A hard or raised surface for attachment seems favourable for stromatolite growth. Similarly, tree stumps submerged in a hypersaline lake of the Lower Cretaceous Purbeck Formation of Dorset, UK, have provided attachment areas on which stromatolites have formed. The stromatolite mounds shown here cover two stumps and a fallen trunk. Some concretionary carbonates in shale may be difficult to distinguish from primary sedimentary cycles of carbonate and clay. Sequences like this elsewhere may not always be correctly interpreted.
Detailed evidence comes from trace-fossils.
Primary gypsum nodules in a modern sabkha on the Mediterranean coast of Egypt. In the clayey sand a few centimentres below the surface there are these white nodules with sutures between them. These are probably Recent anhydrite nodules like those described in previous literature from the Abu Dhabi area of the Emirates. These are not crumbly and prone to distintegrate as is usually the case and very much resemble ancient nodules of anhdyrite or secondary gypsum. In some ancient sequences where there has been replacement of quartz from the outside and dissolution of the calcium sulphate these can form geodes .