When the Gualala Basin was formed 100 million years ago, the floor of the basin was filled with basalt The basalt flowed out onto the floor of the basin from cracks in the rocks in the bottom of the basin. This same type of basalt underlies all of the world’s major ocean basins. During the next 90 million years, the floor of the basin continued to subside and sediments were carried into the basin by rivers that drained the nearby land areas. The sediments were stacked into the basin like pancakes on a large platter, with the oldest sediments on the bottom and the youngest on top. Eventually, a thickness of over 30,000 feet of sediments were piled into the Gualala Basin. With time, heat, and pressure, these sediments became the hard sedimentary rocks of the Gualala, German Rancho, Galloway and Point Arena Formations. The rocks of the Gualala and German Rancho Formations were deposited when the basin was still in southern California or further south, and the Galloway and Point Arena Formations were added as the basin was being carried north by the San Andreas Fault. About 5 to 10 million years ago, the rocks in the Gualala Basin began to be squeezed, folded, uplifted and eroded, so that even some of the deepest rocks in the basin are now exposed at the surface, as shown in the diagrammatic cross section of the geologic map.   


The diagrammatic cross section generally follows the Sea Ranch bluff and the beaches have been projected into the line of section. The rocks have been folded so that the oldest rocks are exposed in the center of the Black Point Anticline and the rocks from Pebble Beach to the north boundary of the Sea Ranch are tilted to the northwest. The prevailing dip of the sedimentary layers is shown by the short red lines. The white lines show the rocks that have been eroded during uplift of the basin over the last few million years.


Most of the rocks in the Sea Ranch area were formed between 50 and 100 million years ago. These are relatively young rocks  compared to the 4.5 billion year age of the earth. If the age of the earth were compared to a human lifespan of 100 years, these rocks were formed when the earth was over 98 years old. 

Sea Ranch

Geologic Background