628 



SCIENCE 



[N. S. Vol. XXIX. No. 746 



Weathermg and Erosion as Time Measures: 



Fbank Leveeett, Ann Arbor, Michigan. 



The paper aimed to set forth the use that may 

 be made of weathering and erosion in determining 

 relative age of the several drift sheets. It alao 

 dealt with the most important qualifying condi- 

 tions that affect estimates. 



At the close of the reading of this paper, at 

 12:30 o'clock, adjournment was taken, discussion 

 being postponed to the afternoon. 



The society convened again at 2 o'clock, Presi- 

 dent Calvin presiding, and took up the discussion 

 of Mr. Leverett's paper, the participants being 

 Professors A. Penck, S. Calvin, F. Leverett, G. 

 F. Wright and G. K. Gilbert. 



The next two papers were then read: 



The Glacial Phenomena of Southeastern Wiscon- 

 sin: Wm. C. Alden, Washington, D. C. (Intro- 

 duced by T. C. Chamberlin.) 

 A graphic presentation by a map 9 X 10 feet, 

 scale one mile per inch, of a detailed study of the 

 deposits of the Green Bay and Lake Michigan 

 glaciers and associated phenomena of late Wis- 

 consin glaciation of southeastern Wisconsin. The 

 map covers an area approximately 8,600 square 

 miles, which has been under study by the author 

 under the direction of Dr. T. C. Chamberlin, dur- 

 ing the greater part of the last ten years. The 

 presentation comprised such description as the 

 time permitted of the conditions affecting the 

 advance of the two glaciers, their relations to each 

 other, the character, distribution and mode of 

 formation of the several deposits, terminal and 

 recessional moraines, outwash deposits, ground 

 moraines, drumlins and eskers, the lithological 

 composition of the drift and its significance. 

 Evidence was presented by a deposit of red till of 

 a later readvance of the two glaciers southward 

 to the vicinities of Milwaukee and Fond du Lac. 

 The shore lines and deposits of Lake Chicago and 

 succeeding glacial lakes were also shown. 



Concerning certain Criteria for Discrimination of 

 the Age of Glacial Drift Sheets as Modified by 

 Topographic Situation and Drainage Relations: 

 Wm. C. Aujen, Washington, D. C. (Introduced 

 by T. C. Chamberlin.) 



The discussion was confined to phases illus- 

 trated by the pre-Wisconsin drift of southern 

 Wisconsin and northern Illinois. 



Character of this drift and reasons for regard- 

 ing the drift exposed at the surface throughout 

 this area as belonging to one and the same sheet. 

 The lithological composition and its significance, 



directions of ice movement, absence of intercalated 

 weathered zones, soils or vegetable deposits. 



Differences in the apparent amount of surface 

 modification of this drift in different parts of the 

 area which might be regarded as indicating differ- 

 ences in age: 



1. Topographic relations and amount of erosion. 



2. Weathering, leaching and oxidation. The oc- 

 currence in places of thoroughly disintegrated 

 drift or residual till; in others, of drift but mod- 

 erately leached and oxidized; in others, of per- 

 fectly fresh unmodified drift at the surface or 

 immediately below the loess. 



The reasons for these differences: 



1. Influence of pre-Glacial topography on drain- 

 age slope and upland of the drift. Influence of 

 the St. Peter sandstone on the pre-Glacial topog- 

 raphy. Relations of surface wash to the ap- 

 parent amount of surficial leaching and oxidation. 



2. The post-Illinoian diversion of Rock River 

 below Rockford, Illinois, and the consequent re- 

 tardation of erosion due to the work of cutting 

 new rock gorges at several cols. Removal of the 

 weathered drift from slopes with preservation on 

 the uplands. 



Necessity for caution in the discrimination of 

 distant drift sheets in the absence of marked dif- 

 ferences in lithological composition or of sections 

 showing overlapping drift with intercalated soils, 

 vegetable deposits or weathered zones. 



The two papers together were discussed by F. 

 Leverett. 



Then was read: 

 Lake Ojihwa, the Last of the Great Glacial Lakes: 



A. P. CotEMAN, Toronto, Canada. 



As the Labrador ice sheet retreated north to the 

 watershed between the Great Lakes and James 

 Bay, the waters now flowing northward were im- 

 pounded, first as a narrow bay of Lake Algonquin 

 opening south past Sudbury, afterwards as a 

 separate lake with an outlet down the Ottawa 

 Valley. This lake probably existed during the 

 time of the Nipissing Great Lakes, and was the 

 last of the ice-dammed lakes. The elevation of 

 its outlet is now 900 feet, but was then much 

 lower. In its bed the " clay belt " of northern 

 Ontario and Quebec was deposited, having an ex- 

 tent of over 25,000 square miles. The maximum 

 area covered by its waters must have been greater 

 than that of Lake Superior; though probably its 

 extent varied greatly in accordance with the posi- 

 tion of the ice front. 



This paper was discussed by Mr. F. B. Taylor 

 and Professor A. P. Coleman. 



The next paper was: 



