THE Simi is essentially a hidden valley. Unseen on approach, because of its enclosing hills and mountains, it suddenly stretches before the eye only when its edge is attained. From the San Fernando valley and Los Angeles plain, to the east and south-east, a main paved highway twists over the Santa Susana pass and descends abruptly to the eastern end of the Simi valley floor. From the west, from the direction of Moorpark and Ventura, a similar route, traversing a low pass, enters the western edge of the Simi valley through the narrows of the Arroyo Simi. No routes enter the Simi except at the eastern and western ends. The northern and southern hills and mountains have, so far, effectively discouraged transverse connections with other valleys on either side (Fig. i). From any vantage point on the surrounding hills the geographic essentials of the Simi are readily apparent. A broad, elongated valley floor, giving way sharply to encroaching hills, is gridded with numerous road lines, most of which are marked by the dark green walls of the parallel tree windbreaks. Partially hidden by these tail trees, the rural and semi-rural dwellings of the valley hug the roads' edges. Large and small land rectangles, outlined by the roads and the tree walls, are given over to a variety of tree crops, truck crops, and grain hays. The flattish valley floor, which is the Simi in a terminological sense, represents the pronounced aggradation of the lower portion of an asymmetrical syncline whose axis trends almost east and west (Fig. 2). The plain consists largely of materials derived from the shales, sandstones, and conglomerates of the northern hills.I More extensive drainage basins to the north, coupled with more easily eroded formations, have supplied the intermittent streams of that section with more abundant materials. As a result, the occasionally debouching waters of the northern hill zone have built extensive fans far south of the northern hill edge. The fans have coalesced laterally to form a continuous alluvial piedmont of very low gradient. The excess deposition from the north has, in general, pushed the Arroyo Simi, marking the thalweg, close against the southern margin of the valley floor. In the south-western portion however both recent and older alluvial fill have pushed the longitudinal drainage line toward the north, so that the Arroyo Simi finally turns northwesterly to leave the valley by way of the narrows. The slope of the valley from north to south and from east to west is so slight as to be almost imperceptible. This apparently flat floor has few landform interruptions (Fig. 3). In the south-eastern portion are several rocky outliers, representing unburied sandstone knobs, which belong geologically to the massive Chico sandstone of the Simi Hills. North-west of these prominent sandstone exposures, where the valley floor begins to assume its greatest