The Geotectonic Factor 329 



ecological conditions came into existence, differing from all pre- 

 vious ones in unique combinations of climatic conditions. 



Frequent slight warpings of the crust have produced less spec- 

 tacular changes in levels in many areas. Many of the land bridge 

 regions such as the Bering Straits area and the Banda geosyncline 

 area (Fig. 130) seem to have been either raised and lowered or 

 alternately exposed and submerged many times (Fig. 143). Com- 

 bined with changes in sea level, slight warpings have caused areas 

 such as the lower Mississippi valley of North America to alternate 

 between being a shallow continental sea and a low elevation ter- 

 restrial area. Such a series of changes coupled with some irregulari- 

 ties in topography could readily produce the succession of islands 

 and connected land shown in Fig. 104. 



Sea level changes contributing to these events have been most 

 extreme during periods of glaciation. When the maximum amount 

 of water was in the form of ice on the continents, the ocean level 

 was lowered some 600 feet. At this low point the present British 

 Isles were connected by a broad land mass with continental Eu- 

 rope (Beirne, 1952), and many present islands of the South Pacific 

 were part of the Asiatic mainland (Umbgrove, 1947) (Fig. 144). 

 The weight of the ice itself caused some warping of the underlying 

 continents, adding another factor to the changes in water level. 



The same orogenic activities which produce mountains on a con- 

 tinent produce many of the islands between continents. Igneous 

 material spewing from volcanoes erupting out of the bottoms of the 

 oceans have built the Hawaiian Islands and others situated far 

 from continents. A combination of upthrust, down-warping, and 

 volcanic activity along what seem to be permanent strips of weak- 

 ness in the crust may have periodically produced series of islands 

 where intercontinental isthmuses now stand. The present Central 

 American corridor between North and South America has probably 

 had this type of history (Simpson, 1950). 



The speed at which these crustal movements progress is not 

 always easy to measure. The volcano Faricutin in Mexico erupted 

 in 1943, continued erupting until 1952, and built up a cone over 

 1,500 feet in height. Thrusts are expressed chiefly as earthquakes. 

 Cloos (1954) pointed out that the unusually severe 1872 quake 

 in the mountains of California produced an increase in elevation 

 of six feet, evidenced today as the fault at Lundy Creek ( Fig. 145 ) . 

 He calculated that a sudden rise like this every few thousand years 

 would produce the 6,000 foot rise of the Sierra Nevada range in 

 about five million years. 



