. Aventures avec les animaux et les plantes. La biologie. Problème I. nous en apprendre davantage sur l'histoire de la Terre jro7n Rocks 523 F"iG. 469 Comment de nouvelles terres ?non. Les plantes comme l'avance dans le lac, il becoines zones marécageuses. Comment fonctionne le marais, dans le temps, tnr ?i dans un pré - ? (New York State College of Agriculture) Fig. 470 Il fut un temps où ces couches de roches doivent avoir été sable et boue imder l'eau. Comment savoir ? yozi (Commission géologique du Canada). Le fleuve Mississippi transporte chaque année une charge de 517 000 000 tonnes de matériau solide. Lorsque le courant de la rivière est ralentie jusqu'à sa bouche, une grande partie de son chargement de sable et de limon (f

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. Adventures with animals and plants. Biology. PROBLEM I. We Learn about Earth's History jro7n Rocks 523 F"iG. 469 How new land ?nay form. As the plants advance into the lake, it becoines swampy. How does the swamp, in time, tnr?i into a -meadow? (new york state COLLEGE OF AGRICULTURE) Fig. 470 At one time these strata of rock must have been sand and mud imder water. How do yozi know? (GEOLOGICAL SURVEY OF CAN- ADA). the Mississippi River carries each year a load of 517, 000, 000 tons of solid mate- rial. When the river current is slowed up at its mouth, much of its load of sand and silt (fine earth) settles to the bottom. The sediment piles up. As the years go on, this deposit may rise above sea level, forming mud flats. Seeds fall on the mud and plants grow. Their roots bind the soil together and make it firm. As the plants die their decaying bodies make the soil richer. Other types of plants appear, finally shrubs and trees. Thus deltas form. You can demonstrate how deltas begin to form and learn about erosion by doing Exercise 3. You may have seen the same process of building land on the shores of a small lake. As mud settles and collects, the shores seem to march out into the lake. In time, where once there was a lake, a forest will stand. See Figure 469. The sediment may form rock. If the land sinks under the floor of an ocean or inland lake, the sediment of sand or silt sinks with it. It will be buried deep be- neath the water and its particles will be pressed together by the great weight of the water above. Even without this enor- mous pressure the sediment slowly hard- ens, for substances dissolved in the water act as a cement that binds the particles together. Thus rock is formed out of the sediment. It is known as sedimentary rock. When the sediment is mostly sand the sedimentary rock is sandstone; when it is mostly silt (mud) the rock that forms is shale. Since the materials deposited by the water change from time to time, both the sediments and the s

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