The Lower Ordovician Prairie Du Chien Group is a mixture of carbonatesiliciclastic sediments, extensively observable over much of southeastern Minnesota. The Willow River Member is the youngest division of the Prairie Du Chien Group, deposited between 485-490 m.y. ago. In Fillmore Co., along highway 52, at the junction of the Root River, a quarter-mile exposure of Willow River Member reveals four distinct assemblages of
stromatolite (i.e. sediment binding cyano-bacteria) growth. The first three layers of occurrence are primarily: Low-relief, continuous, stratiform to psuedocolumnar laminations, ranging in height from 4-7 inches vertically. The fourth assemblage consists of non-linked, discontinuous, dome to bulbous biostromes ranging spatially
(in two dimensions) 8" x l'2" to 1'9" x 2'6". Available morphologies and characteristics of the trapped sediment within the stromatolites imply upper and lower intertidal depositional conditions.