10 PHYSIOGRAPHY AND GEOLOGY 



seen through the clear water which covers it. Aside from the islands, keys, 

 and rocks, which will be discussed later, three features stand out prominently. 

 They are coral reefs, sand bores, and marine ocean-holes. 



Everywhere over the surface of these shallow banks coral heads and reefs 

 are to be found. Along the more exposed eastern face, the coral polyp 

 flourishes and builds extensive barrier reefs making that shore practically 

 inaccessible to shipping, while over the surface of the more sheltered banks 

 individual coral heads and small reefs are constantly encountered. These cause 

 the bottoms to shallow suddenly and are consequently much dreaded by sailors. 

 Dangerous reefs are frequently scattered so thickly over the surface of the 

 banks that it has proved impracticable to chart them. The waters where they 

 occur are avoided by vessels of large draft, and navigation is never attempted 

 save in broad daylight and with a sailor at the bow to notify the helmsman of 

 approaching reefs. 



Sand bars or " sand bores " as they are usually called occur in greatest 

 abundance over the surface of the bank south of the Tongue-of-the-Ocean. 

 In this region they are so numerous as to make it dangerous for even light 

 shipping during times of ebb tide. These sand bores are very low bars of white, 

 coral sand which collect on the banks and frequently rise a few feet above the 

 surface of the ocean. During ebb tide they are laid bare in great numbers, 

 but during high water most of them disappear. They are not fixed in one 

 position, but shift about with the ever-changing currents. 



Submarine ocean-holes, or " blue holes " as they are frequently called, are, 

 as the name indicates, deep holes which open suddenly downwards from the 

 surface of the banks. In the Bahamas, the color of shallow water is green, 

 and of deep water, blue ; so that depth is indicated by color. The presence of an 

 ocean-hole is therefore shown by an isolated area of blue water in the midst 

 of a sea of green; hence the term "blue hole." These ocean-holes vary in 

 diameter from a few feet to a quarter of a mile or more. Their sides beneath the 

 opening frequently flare out like a bottle, and are usually covered with healthy 

 branches of growing coral and many different varieties of marine plants, 

 showing that there is a constant circulation of water. Numerous attempts 

 have been made to ascertain the depth of these holes, but only in a few cases 

 have lines succeeded in reaching the bottom. Agassiz found that some of the 

 holes which he fathomed were at least 300 feet deep. It has been frequently 

 observed that the water boils or rushes through these ocean-holes in harmony 

 with the ebb and flow of the tide, proving that they are connected with deep 



