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a half mile east of south of these potholes is another smaller 
but very perfect one, on the eastern edge of the rocky ledge 
and on the roo-feet contour. It is a foot in diameter or less 
and about 2 feet deep. All three of these potholes are shown 
on the map by round dots. The last named one indicates 
that rapid and extensive currents must have flowed for a time 
even over the high ground. 
The fresh condition of these potholes seems to indicate 
that the lapse of time since they were formed has not been 
geologically great. It does not appear that they can be as old 
as the Tertiary, but rather that they were formed in Glacial 
times, or in the times immediately following. Additional ones 
at West Farms are recorded in Dr. Britton’s paper above re- 
ferred to. 
That the Bronx has been diverted, therefore, from a south- 
westerly course and from a natural drainage valley which is 
parallel with the strike of the country rock, and that it has 
cut across a ridge about 50 to 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 geologic times, there is no doubt. 
But when a diverting cause is sought, it is not easy to find. 
A gravel bar or a morainal deposit in the old channel some- 
where between Bedford Park Station and tide water was first 
thought of as the most probable cause. But exploration along 
the line of the old depression failed to show one. It is true 
that the brook shown on the map headed just above Fordham 
and flowed in the depression before street and railway im- 
provements masked it, but, even if it had eroded in large part a 
supposed gravel barrier, some traces of the latter should re- 
main. 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 per- 
haps a temporary stoppage of the old channel by the ice sheet 
had caused the river to clear of possible gravel, etc. But the 
presence of the potholes militates against this view and indi- 
