THE EFFECT OF LEACHING ON DRIFT PEBBLES 
J. A. UDDEN 
University of Texas — 
How different rocks endure the superficial agencies of leaching 
and solution may be seen from the composition of pebbles in bowlder 
clay where this has been subjected to the solvent action of meteoric 
waters. 
Many years ago the writer took occasion to collect fifty-eight 
lots of 100 pebbles each, from bowlder clay in Louisa County, in 
Towa. By collecting pebbles of one size, about one-half inch, 
and by taking all of this size to be found on a limited space of the 
clay surface, unconscious selection of different kinds of rock was 
avoided. Thirty-seven of these samples were taken from places 
where the drift was in its original condition, unchanged by subse- 
quent weathering or decay. Twenty-one samples were taken from 
the upper part of the bowlder clay, where it had been more or less 
leached and weathered. 
In this region the bowlder clay is overlain by loess. The 
leaching of the clay sometimes extends five feet under the loess, 
but more often less than this. From this upper part of the till, 
calcareous material may be partly or entirely absent, so that there 
is no reaction for carbonates when acid is applied to the clay. 
Pebbles of limestone, which were no doubt originally present, have 
wholly or partly disappeared, and only less readily soluble materials 
remain. The leaching is most complete in the uppermost part of 
the till, and from here to the unaltered material below, is a zone 
in which the leaching is incomplete in varying degree. Limestone 
pebbles in this zone are either etched on the surface, or else they 
have suffered partial internal solution and are porous and even 
crumbling and are often yellow or brown from residual or infiltrated 
ferruginous material. 
Below is a table which gives the percentages of the most impor- 
tant rocks noted in the study of the pebble samples collected. In 
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