CAPE COD GEOLOGY 



101 



Wisconsin Stage of Glaciation 



Nearly all the surface features of Nantucket, except the beaches and sand 

 spits, are parts of the terminal moraine and the outwash deposits of the first 

 substage of the Wisconsin glaciation. The island may be divided longitudinally 

 into three sets of glacial features : the outwash plain, which forms its southern 

 part and includes more than half its surface; the Nantucket moraine, which 

 forms its northern hilly belt; and the fosse or longitudinal valley that lies be- 

 tween the other two areas. For most of the length of the island the northern 

 limit of the outwash plain is fairly well shown by a bluff or ice contact slope, 

 which marks exactly the position of the front of the ice during probably the 

 greater part of the time it was on the island. The area north of this slope was 

 covered with ice that extended northward as far as Labrador. The area south 

 of the slope was free from ice, forming a plain that sloped southward to or toward 

 the sea; and from its analogy with similar plains in front of glaciers in high lati- 

 tudes today we may infer that one who might have traveled over this plain 

 would have been impeded only by the streams of ice-cold water that flowed 

 from the front of the ice. 



THE OUTWASH PLAIN 



The outwash plain that occupies the southern half of the island slopes south- 

 ward except at the east end of the island, where it slopes south west ward. The 

 crest of the ice contact slope stands somewhat more than 60 feet above sea level 

 and the slope descends to the floor of the fosse, which lies between the 20-foot 

 and the 40-foot contour. The crest is notched by creases that were made by 

 streams flowing from the ice and that extend southward, over the plain. Some of 

 the creases have received local names, such as Madequecham Valley, Barnard 

 Valley, Wier Valley. Madequecham Valley appears to be continued across 

 the Nantucket moraine, just west of Altar Rock Hills, to the depression at 

 Polpis Harbor. Immediately south of the town of Nantucket the ice contact 

 slope is scarcely distinguishable, and the deposits there include a sandy till 

 that lies south of the general line of the head of the plain on the east. 



In the western part of the island, near Hummock and Long ponds, the 

 creases are more pronounced and the depressions are deeper, their bottoms for 

 most of their length lying below sea level. They also extend farther north across 

 the fosse into the region of the corrugated Nantucket moraine. The Hummock 

 pond crease is continued in Washing and Maxey ponds and a small pond farther 

 south, all of which appear to occupy ice block holes in a downfolded part of the 

 moraine. 



