MANSFIELD: ROXBURY CONGLOMERATE. ELZ 
shows that in successive periods successive sets of stream threads 
wandered repeatedly over the whole and that as each “patchwork 
addition” was but slight the construction was carried forward with 
substantial uniformity. Every stream has a line of swiftest motion 
along which its coarsest material is dropped, while on either hand 
' the finer material is deposited. The debris slope, then, was built, 
not of bedded sheets, as the alternate occurrence of coarse and fine 
material would seem to indicate, but of interlaced gravel courses 
penetrating a mass of fine material (ibid., p. 618). The thickness 
of the deposit sometimes reaches 500 feet (ibid., p. 628). The max- 
imum size of the gravel is that of cobbles but such material is uncom- 
mon. Individual boulders of larger size, not well rounded, occur 
in various places as if transported thither by river ice or uprooted 
trees. They are often of softer rock, such as sandstone, while the 
gravelly materials are invariably hard. Finer textured gravels are 
relatively abundant (ibid., p. 634). The gravels are disposed in 
courses, following the direction of slope of the plains, and consist of 
materials that have travelled far (ibid., p. 627) and are well rounded.! 
All sections natural or artificial show a no less frequent occurrence at 
one level than at another. There is no observed graded variation in 
size of the gravel up or down in the section but there is a gradual 
diminution in size from the mountains outward, and the rate of diminu- 
tion seems the same at all levels (ibid., p. 633). The gravel beds 
are invariably found elongated in the direction of slope of the plains, 
or approximately that direction, and are prevailingly cross-bedded. 
The sand beds have similar structure but are deeper and broader 
with gravelly courses interwoven (ibid., p. 634). While the coarser 
materials are important and widespread, fine clay constitutes the bulk 
of the mass through which the coarser materials extend in layers of 
varying thickness and at varying intervals (ibid., p. 635). While 
thicker beds of coarser material occur in the lower part of the sections 
they are farther apart than in the upper portion. Occasional thick 
beds appear in the upper levels also (ibid., p. 637). The finer materials 
have a similar arrangement to that of the coarse. Elongated clay beds 
are separated by thin sand beds and occasionally by fine gravel. The 
clay beds are not, however, limited to lenticular shape but frequently 
appear as thin masses spread between layers of sand or gravel (ibid., 
p. 638-639). 
1 The illustration facing p. 634 in Johnson's report shows the pebbles as subangular 
“on the whole. ) 
