Lagoon Edge

Lagoon Edge

The edge of the Muyil lagoon

The hypothesis: the shoreline at Muyil is moving eastward

Originally I hypothesized that the upper sections of the sacbe (1, 2, and 3) dated to the Classic, and that they formed a causeway to the edge of the karstic shelf; that waters of the Muyil lagoon lapped at the foot of the karstic shelf just a few meters away; that during the Postclassic, the lagoon margin slowly silted up (or the water level fell) and that this required the construction of sacbe sections 5 and 6 to cross the mud or swampy edge to the new lagoon edge farther to the east. I also guessed that since the abandonment of Muyil at the time of the Spanish conquest, the lagoon margin had continued its retreat eastward to its present location about 140 m east of Structure 12H-1, the easternmost structure at Muyil. Although the Castillo and Sacbe 1 are Early Postclassic, the hypothesis of lagoon retreat remains viable.

The natural processes at work

In conversations in 1992 with William Ward, of the Department of Geology at the University of New Orleans, co-author of a major geological research report about Yucatan (Weide 1985), the mechanisms for the retreat of the lagoon margin to the east became clearer. First, sea levels (to which the fresh water table at Muyil is closely tied) have been slowly rising in the last two thousand years. This is the opposite of what would be required for a retreat of the lagoon margin. Also, there has been no significant uplifting of the terrain in this span. What does occur, and over the proper periods of time, however, is that vegetation (mangrove) is encouraging slow soil formation at the western edge of the fresh water lagoons along the karstic shelf. Ward has observed this soil formation near Cancun and elsewhere along the east coast of Quintana Roo. In our discussion, he described a complex soil-formation mechanism in which microorganisms in the fresh water table participate in the precipitation of limestone particulates as the water flows from the edge of the karstic shelf. Additionally, dissolved carbon dioxide gas is being released from the fresh water at that point, and may be participating in chemical reactions at the edge of the karstic shelf. Ward believes X-ray analysis of these precipitates in the future may shed further light on the details of these processes. We extracted core samples along the shoreline of the Muyil lagoon, and to the east of Structure 10H-1 near the base of the edge of the karstic shelf. The samples at the lagoon shoreline produced only soft fine-grained limestone mud. Similar core samples to bedrock taken near test pit 29 along a line from the slope of the karstic shelf down to the soil at water level first produced a thin 3-15-cm thick cap of humus, and then an underlying yellow-white limestone mud indistinguishable from that at the lagoon edge 500 m to the east.

The Maya response to shoreline retreat

Because these soil formation processes have been documented by Ward elsewhere on the coast, and are indicated by our core samples, I am disposed to accept the propositions that the shoreline of the lagoon was much closer to the site in Classic times and that the silting-up of the western edge of the lagoon due to soil formation processes active there caused an eastward retreat of the shoreline. As a consequence, the Late Postclassic Maya built a causeway (Sacbes 5 and 6) across the soft and often flooded limestone mud to the later shoreline. I also deduce that the present shoreline shows the amount of additional soil formed since the site was abandoned. The 140-m difference in the shoreline (between Late Postclassic Structure 12H-1 and the location of the shoreline today) over a span of about 500 years calculates to a rate of shoreline retreat of 28 cm per year (28 m per century).