164 The America* Geologist. September, - 
forcing a concentration of energy into the stream, that could 
easily handle flat pieces of shale, abundantly supplied by its 
fissility, from eight to twelve inches in diameter. Bedded ma- 
terial of this size occurs partially throughout the overwash, 
not as iceberg droppings or exceptional phenomena, but as a 
prevalent and characteristic structural condition. 
Delta plains composed largely of sands, with coarser grav- 
els near the head, are of frequent occurrence throughout New 
England. An excellent and typical illustration of a glacially 
built delta-plain is found at Andover, Massachusetts, twenty- 
three miles north of Boston. [Plate XXII.] The plain itself lies 
in the southern portion of the town, known as Ballard vale. 
There extends from New Hampshire across the northeastern 
corner of Massachusetts into Essex county, a well defined 
esker ridge. It crosses the Merrimac river northwest of the 
city of Lawrence, and winds its way into the westerly part of 
Andover, extending in a general southeasterly direction, to the 
Shawsheen river. At the river it turns rather abruptly to the 
south continuing along the west side of the stream. This 
esker, as it approaches the Shawsheen river, divides into sev- 
eral branches locally known as the "Indian Ridges." These 
ridges are remarkable in development and structure, and for the 
present state of preservation. They have long attracted the at- 
tention and study of geologists. Their structure, and correlation 
to the surrounding topography, has been, and is today, a debat- 
able question. The main elevation divides and sub-divides, 
until it becomes a mass of reticulated ridges inclosing all kinds 
of depressions, bogs, and kettle-holes. Approximately at the 
point of primary division in the main ridge, there was a branch 
crossing to the easterly side of the present Shawsheen valley. 
The immediate point of connection with the main ridge is 
slightly obscure, and has been more or less obliterated by the 
erosion of the present stream. This branch from the main 
esker, its subsequent course and its termination, have been 
generally overlooked by those studying the region. Grossing 
to the easterly side of the stream, one comes abruptly upon the 
truncated portion of a well defined esker ridge. Its esker char- 
acter, strictly speaking, is not long preserved. It bears to the 
cast in a meandering course, advancing somewhat up the gen- 
tle slope of the valley wall. Within an eighth of a mile it has 
