170 SCENERY OF SWITZERLAND. 
tween two harder ones A and C will here and there 
be brought to the surface. 
In such a case, owing to the greater softness 
of the stratum B, secondary streams will often cut 
their way back as in Fig. 39, FF, thus forming 
longitudinal valleys parallel to the ridge, the sides 
being formed by the harder strata AC. Such valleys 
(Fig. 40) are common in the Jura. 
Sometimes there may be two or even three such 
“Combes” along a main valley, as for instance 
(Fig. 41) between Mont 
Tendre and the valley 
of the Orbe, where we 
have four ridges of 
harder strata, Urgo- 
nian, Neocomian, Va- 
langian, and finally 
Portland rock enclos- 
ing three combes due to the existence of softer layers. 
It is obvious that in this case the transverse 
valley DE (Fig. 39) is older than the longitudinal 
valley FF. 
A glance at any geological map of Switzerland 
will show that many rivers run along the boundary, 
that is at the outcrop, of strata. 
The long lines of escarpment which stretch for 
