22 TRANSACTIONS OF THE [OCT. 19, 



ically great. It does not appear that they can be as old as the 

 Tertiary, but rather that thej' were formed in Glacial times, or in 

 the times immediatel}' following it. Additional ones at West 

 Farms are recorded in Dr. Britton's paper above referred to. 



That the Bronx has been diverted, therefoi-e, from a southwest- 

 erly course and from a natural drainage valley which is parallel 

 with the strike of the country rock, and that it has cut aci'oss a 

 ridge about 50-60 feet above its old channel and has assumed a 

 course nearly due south, at an angle of about 40 degrees to the 

 foliation of the gneiss, and that all this has happened in rather 

 recent geological times, there is no doubt. But when a divert- 

 ing cause is sought, it is not easy to find. A gravel bar or a 

 morainal deposit in the old channel somewhere between Bedford 

 Park Station and tide water was first thought of as the most 

 probable cause. But exploration along the line of the old de- 

 pression failed to show one. It is true that the brook shown 

 on the map headed just above Ford hum and flowed in the de- 

 ])ression before street and railway improvements masked it, but, 

 even if it had eroded in large part a supposed gravel barrier, 

 some stumps of the latter should remain. Observation of the 

 valley failed to reveal such and the topographical maps do not 

 suggest them. 



A second hypothesis assumed that the present gorge was an 

 old depression from an earlier drainage period, which perhaps a 

 temporary stoppage of the old channel by the ice sheet had 

 caused the river to clear of possible gravel, etc. But the pres- 

 ence of the potholes militates against this view and indicates a 

 rapid stream flowing over rocky ledges, and heavily charged 

 with grit. 



A third hypothesis, and one that appears to be the most reason- 

 able, is that during the presence of the ice-sheet, a sub-glacial, 

 or perhaps in part a supra-glacial stream down the upper valley 

 of the Bronx found its way out over this ridge and began to cut 

 it down ; being prevented issuing by the old channel because of 

 the presence of the ice. The objections to this are the brevity 

 of the time allowed by these conditions for excavating a gorge 

 60-70 feet deep. The hilltops about the gorge are glaciated, as. 

 Indeed, is the surface of the country very generally In this 

 vicinity. A lobe of entirely stagnant ice in the old channel as a 

 diverting cause is regarded as an almost too temporary affair. 

 The terrace of cobble stones is assuredly connected with the 

 great floods of the ice period in some way and with very copious 

 and swift waters, as their size indicates. 



Finally, it may be suggested as a fourth hypothesis, that the 

 present channel has always been the drainage line of the Bronx 



