MINOR GEOLOGICAL EFFECTS OF THE EARTHQUAKE, 401 
Along the beach or sand-spit which separates the Salinas River from the Bay of 
Monterey at Moss Landing, there was a marked lurching of the spit toward the trench 
of the river as illustrated in plates 1344, B and 135a, z. 
Lurching of soft ground was also exemplified on the tidal mud flats of Tomales Bay, 
and on the “made land” of San Francisco; but there being no trench in these cases, 
the movement caused a ridging of the surface with compensating depressions. In the 
case of the made land in San Francisco, and perhaps generally, the deformation of the 
surface due to lurching was complicated by the settling together of the loose material. 
CRACKS AND FISSURES. 
The cracks in the ground which appeared at the time of the earthquake fall into 
different categories. Of these there are two distinct classes: 
1. The crack or fissure of the main fault, which is a superficial expression of the deep 
rupture of the earth’s crust that caused the earthquake. Associated with this as a sub- 
class are the auxiliary cracks and fissures which are superficial expressions of branch 
ruptures or subparallel ruptures, generally close to the main rupture in the Rift zone. 
In this class would also belong any cracks due to supplementary faulting in the general 
zone of disturbance, if such supplementary faulting exists, which is doubtful except in 
special instances. 
2. The second general class includes those cracks and fissures which were caused by 
the earthquake, as a result of the commotion of the ground, and have, therefore, been 
designated as secondary. 
The main crack, or fault-trace, and the auxiliary cracks satellitic to it, have been 
described in the section of the report dealing with the earth movement along the fault. 
The secondary cracks, inasmuch as they are an indication of the intensity of the shock 
at any locality, have been described or referred to in the section dealing with the dis- 
tribution of intensity. A brief review of the phenomena of cracks in the ground, apart 
from the main fault-trace and the auxiliary cracks in the Rift zone, will, however, be 
given, even at the risk of some slight repetition. 
Since some of the cracks to be referred to can not with certainty be placed in one or 
the other of the two fundamental classes above indicated, it will be found convenient 
not to force that classification in all cases. Along the zone of the Rift there were many 
secondary cracks, as well as those classed as auxiliary; but it was not in every case 
possible to discriminate between them. These secondary cracks occurred both on hill 
slopes and in alluvial bottoms. On the hill slopes they were very commonly associated 
with landslides, or marked the inception of landslides; and these have already been 
discust. On the bottom lands of streams or embayments in the Rift zone, cracks in the 
ground were exceedingly common for the entire length of that portion of the Rift along 
which the fault extended. In very many cases these cracks were associated with the 
lurching of soft incoherent materials, just as the cracks on the hillsides were associated 
with more common phases of landsliding. There were also, however, many cracks 
quite dissociated from the deformation of the surface due to lurching, although there 
was doubtless in these cases an ineffective tendency to lurching. 
Beyond the zone of the Rift, cracks were observed at many localities. These were 
most common on the bottom-lands of the streams, notably the Kel River (plate 1384, B), 
the Russian River (plate 139, 8), Coyote Creek (plate 140a, 8), and other streams at 
the south end of the Bay of San Francisco, Pajaro River (plate 1418), San Lorenzo River, 
and the Salinas River. Many other smaller streams might also be mentioned. In 
these cases the cracks were usually associated with the phenomena of lurching of the 
alluvial deposits, though many cracks also occurred where no such association was ap- 
parent. ‘They were in nearly all cases found to be parallel or sub-parallel to the nearest 
2D 
