From
World War II, commonly known as just a
hedgehog, although it is not to be confused with the
anti-submarine weapon by that name.
A Czech Hedgehog was a man-made obstacle approximately 6 feet in height designed in World War II to impede or halt altogether the movement of landing boats onto the shores of France and Belgium. Hedgehogs consisted of three steel rails which were riveted together in the form of a tripod (kind of resembles the jacks from the childrens game) and were used as part of the defense of The Atlantic Wall in conjunction with "Belgian Gates" and log ramps. Some areas of France and the Netherlands were covered with more hedgehogs than other beaches. Depending on where they believed the Allies were going to land
The hedgehogs were also designed to also rip the hull of any ships who came to close to the shoreline or to just pierce through the ship if it unexpectedly rammed into the hedgehog.
They were usually laid out in shallow water, which is why D-Day was set on a day when the tide was low, so that most landing craft could avoid ramming into one.
Although it is arguable how effective these obstacles were, they did in fact make it easier for the Germans to shoot at the landing craft and the few amphibious tanks brought over during the initial attack. However, the hedgehogs also served as protection for Allied soldiers who hid behind them for protection from machine gun fire.
It also forced the Allies to land on a day of low tide so that landing craft could avoid them much easier.
As the Allied Forces were slowly overtaking the beachheads, combat engineers began blowing up all obstacles on the beach to allow landing craft to safely arrive to shore.
name of obstacle and description of them as jacks taken from http://www.sungenweb.net/lst_normandy.htm