Major advances in architecture, art, and technology were made during the subsequent Old Kingdom, fueled by the increased agricultural productivity and resulting population, made possible by a well-developed central administration. Some of ancient Egypt's crowning achievements, the Giza pyramids and Great Sphinx, were constructed during the Old Kingdom. Under the direction of the vizier, state officials collected taxes, coordinated irrigation projects to improve crop yield, drafted peasants to work on construction projects, and established a justice system to maintain peace and order. Along with the rising importance of a central administration there arose a new class of educated scribes and officials who were granted estates by the pharaoh in payment for their services. Pharaohs also made land grants to their mortuary cults and local temples, to ensure that these institutions had the resources to worship the pharaoh after his death. Scholars believe that five centuries of these practices slowly eroded the economic power of the pharaoh, and that the economy could no longer afford to support a large centralized administration. As the power of the pharaoh diminished, regional governors called nomarchs began to challenge the supremacy of the pharaoh. This, coupled with severe droughts between 2200 and 2150 BC, is assumed to have caused the country to enter the 140-year period of famine and strife known as the First Intermediate Period.