158 
bridge, with stone abutments” by the designation orn ‘Boulder 
Bridge,’ the location of the pot-holes in the hemlock grove 
within the Garden boundaries may be readily recognized. 
After a lapse of about fourteen years they were further de- 
scribed and discussed, by Professor James F. Kemp, in a paper 
entitled ‘‘The Glacial or Post-Glacial Diversion of the Bronx 
River from Its Old Channel.’”’* This paper, and the one first men- 
tioned, include descriptions of the general geology of the region 
and discussions of. the probable geologic age of the pot-holes, 
and those who may be interested in the details of this phase of 
the subject may refer to them for information in this connection 
The present contribution will be restricted to descriptions and 
illustrations of three pot-holes within the Garden area. These 
are, apparently, known to comparatively few persons, while to 
the c ce visitor, even when noticed by him, their origin is not 
always understood and their significance is seldom appreciated 
P oles are bowl- or basin-like depressions in rock, caused 
by the abrasive action of gravel or cobble stones when churned 
around in the depressions by rapidly moving water. A pot-hole 
has its beginning in an irregularity or inequality in the rock bed 
of a stream, in which gravel or perhaps a single cobble stone or 
other rock fragment finds logdment. If the conditions are favor- 
able, so that the foreign material is free to move and the current 
of the stream is sufficiently rapid to churn or swirl it around, the 
original inequality in the stream bed becomes deeper and more 
or less circular in shape, from the abrasive action of its contents, 
and a pot-hole is formed. A pot-hole, therefore, no matter 
where it may be located, is definite evidence that the rock in 
which it was excavated must have been, at some time, a part of 
the bed of a rapidly flowing stream. 
There are three well-defined pot-holes within the boundaries 
of the Garden, each at a different elevation above the present 
level of the Bronx River. The lower two are located in the hem- 
lock grove and the highest one in the western border of the 
herbaceous plantation. 
* Trans. N. Y. Acad. Sci. 16: 18-24. 1896. Bulli. N. Y. Bot. Gard. 17: 78-85. 
97. 
