They may shelter from rain under overhanging rocks and behind boulders. The boulder gardens consist of several large boulders linked by winding pathways. It nests in open or hidden crevices among cliffs and boulders. Retreating glaciers leave behind silt, clay, sand, gravel, and boulders called drift. This category includes pebbles, cobbles and boulders. The area is strewn with glacial erratic boulders. The Olmec colossal heads are stone representations of human heads sculpted from large basalt boulders. It has uneven bottom covered with boulders of volcanic rock. Broken trees, boulders, and mud were deposited. Artificial climbing walls allow boulderers to climb indoors in areas without natural boulders. Parts of Foothill Boulevard were buried under of mud, boulders and debris. The climbing of large boulders is called bouldering. Steelhead use aquatic obstructions like vegetation, boulders, and fallen trees as protection. It was made of oak piles driven into the seabed with boulders stacked between. It is situated at the top of stone boulders.