MANSFIELD: ROXBURY CONGLOMERATE. 125 
small or large are almost invariably well rounded and water worn. 
In the occasional exceptions the deposits are not only unstratified but 
earthy, and the stones angular and subangular, some even showing 
faint ice-markings. Such deposits are associated (ibid., p. 184) with 
heaps of well-worn sand and gravel, showing that they belong to the 
same series. Clays are sometimes associated with the sand and gravel 
but there are no organic remains in either. 
Speaking of the shapes of drift fragments, Stone remarks (p. 40) that 
in glacial gravels we find all degrees of water wear. Most of the stones 
and grains found in kames and osars show a very large amount of 
attrition and rolling and are very much rounded. 
The intimate relation of till and glacial gravels is shown by Penck 
(p. 132):— the till with angular, striated boulders overlies and is inter- 
bedded with coarse gravel, sand, and clay, both horizontally bedded 
and showing false stratification. He cites (ibid., p. 137) the obser- 
vations of several writers to the effect that striated pebbles soon 
lose their glacial markings when subjected to the action of water. 
One of the most characteristic features of glacial regions is the 
effect of the ice movement on the underlying rocks, which frequently 
acquire a smoothed and often highly polished appearance. The 
whole surface is marked with striae, especially the close grained rocks, 
such as limestone. Sandstone and highly jointed rocks are usually 
much less marked and often show a broken and shattered surface 
below the till (J. Geikie, p. 18-19). 
Another feature of glacial action is to be noted in regions where 
glaciers descend to sea level, or enter large lakes. The icebergs 
that float away from the end of the glacier are more or less charged 
with debris, embedded within their mass. As they melt, the morainic 
material is dropped upon the bottom of the lake or the sea. It may 
often happen, therefore, that coarse, angular material, bearing glacial 
markings, may be dropped among the finer sediments at some distance 
from shore and may thus become embedded in a well-stratified, argilla- 
ceous matrix, in which may also be contained the remains of marine 
or lacustrine organisms. Stranded icebergs produce local contor- 
tions and interruptions in the bedded deposits on which they rest. 
Sea ice frozen to the shores may bear away great quantities of loose 
debris, when the floes break up and are driven away by the wind. Tarr, 
sailing for a thousand miles along the American coast north of the 
straits of Belle Isle, much of the time among ice-floes, estimated that 
about one per cent of the cakes carried debris of some kind, while in 
some cases ice cakes were black with it (A. Geikie, p. 578). Similar 
