88 .IOWA ACADEMY OF SCIENCES. 



CRITERIA FOR THE DISCRIMINATION OF DIFFERENT DRIFT 

 SHEETS. 



Forest and Peat Beds. — Among the common and obvious evi- 

 dences of interglacial periods none are more widely recognized 

 than buried forests and peat beds. These constitute the one 

 phenomenon which appeals alike to layman and geologists, 

 and buried forests are a constant element of wonder in regions 

 in which they are common. Their wide recognition and fre- 

 quent citation has probably been out of proportion to their true 

 importance. It is recognized alike by advocates of one and of 

 more than one glacial period that not all forest beds may be 

 cited as legitimate evidence of important interglacial intervals. 

 The admitted fact that forests may and do crowd up to the edge, 

 and even grow upon, the ice of some of our largest glaciers, 

 makes it evident that any temporary readvance of the ice would 

 be apt to cover up a forest bed. Whether the vegetation would 

 follow the edge of a continental ice sheet as closely as a smaller 

 glacier is unknown, but may fairly be considered doubtful. 

 The real significance of a forest bed, however, arises not from 

 the fact that it shows that during the ice period there was a 

 retreat of the ice for a period long enough to allow vegetation 

 to gain a foothold over areas later reburied by the ice, but 

 from the light which they sometimes throw upon the climatic 

 and physical conditions prevailing during the interval. If the 

 vegetation includes plants indigenous to warm or temperate 

 climates, it indicates a considerable climatic change, which can 

 hardly be assumed to mean anything but a considerable time 

 interval. The only escape from this conclusion is to assume a 

 change in the habit of the plant in question; which would need 

 independent proof but might be indicated by its associations. 



The vegetation may, however, be of such a character as not 

 to prohibit the assumption of a cold climate and yet its disposi- 

 tion be such as to indicate a relatively long and quiet period of 

 accumulation, and, inferentially, a freedom from glacial condi- 

 tions. A case in point is the Oelwein peat bed as pointed out 

 by Professor Macbride at the last meeting of the academy. * 

 The evidence in such a case is obviously, while still important, 

 of less value than in the former. 



It is conceived that if a forest bed, even if it showed only a 

 boreal or possibly boreal vegetation, could be proven to occupy 



*Proc. Iowa Acad. Scl., Vol. IV, pp. 63-68. 



