SANTA MARIA OIL DISTRICT, CALIFORNIA. 
22 
longitudinal trough running for miles parallel with this range and the 
Los Alamos Valley and cutting across the ends of the above-mentioned 
ridges at right Angles to them, at a distance of one-half to 1 mile from 
the valley. It notches all the ridges and leaves an individual row of 
knobs 100 to 200 feet in relief bordering the valley. This depression 
is not a continuous drainage feature, but is stratigraphically of impor¬ 
tance as approximately marking the contact between the Monterey 
shale and the loose Fernando sand. On the south side of the summit 
of the Purisima Hills the lateral ridges extend a long way with a uni¬ 
form gentle slope, like remnants of an eroded inclined plateau. At 
their base, some miles from the summit, and usually from 500 to 1,000 
feet below, these southern slopes merge into an undulating hilly 
plateau 'that has the appearance of being buried under soft recent 
sand. The range is broadest at the east end, where it consists of a 
number of parallel ridges. The point of convergence of some of these 
is Redrock Mountain, which is 1,968 feet high and the highest summit 
in these hills. Thence westward the hilly zone narrows into a single 
central ridge and its offshoots, and gradually pinches out, finally giv¬ 
ing place on the south and west to a broad terrace in which its hilly 
character is lost. The summit of the main ridge of the Purisima 
Hills west of Redrock Mountain gradually declines in height and for 
most of the way it is remarkably even, the elevation varying between 
1,200 and 1,000 feet. At the elevation of 1,000 feet it grades into 
the smooth terrace called Burton Mesa. 
BURTON MESA. 
Burton Mesa is a marine terrace covering more than 50 square miles, 
which slopes, with an average gradient of 2|per cent, away from the 
west end of the Purisima Hills, reaching the sea within 7\ miles. It 
is composed of Monterey shale, in the main rather gently folded, 
which has been planed off and covered with a thickness of about 25 
feet of horizontal gravel and loose sand. From the elevation of 1,000 
feet, where the continuous sheet of sand overlaps on the end of the Puri¬ 
sima ridge, down to the 600-foot level the distance in a west-south¬ 
west direction is three-fourths of a mile and the slope 10 per cent. 
Within the next three-fourths of a mile a drop of 100 feet occurs, the 
slope being per cent. Beyond lies the main level stretch of the 
plateau for a distance of 5 miles, with no greater slope than three- 
fourths of 1 per cent until the elevation of 300 feet is reached, in the 
southwest corner of the mesa, where there is an abrupt change to a 
10 per cent slope, the distance down to elevation 100 feet being only 
one-third of a mile. Below the 100-foot level there is a bench with 
a 3 per cent grade as far as the edge of the cliff which faces the sea, 
and which is in most places about 25 feet above the water. North 
