460 SHIMER. 



if not all, of the vertical rods penetrated the glacial clay, one to a depth 

 of eighteen inches. The lowest horizontal sticks preserved were about 

 a foot and a half above the glacial clay, and hence between two and 

 three feet below the top of the four-foot bed containing the warm-water 

 fauna. The vertical rods would naturall}' not have been driven 

 deeper than to allow the horizontal sticks to rest upon the surface 

 of the mud. 



It is possible that there were horizontal sticks lower than this which 

 were destroyed before they were covered by the preserving mud or 

 had left their impress upon the vertical rods. Possibly, also, the 

 horizontal withes were not driven deep enough to rest upon the mud 

 surface. It would seem that one or both of these suppositions might 

 be true. The forcing of two inch rods, bluntly and roughly sharpened 

 by a stone ax, as the Harvard specimen shows, into the stiff glacial 

 clay to a depth of eighteen inches, would be a difficult thing to do. 

 Moreover a penetration of the clay to a depth of eighteen inches would 

 suffice to support the weir. These probabilities would add, however, 

 only about a foot and a half to the thickness of the sediment deposited 

 since the weir was erected. 



If we consider the lowest preserved horizontal sticks as originally 

 the lowest and as resting upon the surface of the mud when erected, 

 then about thirteen feet of shells and mud had been deposited between 

 the time when man planted the fish-weir and when he blotted out the 

 Bay. If we consider the probability that there was practically no 

 silt present when the weir was erected it would mean the deposition 

 of fourteen feet, eight inches of sediment between that time and the 

 artificial filling of the Bay. The top of the vertical rods preserved 

 was ten to twelve feet below the junction of the silt and fill. 



How long a time was consumed in the deposition of these thirteen 

 to fifteen feet of silt and shells is largely a matter of conjecture. 

 It has been estimated that the Mississippi River deposits a foot of 

 mud in two hundred years. A similar rate here would have required 

 2500 to 3000 years for the accumulation of this thickness. The 

 streams in the Boston area would have carried annually much less 

 sediment than the Mississippi. The amount retained in the Back Bay, 

 however, would be a balance between the amount of mud delivered 

 into this inland bay protected by many islands and the strength of the 

 tidal scour. That the tidal scour was stronger during the existence 

 of the warmer climatic fauna is shown by the presence in this Back 

 Bay area of an abundance of oysters. These need a bottom free from 

 mud or slime. A stronger tidal scour during the formation of this 



