22 



CAPE COD GEOLOGY 



continent. The beds of the same age on Marthas Vineyard, at,Gay Head, have 

 undergone so much deformation and dislocation that their present elevation 

 above the sea, in the crest of folds, does not indicate that they were laid down 

 above sea level. The depth below sea level and the distance from shore at which 

 the beds of greensand and their fossil crabs, fish, and marine mammals were 

 laid down are not certainly known. The deposits of greensand that are now 

 being formed on the sea floor lie so far out at sea that relatively little mud and 

 sand reaches them from the shore. Pure greensand is laid down in water having 

 a depth as great as 900 fathoms, but it may accumulate in shallower water 

 at places where mud and sand are excluded from it by the nature of the shore 

 or by the absence of offshore currents to carry out debris from the land. The 

 presence of grains of quartz in the Miocene beds at Marshfield and Gay Head 

 show that those beds were laid down in relatively shallow water. The bed of 

 greensand at Nonamesset, which lies nearer the theoretical shore line of the 

 Miocene sea, contains much more quartz and coarser grains of it than the bed at 

 Gay Head. Conditions for the deposition of greensand containing terrigenous 

 material would probably be provided by the submergence of the lowland of east- 

 ern Massachusetts and Rhode Island to a depth that would carry the shore line 

 in the longitude of Worcester up to the present 500-foot level. Such a shore line 

 would extend nearly northward, passing through the eastern parts of the Kent, 

 Burrillville, Blackstone, Marlboro, and Groton quadrangles. The remnants of 

 the beds of greensand at Marshfield and Gay Head now lie about 45 miles sea- 

 ward from the 500-foot contour line, and the depth of water over them, under the 

 conditions here assumed, would have been somewhat more than 450 feet. Most 

 of the present coast of eastern Massachusetts would then have been submerged. 

 The casts of mollusks and crustaceans and the bones of cetaceans found at 

 Gay Head and Marshfield suggest the depth of the water there, but their probable 

 mode of burial should be considered before conclusions based on them are formed. 

 The bones of the cetaceans are strewn through the deposits in a way to indicate 

 that they were laid down at about the same time as the foraminifers that make 

 up most of the greensand. The remains of crabs found in the greensand were prob- 

 ably also entombed with the growth of the deposits. Shallow water is preferred 

 by species closely allied to those found in these beds of greensand. Of the mol- 

 lusks, particularly the clam-like forms, one of which (Macoma lyelli) is found 

 with both valves intact, in the attitude of life, in the greensand at Gay Head, 

 it may be said that they might have lived beneath the surface of the bed of green- 

 sand after the water above it had become shallower than that in which it was 



