HYPOTHETICAL LANDSLIDE ORIGIN FOR BROWN'S FLAT, SAN GABRIEL MOUNTAINS, CALIFORNIA: EVIDENCE FROM STRUCTURAL DATA

Brown's Flat is a large plateau (300,000 m2) in the San Gabriel Mountains located about five miles southwest of Mt. San Antonio. It is a striking feature in this mountain range due to the sharp contrast in topography between it and the surrounding ridges and valleys. This broad flat area protrudes from the side of a valley wall, approximately 180 meters below the ridgeline. No known literature discusses this inaccessible part of the San Gabriel Mountains. The purpose of my study was to investigate the origin of this topographic anomaly using structural data derived from the underlying crystalline bedrock.

The methods used for gathering data consisted primarily of making numerous visits to the field site and measuring the orientations of the foliation and fracture planes using a Brunton compass. These data were then plotted on topographic base maps and stereonets. Outcrops were difficult to access due to excessive vegetation cover, but metamorphic bedrock was well-exposed in a few locations. Two such places were the road leading to the radio towers east of the flat and a major canyon (nicknamed “Alpha Canyon”) traversing the flat on its northeast flank. Wherever the rock is exposed, it consistently shows the same kind of geometry in the planar structures contained therein. The foliations generally strike northeast, with dips ranging from 13 to 70 degrees northwest (towards or beneath Brown's Flat). About 25% of these dip shallower than the topographic slope. Crosscutting fractures have random strikes, but consistently dip steeper than 65 degrees.

The most logical conclusion to be derived from this information is that the shoulder of the mountain that now makes up Brown's Flat was once a part of the ridgeline above it. However, due to probable failure along daylighting surfaces, it appears as though the high angle fractures may have weakened the rock sufficiently to cause a piece of the ridgeline to break off and slide northwestward along the foliation planes. This slump then leveled off below, forming the anomalous plateau now known as Brown's Flat.