MIOCENE 5 1 1 



The Miocene Epoch 



American. — At the opening of the Miocene, the coast-line of 

 the Atlantic and Gulf occupied nearly the same position as at the 

 beginning of the Eocene, differing only by the presence of a nar- 

 row strip of coast and of the Florida island, which had been pro- 

 duced by the slight movements during or at the end of the Eocene. 

 The whole thickness of the Miocene strata is not found everywhere 

 along the coast ; in Maryland and Virginia a slight transgression of 

 the sea occurred, and Upper Miocene beds were deposited upon 

 Lower Eocene. Miocene beds occur in Martha's Vineyard, are 

 apparently concealed beneath the sea along the New England 

 coast, and are continuous southward from New Jersey. In that 

 state they have a thickness of 700 feet, thinning to 400 feet in 

 Maryland, but attaining a thickness of 1500 feet in Texas. In the 

 north the strata are unconsolidated clays and sands, but in Florida 

 they are largely compacted limestones, and in Georgia, limestones 

 and conglomerates. The Mississippi embayment was much nar- 

 rowed by the Eocene uplift, and Miocene strata have not been 

 found in Tennessee or Arkansas ; in eastern Texas they are cov- 

 ered by newer deposits, but their presence is revealed by deep 

 borings. 



The older Miocene of the Atlantic and Gulf ( Chattahoochee and 

 Chipola stages) had a warm-water fauna very similar to that of the 

 West Indies and Central America ; some of the West Indian spe- 

 cies ranged as far north as New Jersey. The newer Miocene, on 

 the other hand {Chesapeake stage), shows a very marked faunal 

 change which points to the influx of cooler waters. This change 

 was probably due to the increasing size and elevation of the Florida 

 island, and the formation of the " Carolina ridge " in the bed of 

 the Atlantic, which diverted the Gulf Stream, pushing it farther 

 away from the coast than it had been before, or than it is now. 

 This allowed a cooler current from the north to follow the shore all 

 the way down to the Gulf of Mexico. Central America was still, 

 for the most part, under water, and rocks of Miocene age make 

 up most of the interior mountain ranges of Costa Rica. Miocene 



