354 A. C. Lane — White Mountain Physiography. 



so that lines of simultaneous water-level (niveau=equipo- 

 tential lines) will feather out as they go north. If, 

 however, the Boott Spur and the other ^4awns" mark a 

 period of relative permanence in the ice, a pause in retreat 

 or re-advance, then the niveau line corresponding thereto 

 might be extra well marked. The sea-level for the 

 725-foot terrace at Bartletts could not have been much 

 below this datum, (for it is less than 20 miles of wide 

 valley until it opens out on the old shore-line.) Thence 

 the sea-level drawn toward the ice at the time sloped off a 

 little, and if the sea-bottom rose rapidly as the pressure 

 of the ice was removed, so that they had already begun to 

 rise, then present traces of that level should slope even 

 more away from the mountains. The sea-bottom terrace 

 would slope still more. Thus, well developed post-glacial 

 terraces should slope more than the earlier ones, to which 

 Barrell gave a slope of 7 feet to the mile. Thus, the 

 sea-bottom terrace corresponding to the Bartlett 725-foot 

 level might well be under the 575-foot level, the 600 feet 

 so prominent around Lake Winnepesaukee. But I did 

 not try to disentangle the lower levels on which Gold- 

 thwait is now at work. When to the shifting volume of 

 water in the ocean and the gravitative effect of the ice, 

 w^hich we know must have had an effect, is added the 

 compressive effect of an ice sheet of which we do not 

 know the thickness or the effect, only a very careful study 

 of the results may perhaps give us a clue to the efficient 

 causes. But it does seem worth mentioning that an ice 

 sheet which was just thick enough by its swathing effect to 

 produce the high-level 'Pawns'' like Boott Spur, was 

 probably also competent to produce just such a depression 

 as we find indicated by the highest post-glacial sea-levels 

 around the southeast flank of the mountains. 



