With Japanese shipping becoming more and more hard-pressed, enemy use of "home-made" offshore obstacles requiring little or no imported material is certain to become increasingly widespread. Concrete, cement, and angle-iron obstacles still will be encountered, but many beaches in outlying Japanese-held territory cannot be supplied with these luxury items. Instead, obstacles requiring either no imported material at all, or little more than barbed wire and nails, will be employed in the preparation of outlying beach and offshore defenses. The following sketches, based on U.S. Navy analyses of low oblique photographs, show the principal types which will be encountered.

Figure 10. Rock Mounds.

These mounds, approximately 3 feet high and 10 to 12 feet square, are staggered at 10 to 15 feet intervals on the outer edges of reefs. Such mounds may contain mines.

Figure 11. Wire Fences.

Fences of this type consist of barbed wire strung on a series of upright posts, often interspersed with "spiders" and light chevaux-de-frise.

Figure 12. Fish Weirs.

Constructed by natives and not intended primarily for defense, these coral-rock fish weirs are arrow-shaped. The arrow averages 200 yards in width at the base of its head, while the length of the shaft may be as much as 500 yards. The weirs, which usually point seaward, may be employed on reef flats off any type of shore line, and anywhere from close inshore to the extreme outer edge of a flat. A variation, termed a "turtle trap," occurs in a saw-tooth arrangement along the outer edge of a reef.

Figure 13. Post Lines.

Two general types of posts have been observed: short, stout vertical logs spaced at intervals of approximately 5 feet along the shore; and taller posts—similarly spaced, but of lighter construction and inclined seaward—usually situated further offshore.

Figure 14. Rock-filled Cribs.

Log cribs, approximately 10 feet long and 4 feet wide, have been filled with coral rocks, and spaced at intervals of about 5 feet.

Figure 15. Tetrahedra.

Most of the examples of this type appear to be made of wood, although some may be made of steel. They often are partly, or completely, filled with rocks.

Figure 16. "Spiders."

A variation of a tetrahedron, a "spider" is constructed of logs or poles, either braced by cross members at the base and weighted with rocks, or anchored with its ends buried in the sand. "Spiders" are spaced at intervals of from 15 to 30 feet, and often are incorporated in wire fences.

Figure 17. Rock-filled Pillars.

These forms are connected by barbed wire, and may also be connected by a cable.

Figure 18. Log Barricades.

Barricades are constructed of logs or poles, and sometimes are strung with barbed wire. Spaced at intervals of 6 to 10 feet, they usually occur just offshore.

Figure 19. Rock Walls.

Rock walls, approximately 4 feet high and from 3 to 4 feet wide, have been observed in sections as well as in continuous lines. In some instances they may be topped with wire.

Figure 20. Rock-filled Mesh Wire.

The Japanese fill rolls of mesh wire with rocks, and arrange them in a line along the outer edge of a reef. (Obstacles of this type were encountered at Guam.)