PART III: RELEVANT GEOLOGICAL FACTORS 



14. Borrow source exploration programs are designed to obtain informa- 

 tion on several geological aspects of the study area that define the presence, 

 character, and surroundings of potential borrow sources. The most important 

 of these are historical geology, geomorphology , stratigraphy, and lithology. 

 While some information on these factors can usually be obtained from maps, 

 charts, and literature sources, sufficient data for project design will 

 require a field exploration effort. In the following discussion, each of the 

 relevant geological factors will be considered in turn. 



Historical Geology 



15. Historical geology is the study of the origin and development of 

 geologic features through time. An understanding of the role of historical 

 processes and events in the formation and modification of sediment deposits in 

 the exploration area is a valuable asset in program planning and interpreta- 

 tion. Although earth history encompasses a vast range of time, most of the 

 unconsolidated surficial and shallow subsurface of the shore, shoreface, and 

 continental shelves (which are of major interest in developing borrow sources) 

 were formed and developed during the geologically recent Pleistocene and Holo- 

 cene epochs of the geological time scale. 



16. The Pleistocene Epoch, popularly known as the ice age, covers a 

 period of time in which major climatic fluctuations caused repeated episodes 

 of glaciation and deglaciation on a worldwide scale. The subsequent Holocene 

 Epoch covers the time between the last major glaciation, known as the Wiscon- 

 sin glaciation, and the present. 



17. Because of the immense amount of water trapped in the great conti- 

 nental and numerous alpine glaciers during glaciations, sea level was reduced 

 to well below the present stand. Sea level recovered during the succeeding 

 deglaciation, at times rising to well above the present level. During the 

 late Wisconsin glaciation, sea level fell more than 100 m below the present 

 level and large portions of the continental shelves were exposed to subaerial 

 conditions for an extended period of time. With the waning of the late Wis- 

 consin glaciation, water unlocked from the glaciers returned to the sea and 

 sea level rose progressively to resubmerge the shelf areas to their present 



10 



