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. Related Links:
Geologic Background,
Sea Ranch |
Sea Ranch Geologic Background |