Terrace Heights is situated over various layers of geologic material. Beneath the modern soil lays the dark, hard, iron-rich substrate called the Columbia River Basalts. The Columbia River Basalt formed when lava poured out of fissures in the ground across eastern Washington during the Miocene (17-20 million years ago), erupting intermittently for over 10 million years. Many layers of basalt successively flowed over one another, back when the area was still flat. Subsequently, the Cascade Mountains actively erupted, depositing ash, cinders, pumice, and mudflows that eventually inter-fingered with the alternating basalt layers. Streams carried this light-colored, heterogeneous volcanic material , now called the Ellensburg Formation, towards the eastern lowlands, depositing thick layers over the basalt.

The Yakima River flowed over the basalt surface as tectonic forces caused enough steady north-south pressure to fold the basalt like an accordion from Toppenish to Ellensburg, forming ridges and valleys. Yakima Ridge, is part of the long, parallel ridges of the Yakima Fold Belt. Folding and uplift occurred so slowly (approximately 6 – 12 inches per 1000 years) that the Yakima River wore through the resistant basalt where it flowed, cutting through the end of Yakima Ridge to form Selah Gap.

As the ridge rose and river cut down, the Yakima River deposited a flat layer of cobbles, gravels, pebbles, and silts onto its floodplain, which eventually rose in elevation due to uplift, out of reach of the river. The resulting landform is called a terrace, an old floodplain perched at a higher elevation in relation to the modern floodplain. Terrace Heights is named appropriately, as successive river terraces comprise the flat, bench-like upland areas occupied presently by homes and orchards.

Another interesting geologic deposit in the Terrace Heights and Yakima area resulted from repetitive catastrophic flooding across eastern Washington. During the Pleistocene (10,000-18,000 years ago), a large glacial meltwater lake named Lake Missoula, became impounded behind an ice-dam. The dam broke, releasing tremendous amounts of water, up to 300 feet deep, scouring the landscape of eastern Washington and eventually draining to the Pacific Ocean. The ice dam broke repeatedly throughout the Pleistocene (up to 89 times!), causing the same effect. At Wallula Gap, south of the Tri-Cities, the constricted topography trapped the flooding water, allowing it to back up into the Yakima area where sediments settled onto the hillsides, terraces, and valleys.

More recently, during the late Pleistocene (10,000 years ago), the continental and alpine glaciers melted back which released large amounts of water and trapped sediment. Windblown glacial dust, called loess, was deposited in a thick layer across eastern Washington. Loess comprises the primary component of the rich, silt-loam soils of Terrace Heights.

The climate of Terrace Heights corresponds to that of Yakima, shielded from easterly Arctic winds by the Rockies and protected from fronts to the west by the Cascades. Average winter temperatures average 32° F. and summer temperatures average 68° F. A large difference exists in the daily temperature range because of the inland location, isolated from the temperature-moderating, moist marine influence. In the rain shadow of the Cascade Mountains, Terrace Heights climate is dry and classified as semi-arid with only 7-8 inches of rainfall per year. The majority of precipitation falls as snow in the winter, averaging 25 inches per year. Only 30% of the precipitation falls between April and September. Winds blow predominately from the west and northwest, averaging 8 mph.

A combination of time, geologic material, and climate led to the formation of Terrace Heights’ soils. In semi-arid climates such as ours, soils form extremely slowly , are very fragile, and can naturally support only certain species. Like most of eastern Washington, the natural vegetation classification of Terrace Heights is shrub-steppe, which includes many varieties of sagebrush, grasses, and forbes. Where springs emerge and stream channels dissect the ridge, lush riparian vegetation grows. Riparian species include cottonwood, willow, alder, and elderberry. Terrace Heights is now a lush, green landscape thanks to irrigation from the Yakima River and wells.

Prior to the arrival of white settlers in the mid-1800s, the land was used by native Americans for hunting, gathering, fishing, and camping. Settlers began irrigating shortly after their arrival, transforming the landscape to homesteads, ranches, gardens, and orchards. The well-drained loess-covered terrace soils needed only water to become productive, so extensive irrigation networks were constructed along the terraced contours of Terrace Heights. For many years Terrace Heights was a rural agricultural area. Over time, changing economies transformed the landscape, as failing orchards were uprooted and replaced with residential sites. Economic development is presently booming in the area which will further transform the landscape with new businesses, homes, infrastructure, roads, and an increasing population.

Lenfesty, Charles D. and Thomas F. Reedy. 1985. Soil Survey of Yakima County Area, Washington. Soil Conservation Service.

McCulloch. Mac. 1990. Gateway to Time; Mile by Mile Guide to the Yakima Canyon. Shields Printing; Yakima, WA.

Thanks to Billie Chamberlain for background info regarding the changing landscape of Terrace Heights.