Four Decades of Landscape Change on a Granite Dome (Enchanted Rock, Texas): A Photographic Field Work Analysis

Abstract

This study focuses on photographic analysis of Enchanted Rock, a granite dome in central Texas; fourteen photo pairs compare contrasting views after—thirty-six to forty-six years, and assess changes in geomorphology, sediments, and vegetation on miniature landforms, especially erosional depressions—gnammas and vernal pools. Several distinct landscape changes occurred during the study period, but these affected sites unevenly. Vegetation displayed noticeable differences in plant density and species, particularly a pronounced increase of Opuntia cacti and woody shrubs on several depressions; these changes, however, are not considered permanent, and are interpreted as representing periodic fluctuations due to climatic oscillations such as recurrent drought. Among geomorphic features, a rock pedestal and a tafoni panel—displaying friable, crumbling, material—are seemingly being rapidly obliterated by weathering; their low compressive strength (Schmidt hammer) R values support this idea. In contrast, a large, isolated, boulder near the dome base did not—as anticipated—display any obvious changes. Large stones and gravelly sediments, accumulated on vernal pools, or deposited along rill channels between depressions, showed the most conspicuous alterations. The observed changes are arranged along a presumed developmental sequence portraying subsequent geomorphic stages during pool development, which occur as detached stones gradually weather and eventually disintegrate; both coarse and fine sediments are transported downslope during intense, infrequent rainfall events. Pools, gnammas, rills, sediments, and vegetation function on Enchanted Rock as interconnected geographical units and components of an elaborate drainage network, steadily affecting each other over space and time.

本文对美国德克萨斯州中部的花岗岩穹隆“迷魂岩”进行了摄影分析。通过14组照片对, 比较了36到46年后的视图, 评估了微观地表在地貌、沉积和植被上的变化, 特别是溶蚀和春池形式的侵蚀洼地。在此期间发生了几个明显的景观变化, 但其影响在空间上有差异。植被的密度和物种有明显差异, 尤其是仙人掌和部分洼地的木本灌木有显著的增加。然而, 这些变化并非是永久性的, 而是气候振荡(例如, 反复干旱)导致的周期性波动。在地貌特征上, 风化迅速地清除了一个岩石底座和一块风化板(易碎和破碎物质), 其较低的抗压强度(施密特锤)R值支持了这一观点。穹隆基座附近的一块大孤石并未像预期的显示出任何明显变化。堆积在春池或沿洼地细沟的大石头和砾石沉积物, 有最明显的变化。变化沿着假定的发育阶段而排列, 表现出春池发育过程中的后续地貌阶段——开裂的岩石逐渐被风化并最终解体；在强烈而罕见的降雨事件中, 大小沉积物都向坡下输送。春池、溶蚀、细沟、沉积物和植被, 在“迷魂岩”上形成连续的地理单元, 成为复杂排水网络的组成部分, 并在时空上不断相互影响。

