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on probably from the southern ridge, as it is working on along the ditches 
still, as that ridge itself may have been washed up before the first reces- 
sion took place, from the depths of Lake Michigan. The peat on that 
marsh now is a quite new or fresh formation, having come from the roots 
of the vegetation that sprang up when the water had largely receded. 
But these operations of nature, of large interest always, and especially 
when we can observe them for a few years, are mostly speculative, rather 
than certain. The writer of this has had an opportunity to observe 
through the space of forty years, and therefore to know with certainty 
with what great rapidity, in some places, during heavy rainfalls, sand 
will be washed over a large area of bottom land. He has seen prodigious 
quantities of sand and gravel removed quite a distance by successive rain- 
falls. 
PRELIMINARY WORK FOR THE APPROXIMATE DETERMINATION OF THE TIME 
SINcE THE RETREAT OF THE First GREAT IcE SHEET. 
By GLENN CULBERTSON. 
It was with the desire of obtaining a close approximation to the time 
which has elapsed since the retreat of the Kansan or first great ice sheet 
that, during the past summer, the two most important waterfalls—Clifty 
and Butler—in the vicinity of Madison and Hanover, Jefferson County, 
Indiana, were visited by me and the work to be described was performed. 
The well-known Clifty Falls, over which the water leaps a vertical 
distance of seventy feet, was the first visited. Into drilled holes steel 
rods were driven vertically to the depth of twelve or more inches in the 
solid limestone of the stream bed at a distance from the precipice over 
which the water falls, and accurate measurements from the rods to the 
edge of the precipice were made and recorded. 
Butler Falls, which is located about one-half mile south of Hanover, 
and over which the water falls eighty feet, was also visited and similar 
measurements made and recorded. 
The falls in both cases are caused by the presence in the stream beds 
of very durable strata of limestone, chiefly of the Madison and Clinton 
formations, and which is of very uniform texture and hardness over the 
> 
region referred to. The rate of valley growth toward the head is governed 
by the erosion or undermining of these rocks. 
