112 CAPE COD GEOLOGY 



SAND-CARVED PEBBLES OR GLYPTOLITHS 



At many places on Nantucket, particularly near the tops of the wind-swept 

 gravelly bluffs that face the harbor and at places west of Sherburn bluffs where 

 glacial pebbles and larger stones are exposed to a natural sand blast, hundreds 

 of pebbles and stones show faceted surfaces, some of which have intersecting 

 planes, forming so-called edged pebbles, or glyptoliths. Most of these pebbles 

 and stones display a high polish. The under sides of many of them have water- 

 worn or glaciated surfaces; the under sides of others are polished, proving that 

 they have been turned over since they were first exposed to the natural sand blast. 

 Some of these sand-polished pebbles are shown in Plate 12. These pebbles were 

 found in the course of a survey on bluffs where the wind was blowing up sand 

 and where sand carving is actively in progress. Figures showing some of these 

 sand-blasted pebbles are given in Shaler's report on the geology of Nantucket, 

 where however, their polish is ascribed to the action of glacial streams. The 

 present accepted explanation of the. polish on these pebbles, which occur at 

 many places in glaciated regions of North America and Europe, was advanced 

 by Mickwitz as early as 1876, but it did not find acceptance in this country 

 until after Shaler's report had been published. 



TUCKERNUCK ISLAND 



The geology of Tuckernuck Island duplicates in general that of Nantucket. 

 The northern part of the island is a remnant of the corrugated moraine of north- 

 ern Nantucket. A slight longitudinal valley that extends between East Pond 

 and North Pond separates the moraine from a tract of outwash plain on the 

 south, which forms the greater part of the area of the island. The fosse and the 

 ice contact slope at the head of the plain show that the ice front extended north 

 of west across the island to Chappaquiddick Island and Marthas Vineyard, 

 where similar features occur. 



MUSKEGET ISLAND 



Northwest of Tuckernuck Island, at the present west end of the group of 

 islands and shoals of which Nantucket is the largest mass of land, there is a low, 

 wind-swept island known as Muskeget. On a map made at the beginning of the 

 eighteenth century and accredited to Captain Southack the name "Sturgeon I." 

 (island) is attached to an island west of what appears to represent Tuckernuck. 

 North of this island, on the south margin of a shoal having soundings of 3 feet, 

 the map has the name Muskeget Sand. This map confirms the geological prob- 



