398 



SCIENCE 



[N. S. Vol. XLIIL No. 1107 



This paper shows what is meant by group rela- 

 tionship in physiography; how the notion of group 

 relations among features is of value in descrip- 

 tion, explanation and classification of the features, 

 and how a recognition of such relationships among 

 features may be of assistance in the interpreta- 

 tion of field problems. 



The Pleistocene of Capitol Hill, Des Moines, la.: 



James H. Lees. 

 Some Evidence Begarding the Duration of the 



Yarmouth Inter-glacial Epoch: Geokge F. Kat. 



That the time interval between the retreat of the 

 Kansan ice and the advance of the Illiuoian ice 

 into Iowa was of long duration is suggested 

 strongly by recent studies in the area of Kansan 

 drift in southern Iowa. This view regarding the 

 Yarmouth Inter-glacial epoch is supported by evi- 

 dence as follows: (1) On the Kansan drift where 

 erosion has been slight there is a thoroughly 

 leached, non-laminated, tenacious clay called 

 gumbo, twenty feet or more in thickness, which is 

 thought to have been formed chiefly by chemical 

 weathering of the upper part of the Kansan drift. 

 (2) Diastrophic movements subsequent to the 

 formation of the gumbo, the country having been 

 elevated one hundred and fifty to two hundred 

 feet. (3) A mature topography which was de- 

 veloped by erosion after the diastrophism and, ap- 

 parently, in the main, before the close of the Yar- 

 mouth epoch. 



Valley Trenching and Graduation Plains in South- 

 ern Indiana and Associated Megions: Clyde A. 

 Malott. 



This paper attempts to establish a partial pene- 

 plain in the central Mississippi valley post-Lafa- 

 yette in age. East White River basin of southern 

 Indiana furnishes the type region, where at least 

 three former base levels are in evidence. Through 

 the middle part of this river basin in the region 

 of limestones and resistant sandstones, a gradation 

 plain is evident at about eighty feet above the 

 present streams. This gradation plain traced to 

 the areas of soft rocks corresponds with the gen- 

 eral upland level of the soft Devonian shale and 

 lower member of the Knobstone group of middle 

 eastern Indiana and of the soft sandstones and 

 shales of the productive Cioal Measures of the 

 southwestern part of the state. At a hundred to 

 a hundred fifty feet above this gradation plain is 

 a peneplain of rather general prevalence in the 

 harder rocks of the state. It is represented in the 

 soft rock areas by monadnocks and rugged up- 



lands only. This peneplain is called the Mitchell 

 plain in southern Indiana. Again in the harder 

 rocks is found a yet higher base-level, a hundred 

 to two hundred feet above the Mitchell plain. 

 This level is represented by monadnocks and flat- 

 topped divides. It forms the highest land in the 

 southern part of the state. The age of the grada- 

 tion plain, marked by the lower uplands of the 

 state, is found by tracing it across southern Illi- 

 nois to the Ozark region, where it is seen to be de- 

 veloped at a lower level than the peneplain which 

 has upon it the gravels of supposed Lafayette age. 

 Moreover the Mitchell peneplain can be traced in- 

 terruptedly by monadnocks to the Shawneetown 

 Hills and Karbers Ridge which represent the level 

 of the Lafayette gravel peneplain of the Ozark 

 Plateau. The highest level of southern Indiana is 

 correlated with the Lexington plain of Kentucky 

 and the Highland Rim of Tennessee, and with less 

 assurance with the base-level some two hundred 

 feet above the Lafayette level of the Ozark re- 

 gion. In literature it is placed in early Tertiary 

 age. Evidence of a post-Lafayette gradation plain 

 or local peneplain is found in several places in the 

 Mississippi valley. In the Nashville Basin of Ten- 

 nessee the flat peneplain along Stones River is 

 some eighty to a hundred feet below the frequent 

 Lafayette gravel capped hills, and the stream is 

 also trenched below the peneplain. Again, in the 

 Drif tless area of Wisconsin, broad ' ' basin valleys ' ' 

 are found at one hundred feet lower than the Lan- 

 caster peneplain determined by Grant, Bain and 

 others to be Lafayette in age. These "basin val- 

 leys" no doubt represent a gradation plain, and 

 are in a position similar to the gradation plain of 

 Indiana. Still another instance may be found in 

 the Parker strata of the upper Ohio. Thus, taking 

 all the evidence into consideration, it seems that 

 there is a rather widespread base-level plain of 

 post-Lafayette age over the Mississippi valley. In 

 southern Indiana it was developed long before the 

 advent of the Illinoian glacial ice. 



The Extremes of Mountain Glacial Erosion: Wm. 



H. HOBBS. 



In a series of articles printed in the year 1910, 

 the writer pointed out that the mountain districts 

 which in the past have been occupied by mountain 

 glaciers, represent each a particular stage in a 

 cycle of erosion, or especially of a receding hemi- 

 oycle. The Bighorn range of Wyoming was cited 

 as the best example of the early stage where gla- 

 cial sculpture has modified but a small portion of 

 the inherited upland surface. This topographic 



