GLACIATED ROCK SURFACES NEAR LINN AND NEAR 



QUARRY, IOWA, WITH A TABLE OF THE BEARINGS 



OF GLACIAL STRIAE IN IOWA. 



BY W. H. NORTON. 



Linn is a junction of the Chicago, Rock Island and Pacific Railway, 

 situated four miles northwest of Cedar Rapids. About one mile west 

 of this station a rock cut, formerly used by the Chicago, Milwaukee and 

 St. Paul Railway, scarps the bluffs bordering the Cedar River. At the 

 extreme north end of the cut, 14 feet above the trackway, and about 75 

 feet above the level of the river, there is exposed by the retreat of a cover 

 of yellow till and loess a surface of less than two feet square, developed 

 on glaciated limestone belonging to the Acervularia davidsoni zone of the 

 Cedar Valley stage of the Devonian. Although rubbed down to a fair de- 

 gree of smoothness the rock was not planed level and retains something of 

 an original convexity. An initial depression an inch in depth and four 

 inches wide crossing the surface from northwest to southeast is not com- 

 pletely smoothed, and the northwest side of the hollow is less distinctly 

 glaciated than the opposite. The surface of the knob is covered with 

 shallow striae of varying bearings. On the south side they bear (cor- 

 rected) S. 79° E. On the center and north they bear S. 84° E and E 4° 

 N some striae reaching E. 14° N. The west side of the knob is well rubbed 

 and is evidently the stoss. The south shoulder is not planed. It is 

 worthy of notice that the striae are aligned with the trend of the valley 

 of the Cedar river which here changes its course from east-southeast to 

 south-southwe?t. 



In the spring of 1910 my attention was called by the Division Super- 

 intendent of the Chicago and Northwestern Railway to an exceptionally 

 large glaciated surface exposed by recent stripping at the company's 

 quarries one and one-quarter miles northeast of the station quarry in 

 INIarshall county. The surface exposed measured about 75 feet in width 

 and 275 feet in length. Over the entire space the level rock floor of the 

 oolitic limestone of the Kinderhook had been smoothed and scored with 

 close-set parallel striae. Little if any plucking had been done, and no 

 knobs had been left above the general level. The only depressions noted 



