Page:The Journal of geology (IA journalofgeology11893univ).pdf/270

extreme case about twenty miles. The ice would be so long passing over such distances that we could expect that the basal water would restrict the enlargement of the tunnels sufficiently to show a characteristic development of the gravels, such as narrowness of the osars or gaps without gravels. While in such situations I nowhere find so extreme a development as in the coastal region, yet there are numerous facts that are best interpreted by the hypothesis that the basal waters of the slack water dams in the subglacial tunnels did somewhat obstruct the enlargement of the tunnels; and thus far I have found none inconsistent with that hypothesis.

The critical reader will have noted that the belt of transition of the coastal gravels of Maine is approximately parallel to the ice front at one stage of the retreat of the ice. It is also somewhat parallel to the southern margin of the névé. It has been necessary to consider whether the coastal gravels were retreatal phenomena, connected with some late stage of the ice sheet's history, also what effect would be produced by the retreat northward of the névé line, whether the discontinuous gravels were due to the gradual rise of the sea, etc. The result has been to relegate all the suggested agencies to a subordinate position with respect to the two causes above named—a probable small acceleration of ice flow near the coast and the limited enlargement of the subglacial tunnels over the area whose basal ice was submerged in the sea.