12 PROCEEDINGS OF MADISON MEETING. 



with a maximum of perhaps 20. While both Hues are visibly deflected by the 

 elevations of the land over which they pass, it is seen that the boundary is much 

 the more sensitive of the two to these variations in altitude. The moraine rises 

 from 300 feet at Belvidere, on Delaware river, to 960 feet at Budd's Lake, on Mus- 

 conetcong mountain, beyond which point it descends. The boundary rises from 

 the low ground at Riegelsville on the river to 760 feet on Musconetcong mountain, 

 beyond which it descends ; but at two different points we find its margin indented 

 by trap-ridges of much less altitude than these maximum elevations. This fact 

 seems to indicate that the southern margin of the ice was comparatively thin. 



Duration and Efficiency of the glacial Ice. 



Some additional facts enumerated below seem to show that the work of the ice 

 in this extramorainic area was relatively brief and not very powerful : 



1. Its brevity is indicated by its failure to produce well marked morainal lines. 

 "While an exceedingly slow and uniform advance and a retreat of the same char- 

 acter may be conceivable, yet the chances would be that if the ice covered the area 

 for any considerable length of time morainal lines would be developed. 



2. There are some districts where the signs of glaciation are exceedingly scanty, 

 where foreign bowlders or rounded bowlders or striated bowlders can scarcely be 

 found. An instance in point is the region north of High Bridge, where for several 

 miles no sure signs of glaciation were detected. The angular gneiss blocks which 

 monopolize the surface are still resting on a gneiss basis, as if never disturbed ; and 

 yet we now know from other sources that the whole must have been covered with 

 ice. It is hardly probable that postglacial erosion, however long continued, could 

 wholly remove foreign bowlders from any district over which they had once been 

 spread. 



3. Many of the till deposits, especially those farther from the moraine, are com- 

 posed largely of local material ; that is, material which has not been carried very 

 far. Is not this, in this case, a sign of the evanescent character of the glaciation ? 

 The first work of an invading ice-sheet will of necessity be that of pushing forward 

 the local materials loosened by previous disintegration. If the advancing margin 

 of the ice-sheet is formed, as is probable, from local snowfall rather than from 

 the onward moving ice that was compacted far northward, the foreign material 

 will be somewhat tardy in its arrival. In any case, the longer the glaciation con- 

 tinues the greater will be the quantity, if not the proportion, of foreign material. 

 It is perhaps true that the majority of till deposits contain in the aggregate a greater 

 amount of local than of foreign material, yet the vivid impression left upon one 

 after studying the southern deposits in New Jersey is that the till responds very 

 quickly to any change in the underlying rock, and that nearly all the erratics 

 might have been obtained from latitudes not higher than those covered by the 

 state of New Jersey itself. Comparatively few have to be referred to regions north 

 of the Mohawk or of the Saint Lawrence. 



Lobes of the New Jersey Moraine. 



The moraine in New Jersey may be naturally divided into two portions : 



1. That which was formed by the Delaware lobe of the ice-sheet in the western 

 part of the state. 



2. That formed by the Hudson lobe in the eastern part. 



