PA LEO BOTANICAL STUDIES IN SALT MARSH DEPOSITS 

 WITH SPECIAL REFERENCE TO RECENT 

 CHANGES IN SEA LEVEL 



by 



Elso S. Barghoorn 

 Harvard University 



The discussion in this paper pertains to studies of the plant remains found 

 in a sedimentary complex exposed in two building excavations in the Back Bay 

 area of the city of Boston. This study involved the identification and ecological 

 interpretation of both naturally occurring plant deposits recorded in peat and 

 marine silt, as well as archeological specimens of wood intruded by human 

 agency into the sedimentary complex. The deposits also provided unusually 

 favorable material for histo -chemical study of degradation of plant remains 

 under relatively well understood environmental conditions of deposition. The 

 Back Bay sediments provided a basis of correlating vegetational changes in a 

 complex estuarine environment, featured by a rising sea level, with an absolute 

 chronology secured more recently by radiocarbon dating. The general sequence 

 of events is interpreted as follows: 1) deposition of a marine ( ?) blue clay fol- 

 lowed by sub -aerial erosion of the clay and subsequent intrusion over the eroded 

 clay of sandy outwash(?) deposits. The age of the blue clay is unknown but is 

 probably of late glacial age and is featured by ice rafted boulders; the sandy 

 outwash deposits are probably of periglacial origin. Wind cut pebbles occur at 

 the base of the sands. 2) development of a fresh water swamp forest featured 

 by stumps of trees and shrubs (Quercus, Ulmus, Cephalanthus , Ilex, etc.). 

 The forest was drowned by rising water with development of a fresh water reed 

 swamp (Phragmites-Corex association). The fresh water reed swamp is dated 

 as 5700 ^750 yrs. B.P. and is 20 i 2 feet below present mean low water of 

 Boston Harbor. 3) inundation and termination of the fresh water swamp by 

 marine waters with development of a short-lived salt marsh featured by Spartina 

 alterniflora. 4) deposition ot marine silt until the period of development of the 

 city of Boston by European colonists subsequent to 1630 A. D. 



Within the stratigraphic sequence the following radiocarbon dates are re- 

 corded, with their relation to the encroaching tidal plane: 1) fresh water reed- 

 swamp peat at 20ir 2 feet below mean low water = 5700i 750 yrs B.P. 



2) human activity, represented by numerous stakes and wattles of a presumed 

 fish weir, at or slightly below 13 feet below mean low water - 4500 i 130 yrs. 

 B.P. (Human occupation probably occurred over a period of centuries). 



3) stream rafted stump deposited at 9-10 feet below mean low water = 38 501 

 390 yrs. B. P. 4) current tidal datum plane z yrs. 



Extrapolation of the radiocarbon dates, in conjunction with paleo -ecologi- 

 cal interpretation of the plant remains, preserved in the Back Bay sediments 

 indicates that sea level has risen in the Boston Basin at an average rate of 6.6 

 inches per century during the past 5-6 thousand years. There is, however, no 

 definitive evidence that this rise has been at a uniform rate. 



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