MANSFIELD: ROXBURY CONGLOMERATE. 119 
abandoned shore lines of the Quaternary lakes of the Great Basin. 
Where a river joins a lake or the ocean the heavier and coarser part 
of its load, that which is pushed or rolled along the bottom, is deposited 
and slides down the front of the delta by its own weight. The slope 
of the face of the delta is the angle of repose of this coarse material 
Subject to modification by waves. The finer detritus is carried be- 
yond the delta face and sinks slowly to the bottom, the deposit being 
thicker near the delta and gradually diminishing outward so that the 
slope of the delta face merges in a curve with the bottom beyond. 
As the delta is built outward the steeply inclined layers are superposed 
over the more level strata of the bottom and in turn come to support 
the gently inclined layers of the delta plain, so that any vertical section 
Fie. 2.— Diagrammatic section of delta deposits (slightly modi- 
fied from Gilbert, a, p. 107). . 
of a normal delta shows at the top a zone of coarse material, bedded 
with gentle inclination, then a zone of similar coarse material, the 
laminations of which are at a high angle and at the bottom a zone of 
fine material, the laminations of which are gently inclined and unite 
by curves with the middle zone (Gilbert, a, p. 106). Such a section 
is represented in Figure 2, taken from Gilbert's description (ibid., p. 
107). Sometimes marine sediments are intercalated among the delta 
deposits, as in the case of the Mississippi delta. Again, deep borings 
have been made in deltas without encountering any traces of marine 
conditions. According to Medlicott, the Ganges-Brahmaputra delta 
