ORGANIC REMAINS. 123 



valley and show clearly that there has been no valley excavation there 

 since they were formed. 



The valley of Lost Creek in Lee County, Iowa (see fig. 4), though 

 lying wholly within the limits of the Illinoian drift, bears evidence of having 

 been occupied and largely excavated by a pre-Illinoian stream. It presents 

 a shallow trough cut in Kansan drift, which is covered, but not greatly 

 concealed, by Illinoian drift. Low swells of till formed at the Illinoian 

 invasion occur on its slopes and bottom, thus proving its pre-Illinoian exca- 

 vation. This valley is about one-half mile in width and 30 to 50 feet in 

 depth, and holds this size nearly up to the head, which is found in the 

 marginal ridge of Illinoian drift. It seems probable that prior to the Illi- 

 noian invasion its drainage basin was much more extensive than at present. 

 This affords an illustration of a partially reestablished stream. This valley 

 and that of the lower course of Skunk River are exceptional, for as a rule 

 the pre-Illinoian tributaries of the Mississippi were so completely filled at 

 the Illinoian invasion that the post-Illinoian drainage was opened along 

 new lines. It is probable that the portion of the Mississippi between Mus- 

 catine and Fort Madison, Iowa, is reestablished along a pre-Illinoian and 

 also pre-Glacial line. 



ORGANIC REMAINS. 



As yet no specific identifications of the wood and smaller plants found 

 at the Yarmouth horizon have been made, unless some of those reported by 

 McGree from northeastern Iowa have this horizon. The wood appears on 

 a superficial examination to be coniferous and largely red cedar. It is 

 planned to have careful examinations made in the near future to ascertain 

 the bearing the plants may have on the climate at the Yarmouth intergla- 

 cial stage. It should be considered, however, that the plants found in the 

 peat were presumably living just before the culmination of the Illinoian 

 invasion, and may not afford a true index of the interglacial stage ; for a 

 general lowering of temperature probably preceded as well as accompanied 

 the culmination of the ice sheet. Possibly a deposit will yet be found in 

 which plant remains occur which were buried in the middle part of the 

 interglacial period. The specimens of wood noted in the clay beneath the 

 bed of peat at Yarmouth may have been buried at a sufficiently early date 

 to be unaffected by the Illinoian glaciation. Unfortunately no specimens 

 of that wood are now available. 



