accretions bio-chemical-accretionary-structures cementation columnar cumulative-structures excretions 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 ancient appearance backgrounds botanic botany cycad Cycadophyta cycads durable evergreen feather-shaped firm-fronds frond garden green gymnosperms hardy nature palm-like pinnate plant prehistoric primitive resistant shade shaded temperate textures undergrowth