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(1) Rock fragments more than 10, 25, or 50 cm in size, depending on the classification. Often, 1 m is taken as the upper limit for boulders (larger fragments being called blocks). Sometimes the term is applied only to spherical or rounded fragments.

Boulders are a major component of psephitic rocks (pebbles, shingle, and conglomerates); they occur frequently in alluvium and proluvium of mountainous countries and in glacial deposits. They are used under the name cobble (rubble) in the paving of roads and the production of crushed stone.

(2) Glacial boulders are rock fragments, irrespective of size, picked up and transported by glaciers; they are often rounded, polished, and covered with scratches (glacial striae). They occur in moraines and in glaciofluvial deposits. They are sometimes found in large quantities in areas of former glacial activity.

References in classic literature ?
As I topped a great boulder I saw the herd of plant men surrounding a little group of perhaps five or six green men and women of Barsoom.
I jumped down and gave another ten minutes to the back-breaking work of carrying more boulders from the water to the wall.
Holmes sat down on a boulder and rested his chin in his hands.
Beside him was a small, flat boulder, not larger than the deck of a ten-man flier, nor standing to a greater height than twice his own stature.
In the instant that I arose I had seen the hyaenodon lying asleep beside a boulder a few yards away.
For a time Werper hid behind one of the lesser boulders that were scattered over the top of the hill, but, seeing or hearing nothing of the Englishman, he crept from his place of concealment to undertake a systematic search of his surroundings, in the hope that he might discover the location of the treasure in ample time to make his escape before Tarzan returned, for it was the Belgian's desire merely to locate the gold, that, after Tarzan had departed, he might come in safety with his followers and carry away as much as he could transport.
The crowning feature of the room was a huge fireplace of rough stones and boulders.
Far below the jagged points of the boulders bristled up, dark and menacing.
It seemed to wind around the mound from top to bottom, like a cork-screw, twisting here and there between the rough boulders but always remaining level and easy to walk upon.
She loved the gentle, misty harbor shore and the silvery, wind-haunted sand shore, but best of all she loved the rock shore, with its cliffs and caves and piles of surf-worn boulders, and its coves where the pebbles glittered under the pools; and it was to this shore she hied herself tonight.
Hartung to inquire whether he had observed erratic boulders on these islands, and he answered that he had found large fragments of granite and other rocks, which do not occur in the archipelago.
The sixth volume of the 'Geological Transactions' contains two papers of mine on the Erratic Boulders and Volcanic Phenomena of South America.