During Ordovician-Silurian time, the southwestern Great Basin was the site of a broad, shallow sea in which the Ely Springs Dolomite was deposited. In the northern Inyo Mountains of eastern California (Mazourka Canyon), the Ely Springs consists of three units and ranges in thickness from 55 m to 161 m. Unit 1 was deposited during Late Ordovician time (Champlanian to Cincinnatian); unit 2 during Early Silurian time (early Llandoverian); and unit 3 during Early Silurian (late Llandoverian) through late Early Silurian (Wenlockian) time. Paleogeography of the area was determined by measuring and studying rocks in four sections and using conodont biostratigraphy for time correlations of the depositional environments. Unit 1 (Late Ordovician) of the formation represents restricted subtidal and lower intertidal environments, with a definite shallowing trend that began close to the Ordovician-Silurian boundary. The shallowing trend culminated in the supratidal deposits evident in the rocks of unit 2 (early Llandoverian). The environments of unit 3 (late Llandoverian-Wenlockian) began to deepen again and became subtidal. The alternating paleoenvironments resulted from a combination of uplift and sea level fluctuations due in part to Late Ordovician-Mid Silurian glaciation. Absence of fossilized in situ organisms and the preponderance of laminated, layered and stromatolitic rocks is strong evidence for a restricted environment. The rocks also are dark, have a strong fetid odor and contain cubic pyrite, which suggests reducing conditions. Fossil debris of normal saline organisms found in lenses of subtidal rocks is interpreted as evidence for current action, perhaps generated by storms. Local uplift that developed during deposition of the Ely Springs produced the northward thinning of the formation and a restricted, reducing lagoonal microenvironment. As a land mass was forming to the north of the field area, a basin was forming in the south. Diagenesis played an important role in altering original characteristics of the Ely Springs Dolomite. A 13-m-thick bed of chert breccia, representing unit 1 in section ESA, was created by pressure solution (stylolitization) caused by Mesozoic tectonic stresses. Dolomitization and silicification occurred together as replacement products of limestone, shortly after deposition. Stylolites were produced as overburden and tectonic stresses became significant. Low-grade metamorphism (talc), fractures and calcite veins most likely occurred during the Mesozoic, a time of major tectonic activity and plutonic intrusions.