We’ve learned a
great deal about how ancient Native Americans used Shell Mound as an
important pilgrimage site, but questions still remain. Among mysteries are the
motivations for erecting the huge mound, and functions that might have been
served by its crescent form and partially enclosed ground-level space.
Some possible answers have already been posed, for example the mound may have afforded protection from storm surges and fires. And it may have been a kind of garden for growing useful plants. Another plausible suggestion is that full time occupants of a village may have lived in the ground-level plaza during the cold winter months where they were protected from frigid winds. They would move in the warmer months to the crest of the mound where exposure to breezes provided a partial respite from heat, humidity, and biting insects. The opening on one side of the crescent may have provided access, obviating the need to scramble over piled up shells to reach the plaza.
This panoramic image made on November 19, 2017 shows most of the Shell Mound plaza, and the trees in the background conceal the greater part of the actual mound . The people are standing on the future trail that follows the inner perimeter of the mound. In this view the central area of the plaza appears to be depressed.
Maybe the following qualifies as rank speculation, but Shell Mound might have served a hydrologic function. Recall earlier discussions about the general scarcity of fresh water throughout the “blue desert.”
Shell Mound flooding on July 28, 2018. Flooding was much more extensive than indicated in this photo looking across the plaza, and several frogs were active in the water.
On July 28, 2018 Peg and I visited Shell Mound to photographically document sites planned for placement of informational panels along the trail. Our visit was not a success. In a few months the new section of the trail we had helped to lay out and clear had become overgrown by rank herbaceous vegetation. At times it was difficult to get a sense of whether we were actually on the trail. Biting insects were fierce as usual, and they kept us moving along. Rains had been unusually heavy and to our surprise, much of the amphitheater-like inner part of the complex—the plaza—was flooded, including major sections of the new trail. We avoided standing in the ankle-deep water to take photos, and even if we had done this, it would have been difficult to get an accurate perspective on placement of the panels.
Following are my notes made after we returned from the field:
The presence of apparently abundant frogs was perplexing. I wish I could have caught one to find out what species were present—from their behavior I guessed they were “pond frogs” (genus Lithobates, formerly Rana). And what could have brought them to Shell Mound? Amphibians are intolerant of saltwater and generally absent from sand dunes. So why are these frogs at the end of a dune extending a mile or more into the Gulf of Mexico—a place where standing fresh water is rare and ephemeral? It taxes the imagination to suppose that they all hopped the mile or more down from the other side of Dennis Creek in response to puddles forming in the past few weeks. Perhaps there is a permanent population here that persists in wet spots or burrows and explodes in numbers in the rare instances when heavy rains make conditions favorable by turning low spots into ponds. A permanent population of frogs at Shell Mound could be yet another artifact of the ecological changes brought about by its builders. It seems unlikely that ancient people would have intentionally brought frogs to the site, and that this frog population would have persisted for hundreds of years. Nevertheless, it seems clear that the ancients and their activities are no less responsible for the presence of frogs there today—without the people and the mound they built, freshwater ponds would not exist in this place.
On another visit, Winter Solstice, December 21, 2018, almost the entire plaza was flooded. Tides had not been extraordinarily high, and it seems likely that the floodwaters were fresh.
Flooding of the Shell Mound plaza on December 21, 2018. An informational panel placed along the trail can be seen in the upper left center of the image. Flooding
persisted so that in late April 2019, refuge staff built an elevated boardwalk to keep this section of the trail passable.
Speculation can run amok, and the possible persistence of a population of frogs rekindles earlier vague notions about a possible hydrological function for the mound. In addition to its other services, might the mound have been a built as mechanism for capturing and retaining fresh water? Could there be a freshwater spring under the Shell Mound plaza, and the ancient people built the mound to protect it from inundation by tidewater? Or might the plaza be a purpose-built relatively impermeable basin capable of collecting and storing rainwater? Could the mound be part of a hydraulic system that raises the water table, effectively storing groundwater and releasing it to the plaza?
A tree-dimensional LIDAR image of Shell Mound developed by Dr. Sassaman and his cooperators. LIDAR accurately measures land elevation by penetrating vegetation. In this view looking North-Northwest the partially enclosed basin-like configuration of the plaza is evident.
Still curious, I revisited Shell Mound on December 22, 2020, wondering if I could find any patches of hydrophytic vegetation on the plaza that might denote a permanent seep or pool. Or were any burrows in evidence that could provide refuge for frogs during the dry season. Unlike on nearly the same date two years earlier, this time there was no flooding; the floor of the plaza seemed quite dry, although moister than the adjacent areas of sand dune. Nor was there any evidence of wetland vegetation or signs of burrows of crayfish or other animals. Onsite observations did make it clear, however, that the plaza is not a flat area as originally believed but is clearly a basin. If designed for something other than catching and retaining water, it has the problem of being flood prone. More thoughts about the frogs led to a reconsideration of my earlier hasty guess that they were probably pond frogs (genus Lithobates). Perhaps more plausibly they were spadefoot toads (genus Scaphiopus), frogs that dig spiral burrows in loose soil, live most of their lives underground, and breed explosively in times of flood.
If, as seems likely, the frogs I observed were Scaphiopus, the possibility of a permanent supply of fresh water under the plaza is much diminished. Nevertheless, as suggested in my notes from 2018, frogs would almost certainly be absent from this place were it not for the ancient inhabitants and the structure they built,
The images above were made December 22, 2020 and they illustrate the basin-like configuration of the Shell Mound Plaza. The top image was made from the open end of the crescent looking toward the west and the central area of the mound. The middle image looks to the east, back toward the opening of the crescent. Note the green vegetation in the central area surrounded by higher drier ground surrounding it. The lower image was made from roughly the same place as the middle one and looks toward the southeast where the boardwalk permits passage over an area of occasional deep water.
We’ll probably never run out of questions, but getting some more answers could be relatively easy. A hydrologist with appropriate skills could answer some of the questions about the possible role of the basin. And a herpetologist could collect and identify the frogs and offer possible explanations of how they got there or are able to persist.
No comments:
Post a Comment