Tidal flats are land areas that are covered by sea water during high tide and exposed during low tide. Sediment structures that commonly form on tidal flats include symmetrical ripples due to bi-directional tidal flows, flat-topped ripples when ripple tops are flattened during flow reversals, and flaser bedding where clay drapes form over silty sediments due to variable energy conditions. Burrows left by organisms moving between surface and subsurface areas during tides are also common, as are root casts from salt-tolerant vegetation and animal tracks.

1. Tidal Systems
Tidal flats represent the land area covered by sea water during high
tide and exposed to the air during low tide.

3. Photo by W. W. Little
Oscillation (symmetrical) Ripples
Because of bi-directional flow, symmetrical current ripples are very
common tidal structures.

4. Photo by W. W. Little
Flat-topped Ripples
Tidal ripples are often flattened as their tops are planed upon reversal of
flow direction. In some cases, a small flap of sediment will extend away
from the crest toward the trough in the direction of the reversed flow.

5. Photo by W. W. Little

6. Photo by W. W. Little
Flaser-bedded Mud
Because of highly variable energy conditions associated with tidal
flow, flaser bedding in the form of clay drapes over silty sediment is
common.

7. Photo by W. W. Little

8. Photo by W. W. Little
Interference Ripples
Surface ponding can lead to the development of oscillation ripples.

9. Photo by W. W. Little
Herring Bone Cross-bedding

10. Photo by W. W. Little

11. Dessicated Mud
Dessication cracks in mud can form during low tides.

12. Photo by W. W. Little
Burrowed Mud
Vertically-oriented burrows are common as organisms surface to
feed during high tide and return to the subsurface for safety at low
tide.

13. Photo by W. W. Little

14. Photo by W. W. Little
Root Casts
Salt tolerant vegetation, such as mangrove, commonly grow on tidal
flats, causing bioturbation through root growth.

15. Photo by W. W. Little
Tracks
Animal tracks, including migratory trackways, are common on tidal
flats.

16. Photo by W. W. Little

17. Photo by W. W. Little
Tidal Channels
Much of the drainage on a tidal flat enters the sea through a highly
complex system of tidal channels.

18. Photo by W. W. Little

19. Photo by W. W. Little

20. Photo by W. W. Little

21. Photo by W. W. Little
Algal Mats
Algal structures are common features in tidal zones.

22. Photo by W. W. Little

23. Photo by W. W. Little

24. Photo by W. W. Little
Large-scale Architecture
Tidal deposits are characterized by horizontally-laminated mud with
interspersed sand- or mud-filled channel forms.

25. Photo by W. W. Little

26. Photo by W. W. Little
Tidal deposits are often “bundled” into sand/mud couplets, related to
annual neap/spring tidal cycles.
Tidal Bundles (parasequences)