GLACIAL GEOLOGY 
415 
The occurrence of the ash-bed immediately beneath the Wiscon¬ 
sin till-sheet and directly upon the Yarmouth-Sangamon-Peoria 
loess, which was wind-formed, fixes the time of deposition as also 
interglacial — probably Peoria. This brings the date of deposi¬ 
tion very close to us, geologically speaking. 
In casting about for the location of the nearest possible source 
of such volcanic material we naturally look first of all to the 
Southwest which would be the direction of the prevailing winds. 
Fig. 14. Volcanic Ash from Iowa 
The nearest volcanic eruption of the violent kind that we know 
of is Mount Capulin, in northeastern New Mexico. This is an 
ash-cone whose magnitude far surpasses that of famed Vesuvius, 
It is 3000 feet high and its crater is half a mile across. But 
Capulin is very recent. Its last eruption perhaps scarcely ante¬ 
dates the landing of Columbus on the western continent. There 
are other volcanoes in the Capulin field which might have served, 
for volcanic outburst there was continuous throughout Quaternic 
and part of Tertic time. Spanish Peaks in southeastern Colorado, 
seem too early in their last eruptions. Another possibility is the 
