The San Andreas Rift after passing through the Cajon Pass in Southern California proceeds approximately S 60° E along the base of the San Bernardino Mountains which rise to the north. About eight miles east of the city of Redlands, Mill Creek issues from the San Bernardino Mountains south-west on to the alluvium covered flood plain. Four miles to the west the Santa Ana River cuts north and south through the crystalline rock. These two streams roughly mark the east and west boundaries of the area studied. The southern boundary is in reality the San Andreas Rift although the edge of the Crafton Hills is shown. The northern boundary is the Mission Creek Fault which is followed by Mill Creek as far west as the north-east corner of the geologic map shown in this report. The area studied covers approximately fifteen square miles in area. The maximum variation in elevation is about 2500 feet. The area was chosen to be studied for several reasons. The foremost reason was because the region was most ideal for the study of faulting. The area contains thirteen mapped faults, large, small, normal, overthrusts, old, and recent. Six formations could be studied and research could be done on a comparatively unexplored area. The author also made a very prolonged search for fossil material in the Potato Sandstone, a formation whose age is only known roughly from paleobotanical evidence. This portion of the research was doomed to failure but the study of the region opened up so many problems of great geological interest that the author has never had occasion to regret in the least the time spent.