ST. MARYS OR FORT WAYNE MORAINE. 575 



strike rock slightly below tlie level of tlie base of the ridge. In buried 

 valleys the drift in places reaches a depth of about 300 feet, as has been 

 shown in the discussion of the Wabash moraine. In Allen County, Ind., 

 and in northwestern Oliio the thickness g-reatly exceeds the amount repre- 

 sented by the relief of the moraine, the average thickness at Fort Wayne 

 being about 100 feet on the moraine and 50 feet or more along the lake 

 outlet, while in northwestern Ohio the thickness, as indicated by gas wells at 

 Hicksville and Bryan, is 160 to 200 feet, or at least four times the amount 

 of the relief of the moraine. 



In the portion where the drift is mainly comprised in the morainic ridge 

 there is but little assorted material, the body of the ridge being a stony 

 blue till, but where the drift extends much below the level of the base of 

 the ridge it is apparently composed largely of sand and gravel in its lower 

 portions. 



Gilbert called attention, in his report for the Ohio survey noted above, 

 to the more stony character of the surface portion of the moraine compared 

 with the surface portion of the bordering plain. The difference was 

 attributed by him in the main to changes produced by subaerial erosion. 

 It seems probable, however, that there was also an original or inherent 

 difference, it being natural that the moraine should carry a larger propor- 

 tion of stony material in its surface portions than the plains, or rather that 

 it should carry less fine material, owing to the extraction and removal of 

 such material b}' the water escaping along the ice margin. So far as 

 known the deeper portions of the drift present no marked differences in 

 the two situations. 



The yellow till which constitutes the surface portion of the drift is 

 8 to 12 feet in thickness. In common with the surface clays of the other 

 moraines of this region it is not deeply oxidized, the color being a grayish 

 rather than brownish yellow. 



Gilbert's report contains the results of an actual count of pebbles 

 included in a sample of the surface till from near Edgerton, there being in a 

 total of 155 pebbles 24 chert, 19 hmestone, 22 quartzite, and 90 gneissoid 

 specimens. This coinit displays a very large preponderance of distantly 

 derived material, a much larger proportion than was found by the same 

 observer in deeper portions of the drift, there being near the base of the 

 drift at Toledo 80 per cent of nonmetamorphic or somewhat local pebbles, 

 and only 20 per cent of metamorphic or distantly derived pebbles. In 



