526 ON THE GEOLOGY OF THE ISLAND OF AQUIDNECK. 
When we come to study the disposition of the underlying rock we 
shall be struck by the great amount of perturbation the beds have 
suffered ; they are thrown into real mountain irregularities, Few a 
parts of the Alleghanies are more seriously disturbed, but the sur 
face preserves no semblance of mountain forms. It does not dit — 
fer from the shape which horizontally lying rock would give. a 
If the character of the surface had been given by water action, 
then we should have had something very different from this. The — 
valleys would then have mapped for us the attitude of the strats 
and the elevations would have been much greater and differently 
disposed from what they are now. But under the sheet of ice 
these details of structure lose their value; the ice cannot turn and 
twist as the water does, only a slight deflection even under the 
most important resistance can be permitted. The result necessarily 
is that the glacial stream wears away without much reference 
to the disposition of the beds beneath the surface. This is a point 
of more than local importance; it may serve often as & general 
guide to the determination of the question whether any l 
country shows the result of glaciation. When the topography of 
any region does not express the structure of its underlying g a 
rials, in the fashion common to all surfaces of purely aqueots o 
erosion, then there is reason to suspect the action of ice. So 
suspicion may amount to a certainty wherein the whole topography 
has been created with the great independence of internal condi- 
tions which it here exhibits. S 
There is a general absence of sand and pebbles in the ere 
about this island; this is, probably, primarily due to the yer 
drift bas not been enough washed over by the sea to remo 
arenaceous matter. The sand mass of Nantasket or Lym ne 
alone much exceeds all that exists in the spits and bese R7 
Narraganset Bay. The small extent to which the drift: has | 
robbed of its sands is due to the original slight sub s 
this district; only about ten to twenty feet of emergence aad 
cated, so the region has not been so much washed ie i 
region more to the north. There are great bodies of san 
shores to the north and south ; its comparative per ` 
be explained by the want of recent great movements pi 
and the deep water which prevents its movement along 
from the east or west. As we go south of this region i 
ial period. 
into the area of positive subsidence since the glac 
