GLACIAL GEOLOGY 
413 
GLACIAL GEOLOGY 
Volcanic Ash in Glacial TUI. There are reports of deposits of 
volcanic ash from numerous localities on the Great Plains. So 
far as is known no occurrence is recorded from the prairies east 
of the Missouri River. Recent discovery of an extensive ash-bed 
in central Iowa is therefore of great interest not only because of 
the fact that it is the eastern-most locality for such deposits yet 
noted, but for reason that the geologic time of the volcanic erup¬ 
tion causing it is narrowly fixed. The Iowa deposit immediately 
underlies the Wisconsin drift sheet and immediately overlies the 
Peorian loess. A clue is thus afforded for determing the geologic 
date of the several western ash deposits that heretofore is not 
revealed. 
Of all geological phenomena represented in Iowa those illus¬ 
trating volcanic action are the rarest. Although we already know 
of the presence of very ancient lava-flows which are disclosed in 
deep-well drillings, these are associated with rocks very much 
older than any outcropping in the state. These are the Keewena- 
wan porphyries of the Northwest. From that very ancient date 
to the present time there is not the slightest sign of volcanic ac¬ 
tivity descernible in all the State’s very full geological record. 
The ash uncovered in the city of Des Moines, a few weeks ago, 
is that pumaceous glass variety which, during violent volcanic 
eruptions, is thrown high in the air and is wafted far abroad 
on the wings of the wind. The material is an impalpably fine 
dust. In the instance of Krakatoa, in the Straits of Sunda, in 
1884, similar volcanic glass-dust drifted quite around the world, 
and produced the gorgeous “red sunsets” for many a month after¬ 
wards. Because of the abundance of the ash and of its thickness 
at Des Moines, it is not probable that the site of its origin is so 
far away as the other side of the earth. Most likely the material 
