CAPE COD GEOLOGY 41 



others, a bed of clay was described as lying stratigraphically below the fossilif- 

 erous sand, and the sand was described as having a pebbly iron-stained basal 

 layer. The clay below the Sankaty sand is not so clearly described as to enable 

 one to decide whether it is a continuation of the stratified blue clay of the Gardi- 

 ners type, essentially free from glacial erratics, or an oxidized stony blue clay such 

 as crops out at Squam Head and at points farther north on the same coast. 1 

 Scudder describes the underlying clay as ' 'a thick bed of light-brown sandy clay" 

 and says that the overlying bed of gravel and sand is four feet thick, coarsest in 

 the upper part and more or less ferruginous, hardening on exposure to a rather 

 compact conglomerate. This description agrees well with that of the glacial 

 conglomerate at or near the base of the Pleistocene series in the eastern islands. 

 The quartz-pebble conglomerate in the underlying Cretaceous deposits is not 

 ferruginous. Furthermore, the overlying beds of fossiliferous sand carry small 

 pebbles and particles of feldspar-bearing rock, a characteristic of the Pleistocene 

 series as a whole and a feature not observed in beds of unquestioned Pliocene 

 or older Tertiary sand and gravel in the New England Islands. Shaler 2 was 

 ' 'disposed to believe that this lower sandy clay . . . represents the upper portion 

 of the lower till at Nantucket." Fuller, on physical evidence rather than on the 

 evidence presented by the fossils in the Sankaty beds, correlated the sand in the 

 Sankaty type section with the Jacob sand, which is regarded as transitional 

 between the Gardiners clay below and the Herod gravel above. 



Until recent years the marine fauna of the Sankaty section was invariably 

 regarded as of post-Tertiary age. At one time J. D. Dana erroneously assumed 

 that the beds are postglacial and that they form a part of his "Champlain 

 group," but Upham showed conclusively that the beds were older than the last 

 ice advance to the island. A detailed study of the section containing fossil shells 

 was made during the summer of 1904 by Dr. J. H. Wilson, 3 who appears to have 

 regarded the underlying clay as of glacial origin and "identical with the 'pre- 

 Wisconsin till.'" Twenty-one species new to the locality were discovered, of 

 which a number were identified by Dall. Among the newly found species were 

 Serripes perousii Deshayes, Macoma incongrua von Martens, and Pandora crassi- 

 dens Conrad, the first two of which had not been previously found east of Point 

 Barrow. The form last named is common in the Miocene of Maryland. Dr. 

 Wilson appears to have assumed that the fauna as a whole lived in Pleistocene 



1 Wood-worth, J. B., The glacial brick clays of Rhode Island and Massachusetts, Seventeenth Ann. 

 Rept. U. S. Geol. Survey, pt. 1, ch. 2, pp. 976-977, 1896. 



2 Shaler, N. S., Geology of Nantucket, U. S. Geol. Survey Bull. 53, pp. 33-34, 1889. 



3 Wilson, J. Howard, The glacial history of Nantucket and Cape Cod, p. 90. The Columbia Uni- 

 versity Press, New York, 1906. 



