72 Adventures in Scenery 



and oldest Cretaceous (the Knoxville) rests unconformably 

 upon the Franciscan. Thus a "lost interval" occurs both before 

 and after the deposition of the Franciscan. 



The story is somewhat long and somewhat complicated. 

 The age of the Franciscan is not definitely known, but is pro- 

 visionally placed in Jurassic time. This, it will be seen from the 

 geologic table, is in the Mesozoic era, and after the long Palaeo- 

 zoic era. The unconformity or "lost interval" that preceded 

 the Franciscan marks a long period of erosion of an old land 

 surface. This part of the earth's crust sank and was covered 

 by the sea. An uplift of the sea bottom again brought the 

 region above sea so that it was again dry land, and was again 

 for a long time subject to weathering of the elements and the 

 erosion of streams. How long these "lost intervals" were we do 

 not know. During both of these intervals this part of Cali- 

 fornia was land and became dissected by streams. 



The next great geologic period following the Jurassic is the 

 Cretaceous. In the region of the Coast Ranges the "lost inter- 

 val" represents a long time of degradation of the land before 

 the subsidence which ushered in the Cretaceous. 



An Arm of the Ocean Covered Central 

 California 



Far back in ancient geologic time, during the Palaeozoic era, 

 an arm of the ocean covered much of central and southern 

 California. In the far northern Sierra region Silurian rocks 

 occur, and extending southward are rocks of Devonian and Car- 

 boniferous age. Sediments, outwash from continental lands 

 somewhere, accumulated to thousands of feet in thickness. 

 Many of these deposits were buried later beneath sediments 

 of succeeding periods. In places these ancient sediments have 

 been uncovered by erosion after uplift of the ancient sea bottom 

 and now form the surface rocks. Fossil remains buried long 

 ago in the muds, silts and sands reveal the age of the rocks. On 

 the western slope of the great Sierra range rocks of Carbonif- 



