siliciclastic rocks initially form as loosely packed sediment deposits including gravels, sands, and muds. the process of turning loose sediment into hard sedimentary rocks is called lithification. during the process of lithification, sediments undergo physical, chemical and mineralogical changes before becoming rock. the primary physical process in lithification is compaction. as sediment transport and deposition continues, new sediments are deposited atop previously deposited beds, burying them. burial continues and the weight of overlying sediments causes an increase in temperature and pressure. this increase in temperature and pressure causes loose grained sediments become tightly packed, reducing porosity, essentially squeezing water out of the sediment. porosity is further reduced by the precipitation of minerals into the remaining pore spaces. the final stage in the process is diagenesis and will be discussed in detail below.