18 GLA(JIAL GEA.VELS OF MAINE. 



the plain, the flood crossed recently plowed fields. The snrface was eroded 

 somewhat and was left with mnuerous swells and hollows, up to a foot in 

 dejjtli, yet this small erosion Avas produced b}^ currents swift enough to roll 

 along mud lumps a foot in diameter. About 5 miles below where the flood 

 issued from the narrow valley, it became concentrated Ijetween banks of 

 hail and swept away a house situated on an open plain in the city of 

 Colorado Springs. 



These and numerous similar observations in Colorado, both in the 

 recent water drift and in that of Tertiary age, shoAv bowldei's of consid- 

 erable size surrounded by fine sand and gravel and occasionally embedded 

 in clay. It thus appears that swift currents can flow over a stratum of fine 

 sediment having an even or level surface without eroding it much, due 

 largely to the fact that the lower part of the water is nearly stopped by 

 friction. The stream can not, so to speak, get at the sediment while it 

 remains coherent. But Avlien a stream impinging against a vertical bank 

 undermines a portion of it, the alluA'ium usually loses its cohevence the 

 moment it is precipitated into the water. The particles now being isolated 

 are no longer able to protect one another by mutual cohesion and friction. 



These observations have a bearing not only on the occurrence of large 

 stones and bowlders in the valley drift, but also on the bowlder beds found 

 in ancient rocks. I consider it certain that large stones and even bowlders 

 may be deposited by running water in the midst of sediments as fine as 

 sand, and even in clay. What is required is a rapid current moving over 

 an even surface and acting for a rather short time. The sudden storms of 

 the Rocky Mountains furnish the required rush of water, and it is quite 

 possible that the spring floods of the Valley Drift period also aff"orded the 

 necessary conditions. 



Large stones found in the sedimentary marine clays must have been 

 dropped from above by ice or other floating body. 



TRANSPORTATION AND EROSION BY SPRINGS AND SUBTERRANEAN STREAMS. 



This important means of erosion and transportation has not hitherto 

 received from students of the drift the consideration it deserves. 



The action of subterranean water is not very rapid, but it is persistent. 

 The rain seeping down through the earth dissolves some of its ingredients. 

 At depths below the reach of frost this process slowly enlarges the spaces 



