Pleistocene lake and stream deposits of southeast Washington and the adjacent portion of Idaho are cut by innumerable clastic dikes of an unusual kind. Dikes are most abundant in the Touchet beds and scabland deposits, a few are present in the earlier Clarkston deposits, and rarely, they enter the Columbia River basalt. The processes of formation were long-continued and concurrent with proglacial deposition. An analysis of the sedimentary environment indicates that ice played an important rôle in deposition of dike-bearing deposits.

Many dikes occur in compound units of several or many individuals lying parallel in the same major fissure; they were formed by repeated filling of growing fissures. Fissures were filled from above by one or more of at least four processes; most of the sediment was swept into fissures by streams, lake currents, and waves, some collapsed from fissure walls or poured in from unconsolidated surficial deposits, a small amount was carried by underground currents, and films of clay and silt were spread upon fissure walls in several ways. Wind-borne sediments are indicated but not proven. Two or more processes generally operated together or alternately to fill each fissure; dike material, ranging from clay to fine gravel, may be sorted or unsorted, stratified or unstratified, and the stratification may be horizontal, inclined, or vertical. Any or all possible combinations of the variables may be present even in individual dikes.

Five processes of fissure development are recognized: (1) uneven settling and cracking through melting of buried ice, (2) gravity sliding and faulting on inclined zones of subsurface melting, (3) formation of cavities where ice blocks and layers melted, (4) erosion by underground streams, and (5) faulting and fissuring by landslides in the Columbia River basalts.

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