RATE OF RECESSION 83 



even in deep water, and so the varves at closely situated localities 

 in the middle of the lake sometimes differ greatly in thickness. 

 At locality 52 deposition of sand started during year 6232 at a 

 water depth of about 75 feet, and at locality 57 during year 

 6308 at a depth of about 100 feet. After the critical depth of 

 clay sedimentation had been reached at a given point the lake 

 there was filled up with sand and silt in a few decades (cf. pp. 

 25-28). My attempts to bridge the gap in the Hudson and Mo- 

 hawk Valleys were also without result, in the former because 

 the cla} r s did not offer any exposures and in the latter on account 

 of the general absence of clays. How many varves are lacking 

 in the normal curve cannot, of course, be stated, but their 

 number in view of the facts known seems to amount to 200 or 

 300. If this estimate is correct, the retardation and the readvance 

 should represent at least 400 years. 



Rate of Recession in Northern Zone 



As soon as the recession from this line had started it seems to 

 have become quite rapid, since between localities 61 and 64, i.e. 

 during the first 100 or 200 years, it averaged 615 feet (188 m.) 

 a year. Between White River and Hanover it reached over 812 

 feet (246 m.). After a slight decrease it increased again and 

 reached, between localities 75 and 79, 1,100 feet (335 m.) a year, 

 the fastest rate observed in New England. Then the speed 

 diminished again, amounting, between localities 79 and 86, to 

 about 830 feet (253 m.) a year. When the ice edge had retired 

 beyond St. Johnsbury it probably halted and readvanced. Since 

 I hope to be able soon to collect more material regarding the 

 conditions in this region, at this time it may merely be stated 

 that the ice border seems to have readvanced to St. Johnsbury 

 in the Passumpsic Valley and to the junction with the Pas- 

 sumpsic in the Connecticut Valley. The final retreat of the ice 

 from locality 85 occurred about 280 years after it uncovered 

 locality 86, a mile to the west. The halt seems to correspond to 

 the one registered by moraines at Bethlehem and Littleton, 



