414 
GLACIAL GEOLOGY 
came from some of the later eruptions in the Rocky. Mountains 
or the Pacific Coast ranges. 
That the eruption that furnished the glass-dust for the Iowa 
deposit was a notable one wherever it was, is clearly indicated by 
the depth and volume of the accummulation. In order to attain 
a thickness of six inches, or a foot, the dust-cloud from the vol- 
^ canic vent must have been driven by a strong and prevailing wind. 
No doubt a like thickness of ash spread over a very large ex¬ 
panse of country, probably over hundreds and perhaps thou¬ 
sands of square miles. Except in especially favored localities 
this dust mingled with the soil and soon lost all evidence of its 
existence. The deposit under consideration manifestly owes its 
preservation to the circumstances that the dust settled in a small 
pool of water where it remained undisturbed from further wind 
action and fully protected from the erosive influence of the rains. 
This notable volcanic ash-bed is brought to sky in the course 
of recent extensive grade-cuttings on Fifth Avenue, between 
Grand Avenue and School Streets, in the city of Des Moines. 
With it is also exposed the finest succession of Glacial deposits ever 
disclosed within the limits of the state. The longitudinal exposure 
is over half a mile, and the vertical cut 50 feet as a maximum. 
It is essentially a north and south cross-section of West Hill> 
along a line about half a mile from the Des Moines River. The 
best ash sections are near the corner of Crocker Street; and the 
formation is locally christened for brevity the Crocker Ash-bed. 
The vertical section displayed at the Crocker Comer is as fol¬ 
lows: 
7. Soil, black loamy .... Feet 2 
6. Till, yellow, bouldery, (Wisconsin) . . 10 
5. Sand, very fine, gray (volcanic ash) . 1 
4. Loam, black, pebbleless .... 1 
3. Loess, yellow (Peoria) .... 15 
2. Till, dark red, bouldery (Kansas) . . 6 
1. Shale, variegated (Carbonic), exposed . 4 
The bed of volcanic ash lies mainly in a shallow depression in 
the top of the loess. It is much disturbed and broken, a feature 
probably due to the plowing action of the Wisconsin ice. On the 
same horizon a few rods away is a bed of coarse gravel and 
small boulders containing numerous small logs of charcoal, which 
evidently were once drift-wood. 
