DOME STRUCTURE IN CONGLOMERATE? 
RALPH ARNOLD 
CONTENTS 
Location and Composition of the Domes. 
Eagle Rock. 
Other Examples. 
Summary of Observations. 
Previous Theories Regarding the Origin of Domes. 
Conclusions Regarding the Origin of Conglomerate Domes. 
LOCATION AND COMPOSITION OF THE DOMES 
On the flanks of Eagle Rock Valley, two miles west of Pasadena, 
Cal., are several dome-shaped structures developed in conglomerate. 
They resemble in a general way the granite domes of the Sierra 
Nevada, but are much smaller and also less isolated as regards the 
surrounding topography. The conglomerate is of lower Miocene 
age, is quite regularly bedded, somewhat tilted, and consists of sand 
pebbles and boulders of the granite, diorite, gabbro, gneiss, and other 
crystalline rocks which form the San Gabriel Mountains to the north. 
Some of the boulders are as much as 6 feet in diameter, although the 
average are not over 3 or 4 inches; some of the layers are little 
more than coarse pebbly sandstone. 
. 
EAGLE ROCK 
The largest and most perfect example of a dome is found at the 
east end of the valley, and is called Eagle Rock. Fig. 1 is a view of 
Eagle Rock as seen from the southwest, and shows its position with 
respect to the surrounding topography. The “eagle” may also be 
seen on its 80-foot precipitous west face. The hills west and south 
of the rock are of conglomerate, similar to, except softer than, that 
composing the dome, and dipping in the same direction. The hills 
immediately to the north—to the left of it in this view—are of crys- 
t Read before the Geological Society of America, December 28, 1906, and pub- 
lished by permission of the Director, U. S. Geological Survey. 
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