Page:The Journal of geology (IA journalofgeology11893univers).pdf/392

Then it falls ten feet, and, curving a little to the west, rises thirty feet to where it reaches its greatest elevation, one hundred and fifty feet above mean tide. This is also the elevation of the front of the sand-plain. At this point it breaks up into several more or less clearly defined branches, which distribute the sand to build up the delta in the estuary.

These branches fall off in height towards the head of the sand-plain, as is often seen in similar deposits elsewhere. As it has been shown that the amount of post-glacial erosion has been small, this depression must be due to conditions existing while the ice was present. The first model shows a large kettle now occupied by a pond which lies north of the sand-plain and east of the esker. This depression, being filled with ice after the course of the esker river was changed, must have had an outlet, and as the main body of ice would have prevented the formation of an outlet on the north, it seems reasonable to suppose that this water quietly cut through a slight sag in the esker to the west. This cutting would have continued until the ice-sheet had retreated farther north, and the ice block in the kettle had melted, and its depth would be governed by the amount of the lowering of the water in the estuary, caused by rising of the land.

Two branches from near the north end of the esker run into cusps at the head of a second smaller sand-plain deposit, formed when the ice-front had retreated some two thousand feet, and while the ice remained at this second point there would have been no outlet for the water to the north. The frontal lobes of this second sand-plain are not at all typically developed.

7. Delta streams.—In front of the openings of the esker tunnels will be seen the depositing streams breaking up into many branches, as Professor Russell has described them in Alaska^[1]. Some of them are represented as having already ceased to flow to the edge of the delta, and are fast filling up; others are pushing out their resulting lobes as far as they can reach; while a third class are supplying detritus to those in front, and are building up their channels to give themselves greater carry-