Markes E. Johnson - Discovering the Geology of Baja California: Six Hikes on the Southern Gulf Coast

This book tells the geological story of a place called Punta Chivato on the gulf coast of Baja California Sur, Mexico, through six nature hikes. Here is found a remarkable landscape, which I revisit as often as possible. Although I have carried out research elsewhere around the world and have traveled widely through the rest of the Baja California peninsula, this spot holds a powerful attraction for me. Each time I return, I take away a richer understanding of the complex web of relationships in nature, particularly their linkages to the distant past.

The Punta Chivato region covers approximately 9.5 square miles (25 km2), jutting like an elbow into the Gulf of California above the entrance to scenic Baha Concepcin (). A growing number of homes cluster around the Hotel Punta Chivato on the southeast corner of the peninsula. An airstrip for small planes serves the area, but it is also accessible by road from Palo Verde off Mexican Federal Highway 1. The principal town, Santa Rosala, lies 28 miles (45 km) to the northwest, and the village of Muleg, 12.5 miles (20 km) south, as the crow flies. From January to April, the local climate is welcomingespecially for anyone eager to escape the rigors of a northern winter, as I am. Punta Chivato sits just above 27 north latitude, similar to Sarasota, Florida.

The region includes three small islands, the Islas Santa Ins, one of which is the site of a sea lion colony. Four distinctive mesas, three of which bear formal map names, dominate the mainlands topography. They are more ridgelike than table-topped. These features rise in elevation from 260 to more than 330 feet (80100 m) above present sea level. The largest and most commanding is the broad Punta Chivato promontory, which covers approximately 2.5 square miles (7 km2) and affords sweeping views of the surrounding gulf. The four mesas and the Islas Santa Ins formed an archipelago in the nascent Gulf of California about 5 million years ago, during the early Pliocene Epoch. In response to regional tectonic forces, changes in global sea level, and the ceaseless effects of erosion by wind, waves, and rain, the islands changed their size and shape through time. Gradually they foundered below the gulf waters, eventually to rise again and join, in part, with the peninsular mainland. The story of these ancient islands, rich in marine life, makes up only part of the regions geological history. The origin of the Gulf of California itself is shrouded in the older basement rocks of the Pliocene Santa Ins archipelago and the surrounding landscape.

Because Baja California was and is a popular adventure destination, the peninsula has been vividly described by a wide range of visitorsgeneric vagabonds, novelists, teachers, and naturalists (somewhat like myself), as well as people who achieved extraordinary feats of long-distance hiking and kayakingand lived to tell their story. None of the popular accounts of life, nature, and travel in Baja California, however, has been written from the perspective of a geologist. My viewpoint is informed by geomorphology and historical geology. Geomorphology is the study of landforms and the processes of erosion responsible for their sculpture. Historical geology is a discipline that seeks to order the physical and biological changes on planet Earth through time.

Map 1. Baja California Sur.

Fossils and the layered sedimentary rocks in which they are preservedprovide many of the clues that bring meaning to the pages of this all-encompassing and still unfinished epic. Our planet has a tangible geological history spanning more than 4 billion years, but the gulf coast of the Baja California peninsula is young by comparison. Its geological history stretches back less than 80 million years to the Cretaceous Period, at best. Indeed, rocks younger than 15 million years, beginning with the later Miocene Epoch, tell most of the relevant story of the Gulf of California.

Geomorphology seeks rational explanations to show why a particular valley is located where it is, for example, or why a certain hilltop stands up in bold relief. The landscape of a given district is dependent entirely on its underlying bedrock. Some rocks are harder, denser, or better cemented than others and thus more resistant to erosion by wind and running water. A geomorphologist must be familiar with the full range of possible rock types: sedimentary, igneous, and metamorphic. Baja California provides fertile ground for consideration because, although it is still a comparatively young landscape, it exposes a profusion of rock types to a desert climate subject to occasional rainstorms and regularly shifting wind patterns. Though often tranquil, the gulf coast can face sudden winter winds that feed vigorous and erosive waves. In short, Baja California presents a veritable marketplace of delights in historical geology and geomorphology, because the sparse vegetation and thin soil cover cannot obscure the goods on display. Knowledge derived from paleontology, stratigraphy, and volcanology can help us appreciate and understand this unique gulf coast landscape and its former shorelines.