志留系小溪组（S3+4x）命名剖面来源于张家界市桑植县瑞塔铺，地层形成于志留纪晚期（4.282亿年前），属前滨沙滩至河口湾沉积相。岩性以灰绿、黄绿色厚中层状砂岩为主，夹少量灰绿色砂质泥（页）岩，富含大型虫管遗迹化石，层内斜层理、交错层理发育。 Silurian System Xiaoxiyu（S3+4x） Formation is named after ruitapu,  Zhangjiajie City. The rock was formed 428.2 million years ago from the sedimentation at foreshore beach to estuary. The strata are mainly of grayish green and yellow-green medium-thick layered sandstone, with a little grayish green sandy mudshale containing ichthyolite (fish fossils). It is characterized by abundant sandy wormtrail tube structures as well as developed oblique bedding and cross bedding.