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Show 160 cafion of the Big Tnhnnja or Tuj unga. Towards the point of contact with the granite and the crystalline metaphorio rocks the Miocene Tertiary strata are much raised and contorted; the direction of their dip is easterly, opposite to that in which the Sierra Madre runs. We follow these mountains or Tertiary hills which form the counterforts of the Sierra Madre, from which they are separated hy a kind of large ditch all along the eastern and northeastern extremities of the San Fernando Valley as far as the entrance of the Pacoima or Pacofia Cafion. There they end abruptly; erosions and denudations have destroyed them for a distance of 2 miles along tfie foot of the Sierra Madre^ which here hears the name of the Sierra Pacofia or Pacoma; afterward these same Miocene strata are again met with before one reaches the Grapevine Cafion. SIERRA MADRE, PACOSA OR PACOIMA CASON. The sides of the Pacoima Cafion are perpendicular, and it runs through gray granite rocks, gneissoid in some parts, with serpentinous metamorphio rooks and crystalline limestone at the entrance of the cafion. These rocks are the same throughout the length of the Sierra Madre chain, from the Cajon Pass Cafion, where I observed tbem in the spring of 1854, to the San Gabriel Cafion, in the Big and Little **" Tuj unga Cafions, at La Soledad, or £ Williamson's Pass, and at the San • Francisquito Pass. This gray gran- £ ite, composed of fine particles,, is " § the same granite as that of the J, Sierra Nevada, of which the Sierra tf 4 Madre is a prolonged spar in the « 3 shape of a bayonet- Tike fault. This £ § fault, or rather this bayonet- like • g* rupture, of the primordial crystal- • » • § line rocks, occurs between Tehach-fig ipi Pass and the Caflada de las ^ g Uvas or Cafion of FortTejoh. The * S* Pacoima Cafion. 4 miles to the east « - - - - - ' - • • • » 19 of the San Fernando mission, 19 w narrower, deeper, and more zigzag * g » than the San Gabriel and Tuj unga g R Cafions. Four or five miles from ^ its issue in the San Fernando Plain » jf it becomes quite impassable, in S g consequence of deep, perpendicular g a cataracts which bar it entirely. tf. 3 I &* ve below a drawing of the 2 geological section to the left of the *£* arroyo, at the place where the jg ^ Pacoima Cafion issues into the San *=•£ Fernando Valley. « c The crystalline metamorphio €£ rocks are from 100 to 120 feet in £ 1 thickness; they form the entrance § c to the cafion, and are covered half- "^ j, way up by a very heavy Qauter- • 2 nary drift The arroyo of Pacoima a has carried away much of this S drift, which is very coarse, with * j rounded and quite heavy blocks; J it may be examined all along the < M banks of the arroyo in clins or « bluffs which have an elevation of ij 60 feet, quite perpendicular to the g bed of the arroyo. This Quaternary 1 drift evidently indicates a very ° 5 large cone of dejection9 at the issue 2 of a cafion which existed here dnr- " ing the Quaternary period. This cone is very well marked, more to the northwest, at a distance of a mile from the * See torrents of the Upper Alps, by Surr ell, page 11; " Torrents de3 Hautes Alpes, par Surrell, page 11.* |
