90 MAX AXD THE GLACIAL PERIOD. 



knobs. In the lower portion of the valley no pebbles 

 could be found which were scratched. Up the valley 

 a few miles pebbles were occasionally discovered which 

 showed some slight indications of having been scratched, 

 but which had been subjected to such an amount of abra- 

 sion by water-action as almost to erase the scratches. On 

 reaching Ackley's Station, the stream is found to be cut- 

 ting through a regular terminal moraine, extending across 

 the valley and full of clearly marked glaciated stones. 

 Above this terminal moraine the terraces and gravel 

 ridges which had characterised the valley below disappear, 

 giving place to long stretches of level and swampy land, 

 which had been subject to overflow. 



Something similar to this so often appears, that there 

 can be no question as to its meaning, which is, that dur- 

 ing the farthest extent of the ice the front rested for a 

 considerable period of time along the line marked by the 

 terminal moraine. During this period there occurred both 

 the accumulation of the moraine and of the gravel terraces 

 in the valley below, due to the vast flow of water emerg- 

 ing from the ice-front, esj)ecially during the period when 

 it was most rapidly melting away. Upon the retreat of 

 the ice, the moraine constituted a dam which has not yet 

 been wholly worn away. For a while the water was so ef- 

 fectually ponded back by this as to form a lake, which has 

 since become filled up with sediment and accumulations 

 of peat. From this it is evident, also, that when the ice 

 began to retreat, the retreat was so continuous and rapid 

 that no parallel terminal moraines were formed for many 

 miles. 



Before leaving this section we will summarise the 

 leading facts concerning the glacial phenomena north of 

 Pennsylvania and New Jersey. From the observations of 

 Professor Smock, it appears that, from the southern mar- 

 gin the ascent to the summit of the ice-sheet was pretty 

 rapid ; the depth one mile back from the margin being 



