58 New York State Museum 



may be found resting upon a lower series with beds 

 dipping at another angle and with an erosion surface 

 in between. This is known as an unconformity, and 

 is brought about as follows. The lower beds were de- 

 posited in a more or less horizontal position under 

 water and later were uplifted, tilted or folded to form 

 a land surface. Erosion took place over this land 

 truncating the folded and tilted beds and tending to 

 reduce the area to a low surface. Later the eroded 

 land surface was depressed beneath the water and a 

 new series of strata was deposited upon the truncated 

 edges of the older series of beds. This condition in 

 which one series of strata rests upon the upturned 

 edges of another series is sometimes termed angular 

 unconformity or nonconformity. An unconformity may 

 also be developed by the deposition of sedimentary 

 rocks upon an eroded surface of igneous or meta- 

 morphic rocks. We may find two apparently conform- 

 able series of rocks with an erosion surface between. 

 Such a condition may be produced by the advance of 

 the sea over the eroded surface of a formation which 

 has not been subjected to tilting or folding, and de- 

 position of a new series of strata. This condition is 

 known as parallel unconformity or disconformity. In 

 disconformities the erosion line may be clearly visible 

 or not evident. When visible it may be evident 

 through the inclusion of pebbles of the older rock in 

 the base of the lower bed of the new series. Some- 

 times in the case of marine formations there may be 

 an old soil bed or eolian sands between the two forma- 

 tions. When the erosion line is not visible the dis- 

 conformity is only indicated by the difference in age of 

 the two formations which are recognized by the fos- 



