18 OYSTER BOTTOMS IN MATAGORDA BAY. 



When the tidal currents are reversed and the flow is running up 

 the bay the conditions of silt deposit also are reversed, and were it 

 not for two important factors there would result a general similarity 

 rather than a marked diversity in the aspects of the two sides of the 

 reefs. As has already been stated, the average velocity of the in- 

 flowing current must, from the relative positions of the stream 

 mouths and the mouth of the bay, be less than that of the out- 

 flowing, and it is therefore physically unable to take up and return 

 much of the material carried down and deposited by the latter. In 

 the second place, and entirely independent of the previous considera- 

 tion, the water in the lower bay, coming in large part from the sea, 

 is clearer than that above Dog Island. The streams are the main 

 sources of silt. This is gradually deposited in the course of the 

 water toward the sea, and, once deposited, would require a higher 

 velocity of current to pick it up again than sufficed to carry it 

 originally. 



In the light of this preliminary understanding of the action of the 

 currents and the local distribution of the silt deposits, let us examine 

 the effect upon that growth of oysters which fixes the final character- 

 istics of the beds. Upon the " up-the-bay " side of the reef we find a 

 deposit of silt from the more stagnant bottom strata of water inhib- 

 iting a set of spat at the foot of the barrier while at the same time 

 the flowing surface water is exerting a scouring action on the top of 

 the reef northeast of the crest. The preponderance of oyster growth 

 is therefore at the top of the reef and toward the upper margin of 

 that side, with the result that the margin in question tends to main- 

 tain a uniform outline and an abrupt face. The crest itself lies 

 closer to the northeast margin, because it, too, tends to grow in. that 

 direction from the same causes — the superior scouring action and food- 

 carrying capacity of the currents on that side of the reef. It can 

 never grow to a level much above the low-water plane, because as it 

 rises above that level the oysters are each year killed by exposure to 

 the air for long periods during the low water prevalent in the winter 

 months. On the opposite side of the reef, as we have seen, the condi- 

 tions are essentially different. Immediately upon crossing the crest 

 the outflowing water begins to deposit silt, which falls most abund- 

 antly in the lower levels between the oyster clusters, and the latter 

 soon become, therefore, the only places on that side of the reef pre- 

 senting conditions inviting a new set. Wave action, too, being more 

 energetic near the surface, tends to scour those areas raised somewhat 

 above the bottom, especially those surfaces looking toward the mar- 

 gin of the reef, and silt thus washed away is likewise thrown down 

 in the neighboring pools and crevices. The result is that the original 

 oyster clusters having this advantage gradually grow into clumps, 

 and these, by virtue of the greater cleanliness of their outer ends 



