Boulder detached rock mass larger than a cobble, having a diameter greater than 256 mm (8 phi units or about the size of a volleyball) and somewhat rounded or otherwise distinctively shaped by abrasion in the course of transport; the largest rock fragment recognised by sedimentologists. Alternatively, a general term for any rock that is too heavy to be lifted readily by hand. A boulder bed is a boulder-bearing conglomerate that is a glacial deposit.
In Bangladesh wide areas of greater rangpur and dinajpur districts are covered with boulders of mostly sedimentary origin. On the other hand boulders of the Jaintiapur (Sonatila) and Bholaganj areas of greater sylhet are mostly of igneous and metamorphic origin. Apart from these numerous hill streams also deposit boulders on the streambeds of the hill ranges and in the plains close to these ranges. These hill ranges are mostly located in the Sylhet, chittagong and chittagong hill tract regions. In teknaf, boulders occur in three localities as plainland deposit, and these are: Silbunnapara-Khankarpara-Dalepara-Waliabad in Teknaf; between Moderbunia chhara and Dak chhara near Berdeil village and Inani, ukhia (near Md Shafi Village). Here the boulder deposits of hard calcareous sandstone are found to be scattered in the plainland covering agricultural land and habitations and are found to be protruding on the surface. Apart from calcareous and ferruginous sandstone boulders, sometimes-conglomerate boulders are found on the streambeds. Also on the Teknaf-Cox's Bazar sea beach seven separate occurrences of boulders are present between Moderbunia chhara and Rajar chhara. They are aligned parallel to the shoreline and are present between the high tide and low tide zones. The surface of the boulders are highly altered and covered with shells. [Sifatul Quader Chowdhury]