28 



:measuring time in geology 





® C7 



Age of Sediments 

 400- 500 



Q Million Years 



V V V V V 500 Million Ye 



' V V V V / . V V 



vvv vvvvv_ 



VVVVVVVV 



V vvvvvvv 



V V V V V V V 



V V V V *-.' ■•/ V V 



Fig. 5. Scheme of relative and absolute dating in geology. A pile of 

 older sediments has been folded during an orogeny. At the end of the 

 crcgenetic period magma intruded into the sediments. This now forms 

 the igneous rocks of intrusion A. The folded crust was later eroded and 

 denudated, whereupon a pile of younger sediments was laid down. 

 This contains pebbles of the igneous reck of intrusion A at its base, 

 so intrusion A is older relative to the younger series of sediments. Both 

 the older and the younger rocks were later intruded by a second magma. 

 A relative age of beds of the younger sediments can be established 

 locally by nothing the superposition of individual beds. Regionally a 

 relative age can be established by the use of index fossils found in this 

 series. The same method can also be applied to older series of sediments. 

 Although the series was folded, it is still possible to see what is the 

 upper and what is the lower bed. It still contains recognizable index 



