89 
have therefore been cut into and dissected. The present cycle of erosion 
has not been in operation long enough for small streams to do more 
than cut narrow valleys, while the larger streams have started on a 
career of lateral corrasion and have produced valleys, in some cases, a 
half mile wide or more. 
The whole belt is characterized by the small amount of general 
erosion. Eyen the broader valleys show steep-walled sides and one 
passes with great abruptness from the flat terrace floors of the divides 
into the trench-lke valleys. Furthermore, the original terrace coy- 
ering has been retained over wide areas where not actually cut out by 
the streams in excavating their valleys. The factors determining these 
phenomena are probably the original plane surface of the country into 
which the valleys were cut, the receney of the uplift, and especially 
the low precipitation and low humidity of the atmosphere. 
The lower ‘terraces apparently never oceupied a large portion of 
the ‘coastal belt, and at the time of their formation and maximum de- 
velopment were more in the nature of coastal fringes or comparatively 
narrow valley flood plains. But as we left San Diego, we rose onto the 
high terrace or dissected plateau. This gave us a broad and beautiful 
panorama and it would seem that at the time of its formation and max- 
imum extent if must have occupied the whole width of the coastal belt. 
and probably extended farther out to sea than the present shore. At 
that time the country must have had the aspeet of a practically un- 
broken low coastal plain extending from the mountains to the sea, and 
for many miles both to the north and south of the position of San 
Diego. The streams must have flowed in very shallow troughs. If those 
conditions had been maintained to the present, the wonderful views 
over the bay and along the coast that can be obtained from the Expo- 
sition grounds and from other elevated parts of San Diego would not 
now be available, but the railroad would have no grade to contend with 
such as it encounters climbing out of Soledad valley. 
The Transition Zone.—As the road approaches the mountains, it 
enters a valley, and the relation of the coastal terraces to the range front 
was not well seen, but the transition appears to be abrupt. The hills 
rise rapidly from the level of the highest terrace, and the terrace in 
places extends back of the line of steep-fronted mountain salients and 
appears to pass into lateral valley terraces up the main streams. It is 
soon evident that we have passed into a region of crystalline rocks. 
Whether the mountain front is essentially an old development affected 
by later erosion and other modifying influences, possibly antedatine the 
Tertiary or even older sediments of the coastal belt, or whether it is of 
more recent origin; whether it is essentially of erosive origin or was 
