The Geological Story Briefly Told 65 



Many Unconformities 



Unconformities, representing lost intervals, are common. 

 Within the San Luis quadrangle, in San Luis Obispo County, 

 there are sedimentary formations separated by five unconform- 

 ities marking periods of elevation and erosion. The periods of 

 elevation were often so long that sediments thousands of feet 

 in thickness were removed by erosion. In the vicinity of Santa 

 Cruz, south of San Francisco Bay, there are 14 recognizable 

 formations. Nine distinct and far-reaching disturbances, re- 

 corded in profound unconformities, took place during the 

 deposition of these 14 formations. 



This is a tremendous problem for the mind to grasp. That 

 whole regions sink till they are below sea level and then later 

 rise and again become dry land requires some exercise of the 

 imagination to understand. The length of time is almost incon- 

 ceivable. The land does sink, and it does rise. Movement is 

 slow, but it is none the less real. The coast of California shows 

 by terraces that it has but recently been lifted above the sea. 

 The movement of elevation or subsidence is so slow as not to be 

 perceptible to the eye, and indeed cannot be detected by any 

 measuring instruments. Thus it is seen that time is long in 

 geology. Geological periods and epochs are marked often by 

 unconformities which represent slow earth changes. There 

 is a good deal of geology compressed in the above sentences. 

 Seven sedimentary formations separated by five distinct uncon- 

 formities reads very simply, but it represents a tremendous 

 lapse of time. As we measure time in years it runs so far into 

 the millions that the figures become almost beyond the compre- 

 hension of the mind. The time from the close of the Jurassic 

 period to the present is estimated to be more than 130 millions 

 of years. This the mind can hardly grasp except by comparison. 

 Seven periods of deposition when the land was below sea level 



