84 HISTORICAL GEOLOGY 



Physical History 



Early and Middle Ordovician. — The progressive submergence 1 

 which caused so much of North America to be covered by marine 

 waters by late Cambrian time continued well into the Ordovi- 

 cian, so that during mid-Ordovician time more of the continent 

 appears to have been covered by the sea than at any other time in 

 its history since the beginning of the Paleozoic era. A good idea 

 of the general relations of land and sea at this time may be gained 

 from the paleogeographic map (Fig. 45) . The principal land areas 

 were Appalachia (except the Piedmont Plateau district in the early 

 Ordovician) along the Atlantic Coast; the Labrador region; Lake 

 Superior and southward; west of Hudson Bay; Wyoming to 

 Arizona; Ozark Mountains of Missouri; and certain rather in- 

 definitely known and possibly connected areas to the west of the 

 Rocky Mountains. 



Because Ordovician strata often show a thickness of 2000 to 

 4000 feet, it should not be inferred that the Ordovician sea was 

 necessarily ever 2000 to 4000 feet deep. Even the limestones 

 often abundantly prove by ripple-marks, mud-cracks, fossils, 

 etc., that they were laid down in shallow water. The very char- 

 acter of the thickest materials (old muds and sands) in the Upper 

 Ordovician implies that they could not have been deposited in 

 deep ocean water. Such sediments are not now forming on the 

 deep sea bottom. But how are these statements to be harmonized 

 with the fact that Ordovician strata several thousand feet thick 

 actually exist over considerable areas? During the early and middle 

 portions of the period (with certain rather local exceptions above 

 mentioned) the land gradually became submerged, and stratum 

 after stratum was formed upon the relatively sinking sea floor 

 (or submerging land), so that at no time is it necessary to assume 

 great depth of water. In general, the early to middle Ordovician 

 sea of North America must be thought of as a vast shallow 

 (epicontinental) ocean which spread over most of the slowly sub- 

 merging continent. There were no ocean abysses at all comparable 

 to those of the present Atlantic or Pacific. 



1 This disregards the probable temporary withdrawal of the late Cam- 

 brian sea from the upper Mississippi Valley region as explained in the 

 preceding chapter. 



