20 



BIOLOGICAL REPORT 31 



Ice Sheet, were responsible for the formation of 

 Cape Cod, Nantucket, Martha's Vineyard, and 

 Buzzards Bay and its watershed. 



The structure of the Buzzards Bay estuarine sys- 

 tem is most closely linked to the Buzzards Bay lobe 

 (as it was modified by the Cape Cod Bay and South 

 Channel lobes). The Buzzards Bay lobe initially 

 spread across the area of Buzzards Bay to its far- 

 thest extent at western Martha's Vineyard. At this 

 point the advance halted for a period of more than 

 1 ,000 years during which glacial till was deposited 

 as ice continuously arrived but melted or evapo- 

 rated before it could advance the perimeter. This 

 till consisted not only of soil and decomposed rock 

 but also of bedrock collected by the ice as it flowed 

 southward across New England. During this sta- 

 tionary phase a portion of the terminal moraine of 

 the Buzzards Bay lobe was deposited (Fig. 2.1 ). A 

 series of events followed that made the formation 

 of the moraines of the Cape Cod and Islands re- 

 gion different than most moraine formations through- 

 out the United States (Oldale 1 992). After the pe- 

 riod of melting and retreat, the glacier readvanced 

 over the heavier deposits nearest the glacier face 

 and acted like a bulldozer, lifting layers of the gla- 

 cial deposits and some previous surface material 

 and thrusting them forward in a process called 

 "glacio-tectonics." In the final phase, the margin of 

 the Buzzards Bay lobe overrode the thrusted de- 

 posits and when it melted left a thin veneer of gla- 

 cial till covering the thrusted deposits that form the 

 terminal moraine (Oldale 1992). 



Sloping away from the moraine is an outwash 

 plain deposited as the finer materials were carried 

 away from the ice edge in meltwater flows. Sloping 

 led to a gradation in sediment sorting and elevation 

 moving away from the moraine. Indeed, today the 

 highest elevations in the Buzzards Bay watershed 

 and Martha's Vineyard (30.5-61 m) are associated 

 with the Buzzards Bay lobe morainal deposits. Ra- 

 diocarbon dating (Kaye 1 964) suggests that some- 

 time after 1 5.300 - 800 years B.P. climatic warm- 

 ing caused the Buzzards Bay lobe to rapidly retreat 

 to what is approximately the eastern watershed 

 boundary for Buzzards Bay and the Cape Cod Bay 



lobe to the location of the Sandwich moraine 

 (Larson 1 980). This secondary position of the ice 

 margin was held for a period, and moraines were 

 formed by glacio-tectonic processes and, to a lesser 

 extent glacial till from the continual inflow and melt- 

 ing of ice (Strahler 1 966; Oldale 1 992). Thus the 

 relatively large Buzzards Bay (and similarly the Sand- 

 wich) moraine was formed, 1 .6-3.2 km wide and 

 extending within the watershed from the Elizabeth 

 Islands to the head of the bay. The Cape Cod por- 

 tion is surrounded by outwash on both sides in its 

 northern parts and is relatively high at 30.5-6 1 m. 

 The Elizabeth Islands portion is of relatively low 

 relief, generally less than 1 2.2 m with a maximum of 

 36.6 m. These islands consist entirely of glacial de- 

 bris with eroded boulders from the moraine form- 

 ing a natural rip-rap in the face of advancing sea 

 level (Moore 1963) and providing a rocky sub- 

 strate for colonization by biotic communities. Be- 

 tween the Buzzards Bay and Sandwich moraines, 

 the Mashpee pitted outwash plain was formed, 

 making up much of this portion of Cape Cod (Fig. 

 2.2). 



Further retreat of the Buzzards Bay lobe across 

 current Buzzards Bay to approximately the west- 

 ern watershed margin, coupled with minor 

 readvances, led to smaller moraines, outwash plains, 

 and glacial till deposits on the western shore. The 

 elevation of the terminal and recessional moraines 

 with outwash plain sloping away helped to deter- 

 mine the watershed of the existing bay. The melt- 

 water eroded the outwash plain and generated 

 outwash channels that were later flooded by rising 

 sea level to create the many embayments on the 

 western side of the bay and similarly outside the 

 watershed on the southern shore of Cape Cod on 

 the Mashpee pitted plain (Fig. 2.2). In addition, the 

 presence of outwash adjacent to the shore front tends 

 to result in sandy-or fine-grained sediment bottoms 

 whereas erosion of moraine leads to a rock- and 

 boulder-strewn coast. These substrate conditions 

 laid down by glaciation thousands of years ago 

 continue to affect benthic biotic structure today. 



