Excerpted from Oddities of the Mineral World (Van Nostrand Reinhold Company, 1976.)
No discussion of concretions would be complete if mention were not made of a real puzzler, a truly unique type of concretion that apparently remains unexplained—sand spikes...
Briefly, the locality consisted of a series of low, sandy hillocks and banks near the Mexican-American border close to Mt. Signal, Imperial County, California. When they were first discovered, many of these unique concretions were weathering out on the surface of the ground. It became apparent, however, that there actually were beds of these strange concretions 3 feet to 8 feet underground!
These concretions consist of a ball-like end coupled with a tapering spikelike formation. In uncovering a bed of spikes over 95 percent of the formations were found pointing west. Spikes of similar types or formats seemingly occurred in the same bed or within lenslike concentrations of the sand-spike formations...
They are composed of absolutely nothing but the identical sand forming the soft little hillocks and banks in which they occur—solid sandstone with no fossil material inside. The mineral cementing the grains together is apparently calcite.
What are they? No one really knows—other than they obviously fall under the heading of concretions by virtue of their method of occurrence and basic mineralogical composition. The long points, westerly oriented, are the most provocative aspect of this truly unique locality. Some say that they are fossils of an ancient type of bulbous seaweed that drifted heavy-end eastward on some ancient shoreline... It has also been suggested that they are some type of fulgurite, which is positively incorrect, and not by any stretch of the imagination are they prehistoric man-made artifacts.See also this 1934 article in Rocks and Minerals and this one from the American Journal of Science in 1936.
Via Allan McCollum and The Agateer (newsletter of the Madison Gem and Mineral Club).