TERTIARY FORMATION AT OCOYA CREEK-ROUNDED HILLS. 
165 
its longer axis being parallel with the mountains, and its greatest width being about ten miles. 
South of Posuncula river, the formation disappears, or loses the distinct character it possesses 
along Ocoya creek. It is probable, however, that the same series of deposits exist, but they are 
at a lower level, and are partially overlaid by the recent alluvial wash from the hills and 
granite ridges, and in the valley, by the alluvial deposits, partly of lacustrine origin. It has 
already been shown in the Itinerary that horizontal strata are found at many points along the 
base of the mountains north of the boundaries which have been given. They are found along 
the course of nearly all the rivers that flow into the San Joaquin. Whether these are properly 
but a continuation of the formation about to be described, is not yet determined. 
The position of Ocoya creek will be readily seen by inspection of the map ; it takes its rise in 
the Sierra Nevada, and flows westward, nearly parallel with Posuncula river, towards the 
southern part of the Tulare valley. After emerging from the granite ridges of the Sierra, the 
stream flows directly across the belt of sedimentary formations, and forms a very good natural 
section of the beds. The thickness or elevation of the series also appears to be greatest along 
the course of the creek; consequently it was one of the best localities for the exploration of the 
deposits. 
Our approach to Ocoya creek was over the tops of the bordering hills, and the view presented 
from them was very peculiar and interesting. The eye could wander in all directions over a 
vast area of rolling hills in a continued series, one behind another, and differing but slightly in 
their elevation. The outlines were all in unbroken curves, the surfaces were rounded and 
smooth, and without one projecting rock or angle. 
The entire absence of vegetation (there being neither trees nor shrubs) was also a striking 
peculiarity, and permitted all the outlines and modifications of the surface to be distinctly seen. 
The color of all these hills was one uniform drab or clay color, and it was so distinct that they 
immediately received the title of 11 clay hills.” 
Under the glare of a strong sunlight the surface presents a peculiar velvet luster, very 
striking and distinct. This may have been due to the dried stalks of the grasses and weeds that 
are found standing thickly together over some of the hills. These remains of annual plants 
show that these hills are probably covered with grass during the rainy season, and present a 
beautiful green surface. The stalks of the wild oat, that grows so luxuriantly on the hills of 
the Coast Mountains, were not observed here in any quantity. 
The distinctness with which the ravines and water-courses on the sides of these clay hills was 
presented was remarkable, and afforded a good opportunity to study the action of drainage 
water upon such materials. Every ravine or deep gully at the base of the highest hills 
could be traced upwards by the eye to its various diverging channels; these expanded into a 
thousand distinct branches, extending to the very summits of the hills, and resembling the out¬ 
spreading branches of a tree. The whole surface of the country was seen to be cut and divided 
in this manner by the main ravines, and their direction could be readily traced. It was very 
evident that all these hills were only remnants of a formerly continuous plain or gently sloping 
plateau, the surface of which was at a level with, or higher than, the tops of the hills now re¬ 
maining. All the valleys and ravines that now traverse the formation are evidently the result 
of the excavating and transporting power of water. It may, at first, be difficult for the ordinary 
observer to comprehend how such extensive wearing away and the removal of such enormous 
quantities of earth has been accomplished by the simple action of drainage water. Effects of 
much greater magnitude are, however, visible in all parts of the world upon rocks and forma- 
