A geosyncline may be defined as “a thick, rapidly accumulating body  or  sediment formed within a long, narrow, subsiding belt of the sea which is usually parallel to a plate margin”. (Oxford Dictionary of Geography). Or we   may say a geosyncline is a “very large linear depression or down-warping of  the earth’s crust, filled (especially in the central zone) with a deep layer of sediments derived from the land masses on each side and deposited on the  floor of the depression at approximately the same rate as  it  slowly,  continuously subsided during a long period of geological time”.