compact till is identical to that of the major drumlins in the area. 

 Coarse material is tightly packed with a matrix of fine silt-clay-sized 

 particles. In fact, without the coarse angular pebbles, the fine part 

 of this till could be confused with the glaciomarine clay wken viewed in 

 a small quantity in a sampler. 



The absence of glaciomarine clay overlying the sandy till in some 

 drill holes (CBC, CBD and CBE) suggests that the clay was never deposited 

 here, or that if deposited, it was eroded during a later transgression by 

 the sea. I believe that the clay was deposited on top of glacial mate- 

 rial as found beneath Crane Beach, but was removed by erosion. Perhaps 

 the sandy till, which is so common in the Crane Beach drill holes, is 

 simply the compact till reworked by the sea and mixed with coastal de- 

 posits. If a deeper drilling had been possible, the sandy till might 

 have turned out to be a veneer over the more compact till. 



The glaciomarine clays were deposited .during a high stand of the sea 

 in a blanket of variable thickness over existing topography, lapping up 

 on the sides of drumlins. The presence of glaciomarine deposits in a 

 steep bluff at the southern end of Plum Island above sea level, and at 

 elevations higher than 40 feet near Ipswich, contribute to the uncertain- 

 ty about how these deposits were superimposed on the topography. Glacio- 

 marine clay usually appears at depths below 50 feet (see Table); the 

 same clay outcrops are throughout the Ipswich quadrangle (Sammel, 1963). 

 This gap between surface outcrops and the 50- foot depth may be an ero- 

 sional situation. More observations of btiried clay horizons are needed 

 to determine the configuration of the top of this deposit. 



3. Indurated Clay 



An indurated layer is found on top of the clay layer (see Table) . 

 Five of 10 bore holes containing marine clay showed an indurated layer. 

 This layer is a hard cement- like material that is 3 to 10 inches thick. 

 Once the material has been loosened by the chopping bit it closely resem- 

 bles the softer clays below. The layer is located by the difficulty of 

 drilling through it. The hard clay is washed up the holes in chips or 

 lumps, and becomes softer as it is mixed with water. 



Such a layer may be dewatered by compaction. This is a common phe- 

 nomenon, especially in areas of artificial fill overlying clay deposits. 

 Induration may also be due to exposure to subaerial weathering. An alter- 

 nate mechanism, secondary precipitation, has been suggested by Coleman 

 and Ho C1967) . 



Coleman and Ho studied recent sedimentary sequences in the Atchafalaya 

 Basin, Louisiana. They obtained undisturbed cores of 120-foot borings 

 near the Mississippi Delta. The age of the deposits ranged from 10,000 

 B.P. to present. This extensive study determined that increase in com- 

 pressive strength and decrease in water content in sediments in this area 

 was partly due to the presence of cementing minerals such as iron hydrous 



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