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.