W. B. Wright— A Theory of the Marginal Drift. 551 



4. An attempt is made to interpret the results of a chemical 

 analysis of the silt in the light of its mineralogical composition, chiefly 

 with regard to carbonate, potash, and phosphate. Muscovite, is 

 found to be the main source of potash, and apatite of phosphate in 

 the silt. 



A Theory of the Marginal Drift. 



By W. B. Wright 



TT is a fact well known to geologists that the drifts laid down in the 

 -^ peripheral portions of the area affected by the Quaternary 

 Glaciation differ markedly from those developed nearer in to the 

 glacial centres. Beyond, perhaps, a vague suggestion that this may 

 have been due to different climatic conditions during the successive 

 stages of the retreat, no adequate or even partial explanation of the 

 phenomenon has ever been brought forward. Pursuing a line of 

 thought suggested by Dr. Matti Sauramo, in his paper on the glacial 

 retreat in Southern Finland,^ it now seems possible to offer an 

 explanation based merely on the simple assumption of a greater 

 degree of cold during the maximum of glaciation than obtained 

 during the later stages of the retreat. 



The peculiarities of the marginal drift which call for explanation 

 may be briefly summarized as follows : — 



(1) The absence along the outer edge of the drift-sheet of any 

 terminal moraines, such as were developed during periods of halt 

 or readvance in the later stages of the retreat. 



(2) The attenuated character of the drift and the extreme 

 indefiniteness of its margin, the so-called " feather-edge ". 



(3) The almost complete absence of eskers and outwash fans, 

 such as might indicate a normal amount of melting during the 

 initial stages of retreat. 



(4) The apparent absence of any of the phenomena of marginal 

 drainage. 



(5) The absence of drumlins or more generally of topographic 

 expression of any kind. 



(6) The presence of a covering of loess. 



These characteristics are, of course, mainly of a negative character, 

 but they are none the less remarkable. The first five might well 

 be summed up in one word — feebleness. 



The Fundamental Observation. — The conclusion of Dr. Sauramo, 

 from which it is now proposed to build up an explanation of this 

 feebleness of the outer drifts, is based on a study of the later phases 

 of the retreat. These were in general characterized by active motion 

 and rapid decay, the essential requirements for the production of 

 end-moraines, eskers, and outwash fans, but a return at intervals 



^ Matti Sauramo, " Geochronologische Studien iiber die Spatglaziale Zeifc 

 in Siidfinnland " : Bulletin de la Commission G6ologique de Finlande, No. 50, 

 1918, 



