THE MONTHLY BULLETIN. 135 



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stony ridges with sides nearly preeipitoiis. That part of the foothills 

 along the San Joatpiin A'alley, sontli of Mariposa County, rises rather 

 abruptly and soon becomes too rough and rocky for agricultural pur- 

 poses, except in a few isolated deeply eroded coves and valleys of small 

 extent, widely separated and far removed from transportation. The 

 steeply eroded surface features of this part of the Sierras are due to 

 a greater uniformity in the rocks forming them, while to the north 

 the great variety of rocks weathers irregularly and gives rise to a more 

 subdued and gently sloping landscape. The ft)otliills along the northern 

 two thirds of the great interior valley rise gradually and culminate in 

 the crest of the Sierra range about 40 to 50 miles distant. Another very 

 important feature of the topography of the entire foothill belt which 

 has. perhaps, one of the greatest influences on the type of agriculture 

 possible, is the system of deep, narrow, erosional valleys with their 

 accompanying intervening ridges and divides, which parallel the general 

 ti'cnd of the mountain range, as c(mtrasted with the numerous canyons 

 and narrow valleys which lead directly from the mountain tops to the 

 valleys below in most other regions. 



CLIMATE. 



The climate of this belt of territory is (piite like that of the great 

 valley below. There are two seasons, a wet and a dry, the rainy period 

 extending from about the first of November until April. The summers 

 are hot and dry. with cool nights generally, and the winters are mild 

 aiul pleasant. Fog seldom occurs except for short periods in that part 

 of the belt bordering the valley tloor. The average annual rainfall 

 increases gradually from about five inches in the lower foothills margin 

 at the southern end of the San Joaquin northward to Redding, where 

 it is about 26 inches. It also increases with altitude, the amount being 

 al)Out eight tenths of an inch for each 100 feet increase in elevation 

 to the 6,000 foot contour, where the increase slowly declines as the 

 summit is approached. From the above it will be noted that the 

 type of horticulture best suited to the belt will be governed largely by 

 climatic features. 



Between the elevations of 200 and 1,200 feet, along the entire foothills 

 belt, is a remarkable thermal belt which permits of the growing of 

 citrus fruits Avith equal facility from Tehachapi in the south, to Red- 

 ding, nearly 400 miles farther north; and still more striking is the 

 fact that oranges from the entire belt ripen several w^eks earlier than 

 in Southern California. This phenomenon appears to be due to the 

 great bowl formed by the interior valley, protected by surrounding 

 mountains, moderated by the warm ocean currents washing the coast 

 and protected from the descent of cold air currents from the snow 

 fields by the series of narrow valleys and ridges paralleling the course 

 of the Sierra range, which cause a slow filtering and warming of cold 

 air before it reaches the lower foothills, many miles below. These great 

 natural features act as barriers and have such an intiuence on the 

 climate of the belt that damage from freezes is rarely known. For 

 success with citrus fruits, however, fiat or slightly depressed areas 

 should be avoided on account of restricted soil and air drainage. 



