Lawson. | 



Post-Pliocene Diastropkism. 



'23 



2. The great system of terraces which score the hill like giant 

 steps. All these terraces have steep cliffs at their rear, and several 

 of the steps are of such great dimensions, both vertically and hori- 

 zontally, that they appear as prominent features on the Coast Sur- 

 vey contour map of the hill, scale 4-50^0 — a statement which will be 

 appreciated by those who have attempted to give expression to 

 ordinary terraces by means of contours, even on much larger scales. 

 Many of the terraces are, however, of much smaller dimensions, and, 

 although they appeal strongly to the eye, are not expressible on 

 the map. 



3. The third prominent topographic feature of the hill to which 

 special attention is drawn is the effect of stream action. The lim- 

 ited catchment area of the hill of course precludes the development 

 of large streams. There are, however, several small streams which 

 flow in the winter season. These would seem to be rather feeble as 

 geological agents, but they have, nevertheless, gashed the hill with 

 numerous sharp canons, which cut into it across the upper terraces. 

 The lower terraces, below about 240 feet, are not, as a rule, canoned. 

 The streams from the canons may in many cases be seen to emerge 

 on the 240-foot terrace, and flow out over its surface to the next 

 lower cliff, in a very* shallow trench. This next lower cliff is in sev- 

 eral instances the cliff of the present strand, and, although where 

 the water falls over it an incipient gorge may be observed, yet the 

 recession of the cliffs is scarcely less rapid than that of the gorge, 

 and in some cases the cliffs are receding so fast that no gorge can 

 be developed. The phenomenon of a stream emerging from a 

 canon, running out across a terrace, and dropping into a less pro- 

 nounced canon in the face of the next lower cliff, is observable on 

 several of the higher terraces also, but it is not so striking a feature 

 as on the 240-foot and lower terraces.* The lesson which the phe- 

 nomena teach is, of course, the relative topographic youth of the 

 lower part of the hill as compared with the upper part, where the 

 canons traverse the entire width of the terraces. The canons are 

 all sharp, acutely V-shaped in aspect, and have robbed the hill of but 



*This characteristic of the drainage does not hold on the San Pedro Harbor 

 side of the hill, since there it is flanked by newer incoherent formations, which, 

 have been easily trenched by the streams. 



