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Ages of Rocks
Geologists have defined a Geologic Time Scale to give the approximate ages of rock layers. Here is a brief guide to the Eras (major divisions) and Periods (minor divisions within Eras), with dates in millions of years before present:

EraBegin DateEnd Date

Some of the Periods
PeriodBegin DateEnd Date

Source: Earth, F. Press and R. Siever, Freeman: San Francisco, p. 40, 46, 49.

to emerge from a relatively narrow valley to an open plain. Nearly all the streams in the front range of the SGM debouche into the alluvial slopes of the San Gabriel Valley and neighboring areas.

A point along a ridge between two peaks where the topography resembles that of the seat used by a horse-rider (the original saddle). Along one direction, the altitude is a local low point (between the two peaks). Along another direction, the altitude is a local high point (between the two drop-offs on either side of the ridge).
Trails frequently go to a saddle since they are:
  • low points along a ridge, making for a minimum altitude gain to access the ridge,
  • often points of intersection for trails that go in either direction along the ridge, or descend the canyons on both sides, and
  • usually places with good views.

Saddles are often formed where a fault cuts across a ridge top. The weakened rock along a fault is easily eroded compared to the stronger rocks elsewhere. Examples of fault-produced saddles are Vincent Gap and Wright-Pine Saddle along the Punchbowl Fault, and Red Box along the San Gabriel Fault.
Saddles often are quite windy (e.g. Windy Gap) because the wind is funneled through this gap in the ridge which otherwise blocks any wind that is present.

Relating to Earth movements such as mountain-building and movements along faults.

use trail
A trail that was created by use, as in "a beaten path", and not by explicit trail construction. Use trails generally have no written signs to help find them, but may have cairns or colored pieces of tape tied to branches marking parts of them. Use trails may disappear quickly, either through disuse or when covered by pine needles or leaves at certain times of the year.

water gap
A canyon cut into a mountain range by a still-flowing river. The usual method of creating a water gap is to begin with a stream in a given location and add tectonic uplift, causing the stream to begin cutting a canyon. As long as the stream can keep up, a water gap is formed. If the stream cannot keep up, a wind gap is formed.

wind gap
A water gap that has lost its stream, leaving only wind where the water once flowed. The usual method in California of losing the stream occurs when the stream cannot keep up with tectonic uplift of an area, changing the drainage pattern in that area.

See also Geographic Names Information System (GNIS) Feature Class Definitions.

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Copyright © 1999 by Tom Chester and Jane Strong
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Updated 9 February 2000.