CLAM FARMS — TURNER 471 



GEOLOGY INVOLVED 



With the help of our colleagues in marine geology we arrived 

 at an explanation involving a hypothesis of "hyclrographic concen- 

 tration" which has been subsequently demonstrated to be correct 

 and also applicable to the formation of concentrations of a number 

 of different marine organisms. Fortunately the geology of the 

 Quincy beach was well understood. The upper beach which con- 

 sisted of a mixture of coarse sand and pebbles sloped steeply down- 

 ward nearly to the low-tide mark. Here, it leveled off and the 

 sediments graded through decreasing particle sizes from coarse sand 

 to a very fine silt. The mechanics of the formation of such a beach 

 is as follows: waves striking the shoreline rush up the beach with 

 considerable force, carrying particles as big as large pebbles in the 

 uprush. Some of the water soaks into the beach leaving less water 

 to return in the backwash. As a consequence the backwash runs 

 down the beach with reduced force. The reduction of the force of 

 the backwash causes it to leave the coarser and heavier objects behind, 

 so that the upper parts of the beach remain rocky or pebbly. 

 Lighter particles are transported seaward and deposited accorduig 

 to size as the force of the backwash is lessened. The coarser sand 

 grains are dropped first, forming a flat sandy beach near the low tide 

 while the finer sands and silts remain suspended until they settle out 

 in the relatively quiet waters beyond the level of the lowest low water. 

 The process is repeated during the rise and fall of each tide sloshing 

 materials up and down the beach, separating the particles according 

 to size and density in much the same way that a winnowing machine 

 separates gram from chaff. 



These hydrographic forces act the same way on clams. However, 

 clams differ from inanimate objects in that they grow. ^Vlien they 

 first settle, the clams are microscopic in size and very light. Any 

 disturbance sufficient to stir up the bottom will bring them into suspen- 

 sion and keep them moving until they arrive at some place where 

 conditions are sufficiently quiet for them to resettle along with the 

 silt particles. This, we believe, is what concentrated the clams in 

 incredible numbers in the finer sediments at the lowest part of the 

 beach. The tiny clams sloshed up and down the beach with the rise 

 and fall of the tide, and as they grew they became larger and reacted 

 to the forces of the wash, as if they were coarser sand grains. Thus 

 they tended to settle more shoreward where we found them concen- 

 trated at low tide in November. As they grew larger they became 

 more like pebbles and became concentrated on the sloping apron of 

 the beach. By this time they had grown so large that they could dig 

 in deep enough to avoid the disturbance of the surface of the substra- 

 tum by the waves. Here they remained for 3 years and grew to a size 



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