This article appears in the Park Watch Training Manual and is reproduced with the
author's permision
Modesto Formation
What:
Soft sand, silt, and clay. Includes gravel derived mainly from Tuscan and Red Bluff
Formations. Soils of Upper Member lack a B horizon. (If present, it would be marked by a
slight reddening and an increase in clay content.) Lower Member not present in Upper Park.
Deposited in channels cut into the Red Bluff Formation.
When:
Late Pleistocene, approx. 26,000 to 12,000 yr ago.
Where:
Most of golf course; riding arena; Five Mile area; all of lower Bidwell Park.
Red Bluff Formation
What:
Compact, hard conglomerate. Mainly pebbles and cobbles of metamorphic and volcanic rock,
set in sandy red matrix. Includes layers of sand or silt. Deposited as alluvial fans formed from
uplift of the Tuscan Formation east of the Chico Monocline.
When:
Pleistocene, approx. 0.5 to 1.0 Ma (mega annum. = million years).
Where:
South of Horseshoe Lake; north bank of diversion channel at Five-Mile area. near diversion
weir (a few hundred feet west of footbridge); diversion channel near main entrance to Upper Park.
Rock folded gently in only one direction. Probably formed because of vertical movement
along a hidden fault. The fold forms a straight line along east margin of the Sacramento
Valley, from Chico to near Red Bluff. A widespread system of fractures formed at the same
time and from the same cause as the Monocline.
When:
Most of the fracture system (and thus the Monocline) formed by 1 Ma. (Some fractures are
covered by radiometrically dated basalt, in Deer Creek.)
Where:
From Rim trail northeast of Horseshoe Lake, southwest-sloping Monocline is visible as an
increase in dip (= angle of inclination) of the rock layers in the south wall of Chico Creek
canyon. A few of the fractures are in the Park; they form low benches or small troughs in
Lovejoy Basalt best seen on the flat area north of Salmon Hole.
Tuscan Formation
What:
Layers of volcanic-mudflow debris (= lahars) and sediment derived from the lahars:
conglomerate, sandstone, and siltstone, all containing abundant particles of volcanic rock. The
lahars tend to form cliffs; sediments form gentler slopes below the cliffs. Vegetation on the
permeable sediment but not on the impermeable lahars gives canyon slopes a striped
appearance.
When:
Late Pliocene, approx. 2.8 to 3.3 Ma.
Where:
Most of Upper Park, including the 1995 acquisition. In stream bottom at Five Mile and golf
course, and in bottom of diversion channel near entrance to Upper Park; along rim trail north of
Horseshoe Lake, and in canyon walls in most of the rest of Upper Park.
Lovejoy Basalt
What:
Hard. black, fine-grained rock, heavily fractured. Formed from very-fluid lava flows.
Individual flows 10 to 30 ft thick. Large, eroded blocks have tumbled into Big Chico Creek
channel, especially east of Devils Kitchen.
When:
Miocene, 18 to 19 Ma (based on radiometric age dates from rock within Park).
Where:
The dip (inclination) of the basalt is steeper than the southwest-sloping gradient of the stream.
Because of this, if we travel northeastward, the basalt layer rises faster than the stream does.
Thus the upper surface of the basalt is at stream level a few hundred feet west of Bare Hole.
but east of Bare Hole is at the level of the main road. East of Devils Kitchen, the basalt layer
rises up the walls of the canyon, forming a line of black cliffs.
Chico Formation
What:
Tan to light gray sandstone. Some layers soft and clayey, others hard and cemented.
Deposition in marine environment resulted in trapped salt water that even today continues to
come to the surface at salt licks.
When:
Late Cretaceous. Approx. 65 to 75 Ma.
Where:
Northerly end of Park. including the 1995 acquisition. Chico Formation is about 2000 ft thick in
the canyon of Big Chico Creek, but not all of that is inside the Park.
Mesozoic And Paleozoic Basement Rocks
What:
Ancient volcanic and sedimentary rocks that were metamorphosed before and during formation
of the Sierra Nevada.
When:
Mesozoic and Paleozoic Eras; older than about 135 Ma.
Where:
Not exposed in the Park. Closest outcrops are in canyons 3 to 6 mi. east and northeast
of the upper end of the 1995 acquisition. However, pebbles and cobbles of these rocks,
transported by westward-flowing streams, are present in most of the younger formations.
