and on retreating left behind drift deposits. Marine sediments were 

 deposited over the drift during interglacial stages when eustatic sea 

 levels were high. However, evidence of such past events has been largely 

 obliterated by subsequent glacial erosion, and only a few remnants have 

 been recognized as probable pre-Wisconsin Pleistocene deposits, e.g., 

 the excavation under Boston Commons where Kaye (1964) found evidence of 

 four, and possibly five, ice advances and three marine transgressions. 



The oldest widespread deposit of Pleistocene age in the Boston area 

 is a discontinuous drift layer (Drift III, Table 1) which Kaye (1964) 

 believed was deposited during the early Wisconsin- lowan substage. This 

 glaciation seems to be the earliest event of Pleistocene time which has 

 left an extensive record in the study area. 



After retreat of the glacier associated with Drift III, the Boston 

 area was inundated by the sea and a layer of glaciomarine clay (Clay III, 

 Table 1) was deposited over the drowned irregular topography. Kaye (1964) 

 indicated that a period of subaerial weathering intervened between the 

 deposition of his Drift III and Clay III as evidenced by an oxidation 

 zone in the drift. 



It is generally believed that the deposition of Clay III was followed 

 by a period of emergence during which Clay III and exposed part of older 

 deposits were eroded. Whether the offshore study area was exposed to 

 erosion in this period is not known. Kaye (1964) showed relative sea 

 level following deposition of Clay III was lower than -35 feet but did not 

 discuss a minimum. Judson (1949) inferred that sea level probably stood 

 about 90 to 100 feet below present sea level during this period of emer- . 

 gence, Upson and Spencer (1964) believed that sea level associated with 

 what is presumably the same episode was at least -50 feet lower with 

 respect to the land. 



Another glacial episode (late Wisconsin) followed the period of 

 erosion leaving deposits consisting primarily of outwash in the Boston 

 Basin. Judson (1949) believed that the ice front did not reach the 

 eastern part of the basin. After this glaciation, relative sea level 

 dropped to -70 feet according to Kaye and Barghoom (1964) who date the 

 low at around 10,000 years Before Present (B.P.). 



Since this time, relative sea level has risen and estuarine and marsh 

 sediments have been deposited in the Boston Basin, especially along the 

 embayed stream valleys. At about 3,000 years B.P. relative sea level 

 reached a near stillstand and essentially modem conditions were estab- 

 lished. Development of coastal landforms and marshes, fluvial deposition, 

 and erosion have been the predominant geologic events of the post- 

 transgressive period. In the recent past, engineering activities have 

 also played a significant role in the geological" processes. 



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