Este estudio está centrado en el análisis fotográfico de la Roca Encantada, un domo de granito del centro de Texas; en catorce pares de fotos se comparan vistas contrastadas –al cabo de treinta y seis a cuarenta y seis años, y permiten evaluar los cambios experimentados en geomorfología, sedimentos y la vegetación asociada con geoformas en miniatura, en especial las depresiones erosivas– gnammas y encharcamientos vernales. Durante el tiempo del estudio ocurrieron varios cambios notables del paisaje, aunque los lugares se vieron afectados desigualmente. La vegetación desplegó notables diferencias en la densidad de plantas y especies, algo particularmente pronunciado en el incrementos de los cactus Opuntia y los arbustos leñosos que crecen en varias depresiones; tales cambios, sin embargo, no se consideran permanentes y se interpretan como representativos de las fluctuaciones periódicas causadas por oscilaciones climáticas, como las sequías recurrentes. Entre los rasgos geomórficos, un pedestal rocoso y un panel de tafoni –que muestran contener material friable y deleznable– parecer estar en rápido proceso de obliteración por la meteorización; sus bajos valores R de resistencia a la compresión (martillo de Schmidt) apoyan esta idea. Por el contrario, un bloque rocoso grande y aislado cerca de la base del domo no desplegó –como se anticipaba– ningunos cambios obvios. Los peñascos grandes y los sedimentos de grava que se acumulan en los encharcamientos vernales, o los que se depositan a los largo de los canales de los arroyos entre las depresiones, mostraron las alteraciones más conspicuas. Los cambios observados se disponen a lo largo de una supuesta secuencia que retrata las etapas geomórficas subsiguientes durante el desarrollo de las charcas, lo que ocurre a medida que las rocas desprendidas poco a poco se meteorizan y eventualmente se desintegran; tanto los sedimentos finos como los gruesos son transportados cuesta abajo durante los poco frecuentes, aunque intensos, eventos lluviosos. Las charcas, gnammas, arroyos, sedimentos y vegetación funcionan en la Roca Encantada como unidades geográficas interconectadas y componentes de una elaborada red de avenamiento, afectándose mutuamente a través del espacio y el tiempo.

Key Words:

• gnamma
• photo-monitoring
• repeat photography
• rephotography
• vernal pool

关键词:

• 溶蚀
• 照片监控
• 重复摄影
• 重影
• 春池

Palabras clave:

• encharcamiento vernal
• gnamma
• fotografía de repetición
• foto-monitoreo
• refotografía

Acknowledgments

I dedicate this study to my loving mother, Josefina Sánchez Muñoz (1928–2023), who encouraged me, and eagerly helped examine hundreds of Enchanted Rock photographs. I am grateful to Billie Lee Turner (1925–2020), Professor Emeritus in Integrative Biology, and Emeritus Director of the University of Texas–Austin Herbarium—currently the Billie L. Turner Plant Resources Center—for inspiring me to pursue this project, for kindly sharing his deep knowledge about Texas, and for his outlandish repeat-photography stories in Africa sixty-eight years ago. I benefited from stimulating botanical discussions with Blanca León (Billie L. Turner Plant Resources Center). I thank David H. Riskind, Director of the Natural Resource Program, Texas Parks and Wildlife, Austin; and Sonny Solís, Park Superintendent, Enchanted Rock State Natural Area, for approving research permits (3–91). Mary L. Butterwick (Department of Botany, California Academy of Sciences, San Francisco, California) graciously allowed use of photographic material. I gratefully acknowledge Colleen Lyon (University of Texas Libraries), Peggy L. Gough (University of Texas Press), and Ariana Buchanan and Julia Wickham (LBJ School of Public Affairs, University of Texas) for their cooperation in obtaining permission to use published photographs. I had the unstinting field support and buoyant confidence of Martha Marie Kowalak-Pérez. Andrés S. Pérez-Bergquist and Alex G. Pérez-Bergquist also provided valuable field work assistance. Several hundreds of students accompanied me on field trips to the Rock. Professor L. B. Leopold (1915–2006) persuaded me, long ago, that all earth scientists should develop a lifelong project; thank you, Luna, I honestly didn’t realize this would turn out to be such extended venture.

Disclosure Statement

The author reports there are no competing interests to declare.

Additional information

Funding

This work was supported by the University of Texas Research Institute under SRG Grant-681, and by the Department of Geography and the Environment at University of Texas–Austin

Notes on contributors

Francisco Luis Pérez

FRANCISCO LUIS PÉREZ is Professor Emeritus and Soils Lab Director Emeritus in the Department of Geography and the Environment at the University of Texas at Austin, Austin, TX 78712. E-mail: [email protected]. He has worked for more than forty-five years on the geoecology, geomorphology, soils, and vegetation ecology of tropical mountains; his field work encompasses the high-altitude páramos of the equatorial Andes; the volcanoes of Hawaiʻi, the California Cascades, and the Canary Islands; as well as mountains in Texas, and sierras in southern Spain.