PRINCIPAL TILL DEPOSIT 525 



3. It is far more deeply oxidized by weathering than the latest till. 



4. Numerous sections show l)oth this and a more recent overlying till 

 of different character and relatively slight weathering. 



1. Tracing from Long island. — On Long island and vicinity Puller* 

 found the Gardiner clay widely distributed beneath the Montauk drift, 

 the latter being overlain by stratified sands and gravels, and these in turn 

 by Wisconsin till. The Gardiner clay was later traced from Long island 

 along the south and north shores of cape Cod, where it has similar rela- 

 tions to the till.f The last exposure south of Boston is at Indian hill, 

 Plymouth, Avhere the same relations hold true. North of there the clay 

 is either below sealevel or it has been eroded, for at Saquish head. Dux- 

 bury, and Third cliff, Scitiiate, the drumlins visibly overlie more ancient 

 deposits. I These sections have been examined by the writer, and only 

 traces of the clay have been foimd at the base of the drumlin till. The 

 latter is in lithologic characteristics like the till overlying the Gardiner 

 clay at Indian hill, Plymoiith ; hence it would appear that this till is of 

 Montauk age. 



2. Eelations to overlying clays. — The Montauk till is overlain by fos- 

 siliferous marine clays that are in turn overlain by a younger till. The 

 farthest point south where clay has been observed overlying till is at 

 Warren cove, Plymoiith. In the vicinity' of Boston and in the region 

 northeast of that city the localities where clay can be seen to overlie the 

 principal till dejiosit of the region are so numerous -as to need no indi- 

 vidual mention. These may be found in numerous brick-yards, wells, 

 and road cuts. From Lynn northward the post-drumlin clay is fossil- 

 iferous and has been known as the "Leda clay." In many places north 

 of Boston it can be seen to be overlain by a still more recent till (Wiscon- 

 sin), as explained on pages 544-552. 



3. Degree of oxidation. — Wliile degree of oxidation is not absolute 

 proof of the age of any deposit, it furnishes an indication of the relative 

 ages of deposits of similar texture. A till deposit oxidized to tlie depth 

 of 10 feet is, other things, such as composition and texture, being equal, 

 probably much older than a similar till oxidized only 2 feet. The same 

 statement will hold true relative to clays and, to a more limited degree, 

 of sand deposits. 



The general depth of oxidation (surface yellowing) in the more recent 

 (Wisconsin) till, as exposed in sections widely scattered through north- 



* Bull. (ieol. Soc. Am., vol. 16, 1905, pp. 367-390. 



t Bull. no. 285, II. S. Geological Survey, pp. 432-441 ; Science, new series, vol. xxlv, 

 1906, pp. 467-409. 



t I. Bowman : Pre-Pleistocene deposits at Third cliff, Massachusetts. Am. Jour. Scl., 

 vol. xxil. 1906, pp. 313-325. 



