The Holocene delta of the Guadalupe River is slowly prograding into San Antonio Bay of the Texas Gulf Coast. The delta is part of a modern complex of lagoonal and deltaic sediments deposited along a shoreline dominated by barrier islands. The river deposits its load in a shallow, relatively quiet body of water, and, as the delta progrades into increasingly shallow water, distributary channels have become deeper than the present bay floor. The primary purpose of the detailed study of this modern delta is to establish criteria to aid investigations of similar ancient deltaic sediments and environments where the geologic record is much less complete. Distinctive sediments are recognized from the following six environments of the delta : distributary channel, natural levee, marsh, interdistributary bay, delta front, and prodelta. Sediments are clay and silty clay except for the basal part of the distributary-channel and delta-front deposits, which are fine-grained sand, silty sand, or silt. Silty clays of the natural levee and marsh contain abundant plant material, some thin silt beds, and they are root disrupted, whereas the silty clays of the interdistributary bay are shelly and burrow mottled compared to the shelly but laminated prodelta silty clays. The sands of the delta front develop a geometry which reflects the shoreline configuration. Where the delta is birdfoot, the delta-front deposits are shoestring (bar-finger) sands. The overall configuration of the Guadalupe delta is birdfoot, although occasional small lobate masses develop where the delta has prograded toward the shallower bay margins. Lobate parts of the delta exhibit closely spaced distributaries, coalescing distributary-mouth bars, and sheet sands. The low-salinity fauna is restricted to sediments of the distributary channel, interdistributary bay, delta front and prodelta. No species of the fauna is confined to a particular environment of deposition. Subdivision of the delta into biofacies is based on the diversity and relative abundance of the fauna, mainly Mollusca. Mollusca are most abundant in the burrow-mottled delta-front sands and occur in decreasing amounts respectively in the inter-distributary-bay, small-scale crossbedded delta-front, prodelta, and delta-plain sediments. Ostracods represent a proportionately greater amount of the fauna in the interdistributary-bay sediments. Plant fibers and wood fragments are common in all facies of the delta. The rate of progradation also varies in response to the depth of water. Crossbedded delta-front deposits and laminated prodelta deposits probably indicate a high rate of progradation. Burrow-mottled delta-front deposits underlain by thin prodelta deposits probably indicate a low rate of progradation. The relatively stable tectonic setting compared to the Mississippi delta apparently accounts for the slight overlap of the subdeltas. Marsh overlying natural levee deposits suggests about 1 foot of subsidence of the delta. Subsidence is attributed to compaction of sediments. Characteristics which make the Guadalupe delta a distinctive model are: (1) the river discharge and load overwhelm the weaker currents and waves of the bay, (2) deposition is in such a shallow body of water that waves can rework the bay sediments, (3) the river channels are deeper than the present bay floor, (4) the bay is becoming progressively shallower as the delta progrades, (5) tectonic subsidence is less in the Guadalupe Delta area than in the area where the Mississippi, Brazos, and Rio Grande Rivers are located, and (6) growth is by development of successive subdeltas, most of which are presently deteriorating. Factors that control sedimentation in the Guadalupe delta area should apply to other lagoonal deltaic complexes, to crevasse systems of large deltas, to rivers that deposit their loads in shallow inland seas, and to their ancient counterparts. Upper Cretaceous rocks of the Rocky Mountain area and Pennsylvanian rocks of northern West Virginia have characteristics which suggest conditions of sedimentation similar to the Guadalupe delta. This volume was based on a symposium, Deltaic Sedimentation, which was held at the AAPG/SEPM Annual Meeting in New Orleans, Louisiana on April 1965. Many geologists have become involved in studies of deltaic sediments and sedimentation processes. Some of the papers in this volume are based on detailed local studies of modern deltaic sedimentary sequences, on processes of deposition, and on physical and biological characteristics of the deltaic environments.