364 PRACTICAL LESSONS IN SCIENCE. 



As the continental ice sheet retreated, it left the whole country 

 covered with clay that was so filled with pebbles and boulders 

 that it has received the name of boulder clay, sometimes known 

 as bard pan ; it is so hard, and resists erosion so well that it is 

 often mistaken for rock. The margin of the ice sheet can be 

 traced across the United States with some degree of accuracy, 

 but in many cases the location is doubtful, as the original de- 

 posits, doubtless thin toward the margin, have been so washed 

 away and commingled with other material that the limit of the 

 ice cannot be denned. 



The line that marks in a general way the southern limit of the 

 ice and drift, runs from Cape Cod along the south shore of Long 

 Island to New Jersey, then northwesterly to near the headwaters 

 of the Alleghany river, not far from Salamanca, thence south- 

 westerly through New Lisbon, Danville and Winchester, Ohio, 

 crossing the Ohio river near Cincinnati to Madison, Indiana, 

 thence northwesterly a little beyond Martinsville, thence south- 

 westerly through New Harmony to the Mississippi river near the 

 mouth of the Big Muddy, thence up the river crossing to the Mis- 

 souri below St. Louis, thence up the Missouri to the Osage, then 

 up the Osage for a short distance, then westerly and northwest- 

 erly in a course generally parallel with the Missouri river to the 

 Rocky Mountains, thence southerly nearly to the head waters of 

 the Red river, thence across the mountains and northerly to 

 about the Columbia river, thence southerly including most of 

 the Sierra Nevada Mountains and the northern part of the Coast 

 ranges. The mountain region was hardly covered by an ice 

 sheet, but was rather the seat of numerous local glaciers, much 

 as Alaska and Switzerland are to-day. 



The greater part of this region had been above the sea and 

 subject to erosion since Devonian times, and was deeply scored 

 by drainage channels that in great measure determined the direc- 

 tion of the ice flow, tongues or lobes of ice flowing along the val- 

 leys in advance of the ice sheet ; and these are sometimes called 

 glaciers, as the Green Bay lobe or gla,cier, the Lake Michigan 

 glacier, the Saginaw glacier, the Maumee glacier and several les- 



