Boivlder Trains of the Latest Glacial Movement . 491 
The quartzites on the outer margin of the moraine between 
these limiting points have an abundant distribution. Begin¬ 
ning with the southwest margin in Union township, the outer 
ridge on section 26 shows for a quarter mile about eighty- 
blocks, or one for each rod in extent. They are still more 
abundant over the central portions of the train. On section 33, 
Porter township, on a surface of forty square rods over 100 
quartzites were noted, and at other points in this vicinity the 
local drift exposed aggregates about .four cords per acre. 
Near the east margin of the train there is also a decrease 
of the local material. On the outer crests of the moraine in 
Janesville township, west of the river, the quartzites again 
average one to each rod of drift margin, while east of the river 
they are much less abundant though they are found in roadside 
and railway sections, in the townships of Milton and Harmony, 
as far west as the line of the Chicago & Northwestern railway. 
Relative amount of local drift .—The relative amount of local 
drift at proximal points is of course much greater than this. 
At several points adjacent to the Portland ledges are heaps or 
mounds of bowlders, evidently from preglacial talus accumula¬ 
tions. These mounds rise from ten to twenty feet above the 
marsh level, cover one-quarter to two acres of surface and ap¬ 
pear to be composed of from 25 to 75 per cent, of quartzite 
fragments. The marsh borders adjacent to the ledges bear a 
fringe of bowlders from two to ten rods wide, the blocks aver¬ 
aging from twenty-five to 150 per square rod. 
Drift sections on the roads and railway south of the principal 
ledge area show a quartzite content of from 5 to 20 per cent, 
at points within two miles of the ledge areas. These proportions 
diminish to one and one-half per cent, at points six miles away 
and to about one per cent, at points ten miles distant. Over 
the medial portion of the train this percentage has decreased to 
one-half per cent, and on the periphery of the lobe the relative 
amount of local drift is one three-hundredth. 
The total amount of quartzite drift exposed on the surface is 
closely approximated from observations made upon almost every 
square mile within the area, and is placed at about 35,000 cords 
The estimated amount contained within the drift formations 
