106 BULLETIN OF THE ESSEX INSTITUTE. 



to stream volume, with a tendency to approach the dry 

 talus at one end of the series and the alluvial fan at the 

 other end of the series. In this field tiiere exist several 

 peculiar deposits, usually ridge-like in habit, but diftering 

 from eskers in that they extend east-west, or north-east 

 and south-west while neighboring eskers extend north- 

 south ; and in that they have a typical ice-contact on their 

 northern or leeward sides, and a deltate or lobate topog- 

 raphy on the opposite southern side. There is usually a 

 steep slope from the summit line of the ice-contact slope 

 to the outer margin. The deposits not infrequently have 

 one high apical point along the ice-contact. They are 

 deposits of the subaerial type in most cases, although 

 marginal delta lobes would in other cases point to stand- 

 ing water about their bases. Both the Bridgewater cone, 

 known as Sprague Hill, the deposit at Walpole Junction 

 and that southeast of Waban station point clearly, it 

 seems to the writer, to the subaerial construction of the 

 upper prism of these deposits. 



If the topography of an existing alluvial plain deposited 

 in a water-body may be taken as affording evidence of 

 water-level, the summit line or brow of the lobate margin 

 is at water-level. On the margin of such a deposit, lobes 

 are built by different streams at the same time or by the 

 same stream at different times since a stream may wander 

 from side to side of the fan ; hence, since the water-level 

 may vary, the lobes of such a plain may occur at slightly 

 different levels. The instances pointed out by Salisbury 

 in Lake Passaic, New Jersey, probably fall within this 

 class of effects. The elevation of the summit line of 

 multilobate plains thus becomes of importance in deter- 

 minins: water-levels. It is the southern and outer rather 

 than the northern and iceward margin of the plain which 

 is taken into account. In most plains the level of the 



