angle angles appearance bark colored coloring colors coloured colouring colours crack cracked cracks cut cuts droplets excrete excretions exposed grooved grooves lopped orange perspectives protective reaction response rough sap sawn secrete secretions sticky Stump surface surfaces texture textures timber tree tree-stump trunk uneven wood wooden accretions bio-chemical-accretionary-structures cementation columnar cumulative-structures formations formed forming fossils high-saline-levels lake layered living-fossils living-rocks microbialites microorganisms natural park rock-structure sedimentary-grains shallow-water shaped shapes slow-growth stromatolite stromatoliths structures anatomy biology cells filtering health human kidney magnification microscope science urine