148 PROCEEDINGS OF THE ALBANY MEETING 



has somewhat the appearance of an okl beach ridge, and if so it may represent 

 the highest old shoreline of Hudson Bay in postglacial times. 



This was the end of the track at the time of my visit, but the chief engineer 

 informed me of a very strong gravel ridge at mile 330, whose crest is 430 feet 

 above tide. This ridge may be regarded with certainty as one of the old 

 shores of Hudson Bay. Other old raised shores occur along the railway loca- 

 tion between that place and Port Nelson, at mile 424. I saw no evidence of the 

 presence of a moraine at or near the eastern limit of the Lake Agassiz clays. 



ROCK TERRACES IN THE DRIFT LESS AREA OF WISCONSIN 

 BY LAWRENCE MARTIN 



(Adstract) 



In working out the physiography and stratigraphy of the Tomah and Sparta 

 quadrangles, Wisconsin, for the State Geological Survey, a system of rock ter- 

 races was mapped in 1916 by the author, assisted by F. T. Thwaites. These 

 are in the western upland of Wisconsin, within the driftless area, near the 

 headwaters of the Kickapoo. Lemonweir, and La Crosse rivers, a few miles 

 east of the Mississippi. The rocks in the district include: (a) Oneota (Lower 

 Magnesian) limestone which caps ridge tops in the broad Magnesian cuesta; 

 (&) two cliff -makers in the Saint Croix (Potsdam) group — an upper on the 

 Jordan sandstone, a lower on the Dresbach sandstone; (c) two weaker, ter- 

 race-capping formations between these — the Saint Lawrence and the Franconia. 



The Saint Lawrence terrace and one of the Franconia terraces are the two 

 ])ersistent topographic features within the valleys of the area. There are often 

 four or five terrace steps, the main Franconia terrace being wide enough so 

 that the escarpment at the border of the western upland is usually double, 

 with cliffs and steep slopes of Dresbach and Jordan at the borders of the 

 Franconia terrace and Magnesian cuesta respectively. Likewise the Kickapoo 

 Valley is double, or benched, the Franconia terraces making broad upper 

 shelves, as if a narrower valley had been intrenched below the level of a 

 broader strath. It seems certain, however, that there has been no halt and 

 subsequent uplift in connection with the formation of this and the other ter- 

 races, as Butts implied in the original technical use in Pennsylvania of Geikie's 

 Scotch term strath. 



The terraces and the cliffs in Wisconsin are due to weathering of unequally 

 resistant sandstones. The cliff-making sandstones are coarse grained and 

 thick bedded, the Jordan being case hardened at the top, the Dresbach likewise 

 having the firmest layers above. The terrace sandstones are fine grained, the 

 Saint Law^rence approaching limy shale near the base, and the Franconia 

 being glauconitic, with micaceous sandy shale forming a rather impervious 

 capping for the underlying Dresbach. That the minor features of topography 

 are controlled by rock structure is another argument, not independently de- 

 cisive, to be sure, but to be added to those^ which lead to the interpretation of 

 the upland near the Mississippi River in Wisconsin and adjacent States as 

 cuesta rather than dissected peneplain. A reconnaissance of a considerable 



1 Bull. 36, Wis. Geol. and Nat. Hist. Survey, 1916, pp. 63-70. 



