Geologic Material
	Water flow in the subsurface is controlled by properties of the geologic material. For hydrologic purposes, we will group geologic materials into 3 main groups:
		1. Sediment, soil, alluvium - these are all terms for loose, unconsolidated pieces of minerals and rocks. Sediment is defined by its size:
			Gravels are solid particles ranging in size from a pea to larger than a house
			Sands are solid particles ranging in diameter from 0.075 mm (powdered sugar) to 4.75 mm.
			Silt ranges in diameter from 2 to 62 microns
			Clay is less than 2 microns in diameter
		2. Sedimentary rocks are formed by the lithification of sediment. Sediments are buried and undergo compaction and cementation (minerals form in the open spaces between sediments to cement them together).
			Sandstone is produced by the lithification of sand
			Conglomerates are produced by the lithification of gravels
			Shale is produced by the lithification of clays
		3. Crystalline Rock - rocks with densely packed crystals. Includes plutonic igneous rocks (granite, gabbro) and metamorphic rocks (schist, slate, gneiss)