ECOLOGICAL FACTORS. 



11 



The action of tho wind in shiftino- the sand and thoichy oitlior uproot- 

 in"' or hurviiM'- (lie \ ci-ctatioii is ono of tin* most ol)vious i)oiiits to l»e 

 noted in a shift in«,^ (hnic aiva. Tho heavy winter winds, usuiilly from 

 the northeast or northwest, are the most ert'ectivo in shiftinj^ the sand 

 and in preventing' the existence of vej^etation. A nmcii monr meajj^er 

 vet^etative covering is present on the northern than on the southern 

 slopes of the pai'lially estal)lished (hmes. 



The isolated location of the area is such as to subject it to the 

 unbroken force of the winds. The averai,^e hourly velocity of the wind 

 for Nantucket, where records are accessible, is 11.7 miles per hour. 

 Gale velocities of fioiii 5(» to «'»<> miles ])er hour are not infre(iuent, 

 while velocities as hiuh as 7'2 miles per hour have l)een recorded. 



The soil of the entire area is largely reassorted glacial sand, at least 

 95 per cent of which is light-colored (piartz.. The other con.stituents 

 arc principally magnetite, limonite, feldspar, schist, and garnet. The 

 vigorous growth of the vegetation in the protected locations indicates 

 that tho sand is not as deficient in food s-.dts as is sometimes supposed. 

 The large size and angularit}' of tlu^ grains constitute the character- 

 istic features of the Cape sands. It is noticc'abic that on the bare 

 wind-swept slopes the dune sand has been swept away, leaving the 

 coarser grains, which remain and act as a protective blanket, thus 

 reducinir the movement of the underlving sand bv the wind. 



The followinir table indicates the size of the two kinds of sand. 

 No. 1 was taken from the surface of the windward slope of one of the 

 dunes. No. 2 was taken fioni S inches l)eneath the surface of a typ- 

 ical dune and fairly represents tlu^ bulk of the Cape sands. 



The moisture relations of the dune vegetation are unique. Though 

 the upper layers of sand are usually hot and dr}^ during the growing- 

 period except when rain is actually falling, the moisture comes to 

 within a few inches of the surface, even during periods of protracted 

 drought. The effect of a heavy rain is not noticeably different from 

 that of a very light one, as in either case a few hours of sunshine suf- 

 fice to remove all visible traces of the precipitation, most of which per- 

 colates at once to the lower layers of sand. For this reason the number 

 of rainy days, which averages 96, is of greater moment than the actual 

 annual precipitation of 43 inches, w hich is sufficient under proper soil 

 conditions to produce a luxuriant vegetation. The snows which 



