Courtesy of Dibblee Geological
Foundation
| |
| | Alluvium
Holocene
(+/- 11,000 years ago to present) | | | Fanglomerate
Pleistocene
(+/- 2-1 million years ago) | | | Rincon
Shale
Early Miocene (+/- 24-20 million years ago) | | | Vaqueros
Sandstone Early Miocene (+/- 24 million years ago) | | | Sespe
Oligocene
(+/- 37-25 million years ago) | |
Five major
rock formations are found in the San Marcos
Foothills. They are listed below, from youngest to oldest. The headings show the
name of the rock formation, its geological age, and its color on the map. "T"
refers to the Tertiary period (65 to 1.8 million years ago), and "Q"
refers to the Quaternary period (1.8 million years ago to present). The lower
case letters refer to the name of the formation.
|
|
| Alluvium
(Qa) | |
| Age: | Holocene
(about 11,000 years ago to present) |
| Origin:
| Stream bed debris from floodplain deposits |
| Appearance: | Unconsolidated
muds, silt, sand, and gravel. |
| Features: | Flow
sequences filling stream channels through older rocks. |
| Vegetation:
| Willows and oaks downstream, brush upstream. |
| |
| Fanglomerate
(Qog) | |
| Age: | Pleistocene
(about 2-1 million years ago). |
| Origin:
| Alluvial fan debris
from nearby sandstone cliffs at mountain
front. |
| Appearance: | Poorly
bedded sandstone boulders and cobbles in a loose matrix. |
| Features: | Boulder-strewn,
high sloping mesas dissected by stream channels. |
| Vegetation:
| Grasslands and oak savannah. |
| |
| Rincon
Shale (Tr) | |
| Age: | Early
Miocene (about 24-20 million years ago). |
| Origin:
| Marine: muds deposited on an offshore continental
shelf. |
| Appearance: | Poorly
bedded, unconsolidated gray shale. |
| Features: | Clay
beds swell and erode easily; mudslides are common.Poorly preserved marine micro-fossils
(foraminifera). |
| Vegetation:
| Gently sloping grasslands where grazed,
with some scrub were ungrazed. |
| |
| Vaqueros Sandstone (Tvq) |
|
| Age: | Early
Miocene (about 24 million years ago) |
| Origin: | Marine;
shallow nearshore sands of a warm, transgressing
sea. |
| Appearance: | Massive
to thick-bedded, greenish-gray to tan sandstone. |
| Features: | Resistant
to erosion; forms topographic highs and steep slopes. Fossils include clams, oysters,
barnacles. |
| Vegetation: | Heavy
brush; yucca and chaparral common. |
| |
| Sespe Formation (Tsp) | |
| Age: | Oligocene
(about 37-25 million years ago). |
| Origin: | Non-marine;
gravels, sands, and muds in creeks and streams. |
| Appearance: | Maroon,
red, and green silt and shale
with interbeds of pink, tan, or gray sandstone. |
| Features: | A
slope-former; may erode as landslides in steeper areas. |
| Vegetation: | Brush,
chaparral. |
