194 H. L. FAIRClllLl) POST-GLACIAL I'PLIFT OF X. E. AMERICA 



ice-lobe, as ice-border drainage, and such dams were weak and shifting. 

 In consequence of this fact most glacial waters were epliemeral in life, 

 inconstant in level, and sudden or spasmodic in extinction. The last con- 

 dition is especially important. Glacial sand-plains rarely exhibit any 

 regular succession of terraces, because of the sudden or irregular down- 

 draining of the waters. By contrast, the ocean-level waters of the estu- 

 aries and their branches fell away (relative to the land) by steady, slow 

 decline, and the sand-plains built in these Avaters commonly present a 

 series of terraces, even down to the present stream floodplains. 



Sand-plains built in glacial waters may reveal some evidence of the 

 presence of the ice-margin; shown by ice-contacts, or by kettles, or by 

 included till masses, and less clearly by irregular deposition and poorly 

 assorted materials. Some deltas deposited in sealevel waters might have 

 been built near or even against the ice-front and might contain unas- 

 sorted materials. Any deposits made by glacial drainage, directly from 

 the ice-front or along the ice-border, are not likely to have good delta 

 form or relationship to land drainage, but may be good index of the sum- 

 mit sealevel. 



The most important distinction between glacial sand-plains and estu- 

 ary plains is the fact that the former, in independent, separated basins 

 would have no genetic correspondence in altitude. A series of detached 

 lakes, with accidental correspondence of height over a long distance, is 

 very improbable. But, on the other hand, the plains built in the estuaries 

 have close accordance in altitude over hundreds of miles along an uplifted 

 and tilted water plane. Glacial waters could never have height inferior 

 to the sea. By chance they might be near, slightly above, the sealevel. 

 Usually they were a recognizable distance above the marine plane, the 

 latter being determined by a variety of positive features over long dis- 

 tances and wide areas. A vertical interval with no, or very weak, deposits 

 suggests the relatively sudden drop of the glacial waters to the marine 

 level. 



It is only the sand-plains or shore forms very close to the marine plane 

 which are liable to confusion with the sealevel features. An example may 

 be helpful. 



Many years ago Professor Davis described the delta of the Catskill 

 Creek, built in the Hudson estuary, at South Cairo, some T miles from 

 the Hudson Eiver (Coxsackie and Catskill sheets). The relation of 

 the Catskill tributary valley to the Hudson trunk valley might permit 

 glacial damming in the former. The South Cairo plains have every char- 

 acter and relationship of an estuary delta built in slowly subsiding waters. 

 It may be conceded that we have no absolute proof that the Catskill waters 



