ABSTRACT

The southern end of Laguna Madre (Texas) north of the Rio Grande mouth, is marked by very shallow water, wide tidal flats, lunettes, islands made of beach ridges and lesser features. The number and variety of islands in the lagoon is remarkable.

The lunettes ("clay dunes") are made primarily of quartz sand and coarse silt. They are commonly 15-35 feet (5-10 meters) high, irregular is shape and steep-sided. They were deposited from wide transport, and did not migrate. Those that are islands in the lagoon predate present position of sea level.

Islands made of beach ridges were built from the lagoon side. Photo analysis, field work and granulometry all show that this sand was not moved into these ridges by Gulf of Mexico waves. Trenches in 12 beach ridges showed horizontal bedding but neither low-angle nor steep cross-bedding (hence quite unlike swash-built beach ridges). The ridges were built by wind-tide lag effects, not from the swash. Therefore these beach ridges are a new type, in addition to swash-built, eolian, and storm-surge ridges.

Growth of the ridges appears to be over.