Delta-Plain at Andover, Mass. — Mills. 165 
lost its closely definitive form as an esker, and broadens out 
into a confused, hummocky, kame-like area, as it finally bears 
to the south. A distant view of it presents the landscape of a 
morainic region. In passing directly over this area, however, 
one can trace the indefinite preservation of an esker-like char- 
acter. The width of this region is approximately a quarter of 
a mile. It is a complex mass of loosely bedded gravel knobs ; 
reticulated ridges enclosing small kettles; conical, kame-like 
cones of gravels and sands, and irregular hummocks ; — all of 
water assorted materials. At times, pockets and lenses of the 
finest sands occur. Exposures of the latter often show mar- 
velous examples of cross-bedding, indicating indeterminate 
conditions that could produce such an abnormal succession of 
cross and counter currents. [PI. XXIII A.] In other places, 
sections show a contortion, or forcible thrusting of one strati- 
fication into another, subsequent to their deposition. The lat- 
ter is probably due to some minor movement of the immedi- 
ately adjacent ice. A transverse cut through the ridge, near 
the northerly end of 'this complex area, gives an interesting 
exposure. The body of the deposition is thinly stratified, fine 
sands. Lying unconformably upon the western side of the 
sand beds and embedded in it, is a zone of coarse and rounded 
pebbles. The contact line of the sands and pebbles is suggest- 
ive of an eroded channel or the rounded bottom of a pot-hole. 
The extent of this coarse deposit cannot be accurately deter- 
mined, but is probably very limited. A crosscut a hundred feet 
farther south shows no indication of it. Its localized extent 
along a north and south axis suggests the work of a moulin, or 
of a stream descending through a crevasse. [PI. XXIII, B.] 
An overburdened stream of a glacial mill might churn out an 
excavation and leave behind its denser material, not having 
the power to carry it out of its previously eroded pot-hole. A 
forward movement of the ice, or a closing of the crevasse 
would permit such conditions to exist only temporarily. For- 
mer conditions would be restored, so that the pot-hole and its 
contents would be entirely enclosed in the normal deposition 
of the sub-glacial stream. In fine, that such a complex area 
as a whole, closely in relation to the dying ice-front, was the 
result of deposition under a countless variety of conditions, 
and where shifting changes were, rapidly and forcibly accom- 
