THE PUERCO AND TORREJON FORMATIONS OF THE 

 NACIMIENTO GROUP ^ 



JAMES H. GARDNER 



CONTENTS 

 Introduction 

 History of the Puerco 

 History of the Torrejon 

 Naming of the Nacimiento 

 Geology of the type-locaUty 

 Fossils 

 Correlations 

 Summary 

 Bibliography 



INTRODUCTION 



The formations of the Nacimiento group are subjects of much 

 interest to science because of the character of their vertebrate faunas 

 and the positions they occupy in the time-scale of geologic history. 

 The fossil mammals of the two formations have been carefully dis- 

 cussed by eminent paleontologists and yet but little is known of their 

 areal distribution or stratigraphic occurrence. This paper is accom- 

 panied by the first contribution of detailed geologic mapping in the 

 area of their type-localities, and is the result of research which has 

 brought forward some important facts and thrown considerable 

 light on the problem of their faunal and stratigraphic relationships. 



The Nacimiento group was deposited during that long period of 

 fresh- water conditions which prevailed over the greater part of western 

 North America at the ending of the Cretaceous and the beginning of 

 the Tertiary periods. In recent years paleontologists have considered 

 the group as being earhest Tertiary in age, and thus marking the 

 beginning of the Eocene series. 



It is intended in the following pages to review the formations of 

 this group, their correlations, etc., from the first discoveries to the 

 present time and to set forth clearly the facts of their stratigraphic 



' Published by permission of the Director of the United States Geological Survey. 



702 



