SALIENT FEATURES OF THE GEOLOGY OF OREGON 107 



In the topography of the Oregon coast dunes are especially note- 

 worthy, since Neocene and Pleistocene sandy formations outcrop 

 throughout practically its whole length. These shifting sands have, 

 besides resulting in dunes, caused the formation of a chain of very 

 interesting and picturesque lakes. These lakes owe their existence 

 to the damming up of sluggish streams by the shifting ocean sands. 



Peat bogs are now forming in these dune areas, where large 

 quantities of sphagnum moss and rhododendron leaves accumulate. 

 In the cliffs overlooking the sea, near the top of the section in many 

 places can be seen a bed of two feet or more of fairly solid peat. 

 This may be of Pleistocene or Recent age, or in part both. 



In various parts of the Cascades there are cinder cones and 

 lava streams which have every appearance of being of very recent 

 origin. Some of these may be not much over a hundred years old. 

 Indeed very carefully sifted newspaper stories and accounts by 

 pioneers make it reasonably clear that volcanic activity persisted 

 in the Cascade region until a very late day. The Portland Oregonian 

 reports an eruption from Mount Hood as late as 1865. 



' GEOLOGIC HISTORY 



The basement, upon which the Oregon strata appear to lie may 

 be represented by the Bald Mountain gneiss of the Blue Mountains. 

 The chemical composition of this supposed Archean rock indicates 

 a sedimentary origin. Similar gneisses in Idaho and California 

 possibly suggest a wide extension of a sedimentary terrain subse- 

 quently removed by erosion, or covered by younger deposits. 



In the late Proterozoic southwestern Oregon became a basin of 

 deposition in which the presumably conformable Abrams and 

 Salmon formations were developed. This epoch of sedimentation 

 was followed by a period of folding and accompanied by meta- 

 morphism which altered these rocks to mica and hornblende schists. 

 An erosional interval extending over several geologic periods pre- 

 ceded the depression, in the middle Paleozoic of this same region, 

 allowing the sea to enter from the southwest. The earlier sediments 

 of this marine basin, now tentatively assigned to the Devonian 

 system record a transgressing sea, with frequent oscillations from 

 deep to shallow waters. Little is known regarding the climatic 



