlets Low Places In the Land then become sea bottom and the higher places appear above the water's surface in Long Ridges of rock or as Islands the force of the sea breaks some parts of the coast into Boulders these are gradually pounded into pebbles and sand as waves wash them back and forth some parts of the coast are thus built up by material worn away elsewhere coastlines are always changing the force of the Waves Cuts them into an irregular outline of rugged Headlands Rocky islands and gently curved bays and beaches yet not even The Dashing of all the oceans on all the sea coasts in the world nor the blasts of the strongest wind nor even the gouging and scraping of the largest continental ice sheets and Glaciers nor the dissolving of rock by chemical decomposition and not even the mechanical crumbling of rock into soil none of these things nor all of them together equals the power of running water this great force of running water will be the subject of our next study