122 
IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
The composition of each is identical with that of the other, 
both as regards the size and quality of the component pebbles, 
and as to the nature of the cementing material. The greater part 
of the pebbles are rather large ranging from one-fourth inch to 
three inches in diameter, and on the average are of fairly uniform 
shape. They are of material which is entirely siliceous in charac- 
ter, many being almost pure quartz, translucent cream to yellow, 
while others are of an opaque cherty material ranging from grey 
and brown to yellow, and in many cases jasper-like in ap- 
pearance with marked conchoidal fracture when broken. Nearly 
all are well rounded, of puddingstone variety, bearing every evi- 
dence of being waterworn shore-pebbles. Very little if any an- 
gular or brecciated material is present among them. 
The interstices are filled with a siliceous cementing material 
which is of a typically quartzitic texture throughout, quite similar 
in appearance to the Algonkian quartzite of Minnesota, South 
Dakota and Iowa, which is easily recognized and rarely mis- 
taken by one who has studied the formation at the ledge. It pos- 
sesses binding qualities which are remarkably strong and tenacious, 
which fact must be self-evident, as it would have been utterly im- 
possible for conglomerate of less firm a texture to have withstood 
the enormous pressure and grinding power of the containing ice 
sheet during the process of transportation throughout such a great 
distance as these boulders must certainly have been carried. The 
more enduring granites, quartzites and their like are quite often 
found, having been transported from near the source of the drift 
sheet to its extreme outer margin, but it is quite unusual to find the 
more friable limestones, sandstones and conglomerates at any great 
distance from the place of their origin. 
From the position in which these boulders were discovered it is 
not improbable that they were carried almost if not the entire dis- 
tance as a single mass, becoming separated only in the last stages 
of their journey, or perhaps after they had reached their final 
resting place, by means of disintegrating forces possessed by the 
freezing processes to which they were subjected during our rig- 
orous winters. It is possible that other fragments are present 
but have not yet been found, owing to the fact that no special 
search with this in view was made at the time of their discovery. 
While the boulders themselves are entirely devoid of any evi- 
dence of stratification there can be little doubt that they originated 
as a basal conglomerate formed in the mid-continental seas to the 
