Description of Map Units

Nonglacial deposits Artificial fill (Holocene)—Mud, sand, and gravel of varying proportions, possibly including foreign debris such as concrete, logs, timbers, or brick. Used for highway roadbeds and other construction. Thickness generally greater than 2 m. Mapped where fill substantially obscures or has altered original geologic deposit. Beach deposits (Holocene)—Soft sand, silt, and mud deposited or reworked by wave or tidal action at head of Port Discovery. Contains logs and timbers at or above high tide. Deposits lie within 2 m of high tide. Alluvial fan deposits (Holocene)—Boulders, cobbles, and soft sand deposited in lobate form where streams emerge from confining valleys and reduced gradients cause sediment loads to be deposited. Alluvium (Holocene)—Moderately sorted soft deposits of cobble gravel, pebbly sand, and sandy silt along the floodplain of Snow and Salmon Creeks south of Port Discovery, and along Snow Creek in the northwest corner of the map. Gradational with and including sediment equivalent to unit Qb Wetland deposits (Holocene and late Pleistocene)—Peat, marsh, and bog deposits, with some intermixed sand, silt, and clay. Unit is very soft, water saturated, and occupies shallow depressions in till (Qvt) or low-lying areas of recessional outwash sands (Qvrs). Landslide deposits (Holocene and late Pleistocene)—Diamicton of angular clasts of bedrock and surficial deposits derived from upslope and commonly includes trees. Includes areas of irregular, hummocky topography. Poorly consolidated. Landslide four kilometers south of Port Discovery consists of house-sized blocks of Lyre Formation conglomerate. Glacial deposits DEPOSITS OF VASHON STADE OF FRASER GLACIATION OF ARMSTRONG AND OTHERS (1965) (PLEISTOCENE)—Consists of: Sand-dominated recessional outwash deposits (late Pleistocene)—Stratified unconsolidated sand-dominated sand and gravel deposits. Occupies lower parts of lower Snow Creek-Crocker Lake-Leland Lake valley. May include interbeds of gravel-dominated recessional outwash deposits (Qvrg) Gravel-dominated recessional outwash deposits (late Pleistocene)—Stratified unconsolidated gravel-dominated sand and gravel deposits. Occupies lower margins of lower Snow Creek-Crocker Lake-Leland Lake valley, but lies higher than sand-dominated recessional outwash deposits (Qvrs). May include interbeds of sand-dominated recessional outwash deposits (Qvrs) Fine-grained recessional glaciomarine or glaciolacustrine deposits (late Pleistocene)—Stratified firm fine silt with rounded dropstones of pebbles and cobbles along Highway 20, southeast of Port Discovery. Coarse silt and sand are interbedded. Beds are greater than 3-10 cm thick. Some contacts between beds are convoluted, and sand dikes and blobs are present and cross bedding. Overlies probable till Till (late Pleistocene)—Compact and firm light to dark gray non-stratified diamict containing subangular to well-rounded clasts, glacially transported and …