330 THE CANADIAN NATURALIST. [Vol. VliL 



siderable size. When the glacial river entered the open valley 

 from which the ice had retreated, or in the lower part of its 

 channel, while still walled on both sides by ice, its current was 

 slackened by the less rapid descent, causing the deposition, first 

 of its coarsest gravel, and afterwards, in succession, of its finer 

 gravel, sand, and fine silt or clay. The valleys were thus filled 

 with extensive and thick deposits of modified drift, which took 

 the same slope with the descending current, and which increased 

 in depth in the same way that additions are now made to the 

 bottom-lands of our large rivers by the annual floods of spring. 



The retreat of the ice sheet was towards the north ; and where- 

 ever the natural drainage was in that direction, it would be for 

 a time obstructed by the ice, forming lakes in which the deposi- 

 tion of modified drift would be much different from that which 

 took place when the slope was to the south. In New Hampshire 

 the portion of the Contoocook valley which extends through 

 Hillsborough county was occupied by a lake during a large por- 

 tion of the Champlain period. 



The oldest of our deposits of modified drift are long ridges or 

 intermixed short ridges and mounds, composed of very coarse 

 water-worn gravel, or of alternate layers of gravel and sand 

 irregularly bedded, a section of which shows an arched or anti- 

 clinal stratification. Wherever the ordinary fine alluvium also 

 occurs, it overlies, or in part covers, these deposits. Similar 

 rido-es of gravel have been often described by European geologists, 

 under the various names of kames in Scotland, eskers in Ireland, 

 and asar in Sweden. They have also been described by geolo- 

 gists in many portions of the northern United States and Canada. 

 In New Hampshire kames are of frequent occurrence, sometimes 

 a single one extending in a steep, narrow ridge for miles along 

 the lowest portion of a valley, or elsewhere short, and several 

 parallel to each other, or in very irregular mounds and ridges, 

 with hollows enclosing small ponds. Their position is generally 

 alono- the middle or lowest part of the valleys, which are bordered 

 by high ranges of hills ; but in the south-east part of the State, 

 in some parts of Maine, and in Eastern Massachusetts, where 

 there are only scattered hills, with the valleys not much below 

 the general level of the country, these ridges, of smaller size than 

 in the great valleys, are found extending usually north and south, 

 without special regard to the present water-courses. In the val- 

 leys of our largest rivers, the Connecticut and Merrimack, they 



