CILACIAL CURRENTS SHOWN BY STRI.E. 129 



recession and final melting- of the continental ice-sheet cansed it to extend 

 over lands within the Arctic Circle which had not beeij covered by the ice 

 when it reached farthest sonth. From the melting of its last remnants 

 moistnre-laden winds doubtless have cari-ied poiiions of it across Baffin 

 Bay and Davis Strait to be deposited again in the ice-sheet that still covers 

 the interior of Greenland. 



GliACIAL, CUKRENTS WITHIN THE BASIN OF LAKE AGASSIZ. 



Table of courses of f/ldcUd strice. — The directions of the currents of the 

 ice-sheet are shown by its tracks, the furrows and stripe which bowlders 

 and gravel frozen in the base of the moving ice engraved upon the bed- 

 rocks over which they passed. From these courses of movement of the 

 ice, the areas of its thickest accumulation and consequent outflow are 

 known. In some districts, also, changes in the outlines of the ice border 

 and in its slopes and currents during its final retreat are indicated by 

 deflected glacial stripe which run across the earlier courses. Occasionally 

 two or more sets of striae are found intersecting on the same rock surface, 

 but more frequently the earlier and later sets are preserved on separate 

 portions of the same or contiguous rock-outcrops.^ The testimony of 

 these records is so important concerning the banier which held Lake 

 Agassiz that a table is presented as an appendix of this volume, noting 

 the courses of strire which have been reported upon all the country from 

 Hudson Bay, Lake Superior, and Minnesota westward and northward across 

 the basin of this glacial lake. 



Converging lobes of the ice-sheet in Minnesota and Manitoba. — The south- 

 westward striation in northeastern Minnesota and the southeastward 

 striation in the central and southern part of that State belong to two 

 convergently flowing lobes of the ice-sheet, partly corresponding to its 

 portions which earlier inclosed the driftless area of southwestern Wiscon- 

 sin and united in a continuous area of ice farther south The central 



'A most valuable classitication of the various types of glacial striatiou, iilanation, and emboss- 

 ment, with discussion of their methods of origin and of their significance as evidence of the prevail- 

 ing ice currents and of deflections during the glacial recession, is given in Professor Charaberlin's 

 memoir, " The rock-scorings of the great ice invasions," Seventh Annual Report, U. S. Geol. Survey, for 

 1885-86, pp. 147-248, illustrated by 50 ligures in tlie text, mostly engraved from photographs. 

 MON XXV y 



