Calamitalean stems that are preserved in growth position in Carboniferous sedimentary deposits have been described many times in the literature as either pith casts or stem casts. Many of these in situ stems show branching, which gives some information on their patterns of growth. Their manner of preservation is discussed in the light of a new stand of well-preserved in situ stem and branch pith casts of Calamites discovered in mid to upper Duckmantian sandstone at Brymbo in the Wrexham Coalfield of North Wales. Analysis of a brown mineralized layer surrounding the casts and below the black compression remains of the stem tissues has shown the presence of goethite, muscovite, quartz and kaolin. Deposition of these minerals around the inside of the central stem cavity would have provided rigidity and sufficient support, while the pith cavity filled with sediments. The outer tissues would then have been compressed to form a thin coal layer around the mineralized infill of the pith cavity. Cross sections of stems were found clustered together in relatively small areas, and kernel density map and nearest neighbour analysis suggest that each small patch of these pith casts represents an individual plant spread by rhizomatous growth. Stems found in ironstone nodules are external casts of leafy stems preserved by the deposition of siderite on their surfaces. A length of rhizome found at Brymbo was similarly preserved as a cast in ironstone.