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 sedimentary rocks 

 usually occur in layers or strata (stray'- 

 ta). If you do Exercise 4, you can see 

 strata forming on a small scale. 



