geode - bubble-filling geode - large hand/display specimen which is paler

which is paler

The Allison Member of the Upper Cretaceous Menefee Formation is a cliff-forming yellow brown sandstone

but a decent example if you are explaining the sequence from peat to lignite

The marsh was left high and dry about a century ago when the creek was incised during a flood

an excellent plate

geode - bubble-filling geode - large hand/display specimen which is palerGeodes are bubble fillings that weather out of a vesicular lava. As the flow cools, unescaped gases form voids in the otherwise solid flow. Subsequent hydrothermal activity fills these voids with mineral rich hot water and as it cools, crystals grow on the surface of the roughly spherical opening. In the lava flow, the immediate zone around the now crystal lined bubble is altered and becomes more resistant than the surrounding rock and subsequently