20 RALPH S. TARR 



theory was incorrect, and that these two valleys are really confirm- 

 atory of the glacial-erosion theory. This change of view is the 

 result of a recent study of the valley profiles and a mapping of the 

 moranic deposits of the valleys in question. The latter show that 

 these valleys were occupied by actively moving ice parallel to their 

 axes, while the neighboring higher hanging valleys were not. They 

 were therefore open to ice- scouring. A study of the profiles shows 

 that these discordant hanging valleys have the U -shape of glacial 

 erosion and not the gorge-shape of a rejuvenated valley, the only 

 other explanation that seems a possible one for such discordance. 

 It is believed, therefore, that while the Cayuga valley was pro- 

 foundly deepened by ice-erosion, the Salmon and Six Mile Creek 

 valleys were deepened moderately, and the Fall, Cascadilla, But- 

 termilk, and other valleys practically not at all, since they were not 

 occupied by ice freely moving along their axes. 



There still remains some evidence opposing glacial erosion by the 

 Wisconsin ice-sheet, but none opposing erosion by an earlier 

 advance, unless the fact that no deposits of an earlier ice- advance 

 are found in this region is considered opposing evidence. The most 

 serious objection to Wisconsin ice-erosion is the presence of resid- 

 ually decayed rock at the Portland quarry north of Ithaca. But in 

 view of the striking topographic evidence of ice-erosion of a double 

 period — the condition of double hanging valleys, the steepened slope, 

 the spurless valley wall, and the undissected valley sides — the ques- 

 tion may fairly be asked whether even this evidence from residual 

 decay remnant, in a single place, may not be misleading. In seeking 

 for an explanation of the presence of this decay product in the par- 

 ticular place where it occurs, I have found but one hypothesis that 

 appears possible. The Portland quarry lies just south of the junc- 

 tion of Cayuga Lake and Salmon Creek valleys, down both of which 

 the ice flowed somewhat freely. It is possible that the thrust from 

 the Salmon Creek valley pushed the ice toward the west side of the 

 Cayuga valley, leaving the site of the Portland quarry under a wedge 

 of ice with slight motion, and hence with slight erosive power. Too 

 little is known about the erosive action of ice, and the direction of 

 ice-currents under the guidance of topographic irregularities, to war- 

 rant further consideration of this question, at present. 



