370 Fence Wall Geolof/r/. — Foerste. 
express themselves as beds of streams, long lakes, or valleys. 
In localities presenting great geological diversity and sub- 
jected to long erosion it is often found that the existence of a 
well marked valley, stream or pond, immediately heralds 
some change of formation which will be found on crossing the 
same. Boundaries of geological areas as determined by boul- 
ders are then readjusted also with reference to neighboring 
streams and valleys. 
Where the rocks dip towards their boundaries, the boundary 
streams will usually be found to remain near the fractures 
which determined their course. Where they dip away from 
their boundaries, especially if composed of softer rocks, the 
streams will often wander a small distance from their original 
beds. — As a general rule the dip of rocks in the more recent 
formations is away from, in the older rocks, towards, any 
area of massive igneous rocks penetrating the same. This 
simply means that since such igneous rocks do not always 
show themselves at once at the surface, the element of time 
must be taken into consideration, and as a rule the anticlinals 
of any area are sufficiently eroded to expose igneous rocks not 
originally shown at the surface, before synclinals containing 
corresponding igneous rocks in the same position are suffi- 
ciently eroded to expose them. A certain allowance must be 
made for the probable change of course of any stream since 
its first connection with a series of fractures. 
Such, in general, are the methods used in gaining some 
slight assistance from the boulders of fence walls and from 
topographical features when the drift is too heavy to permit 
the frequent exposure of the bed-rocks. Frequent modifica- 
tions of these methods are used in the field, but the principles 
are all those well understood by the glacial geologist and do 
not need further discussion. The amount of information 
often obtained in this way would no doubt be a matter of 
great surprise to those geologists who neglect all features 
except surface exposures. Yet, since the value of all such 
work is dependent upon the accuracy and distinctness with 
which the original boundaries of bed-rocks, now drift-covered, 
can be traced, such methods find their best practical applica- 
tion where the dip of rocks is strong enough to furnish sharp 
boundaries along their strike (30° to 90°) where the structure 
