Also, many temples and tombs have survived because they were built on high ground unaffected by the Nile flood and were constructed of stone. Thus, our understanding of ancient Egyptian architecture is based mainly on religious monuments, massive structures characterized by thick, sloping walls with few openings, possibly echoing a method of construction used to obtain stability in mud walls. In a similar manner, the incised and flatly modeled surface adornment of the stone buildings may have derived from mud wall ornamentation. Although the use of the arch was developed during the fourth dynasty, all monumental buildings are post and lintel constructions, with flat roofs constructed of huge stone blocks supported by the external walls and the closely spaced columns. Exterior and interior walls, as well as the columns and piers, were covered with hieroglyphic and pictorial frescoes and carvings painted in brilliant colors. Many motifs of Egyptian ornamentation are symbolic, such as the scarab, or sacred beetle, the solar disk, and the vulture. Other common motifs include palm leaves, the papyrus plant, and the buds and flowers of the lotus. Hieroglyphs were inscribed for decorative purposes as well as to record historic events or spells. In addition, these pictorial frescoes and carvings allow us to understand how the Ancient Egyptians lived, statuses, wars that were fought, and their beliefs. This was especially true in recent years when exploring the tombs of Ancient Egyptian officials. Ancient Egyptian temples were aligned with astronomically significant events, such as solstices and equinoxes, requiring precise measurements at the moment of the particular event. Measurements at the most significant temples may have been ceremonially undertaken by the Pharaoh himself. Columns As early as 2600 BC the architect Imhotep made use of stone columns whose surface was carved to reflect the organic form of bundled reeds, like papyrus, lotus and palm; in later Egyptian architecture faceted cylinders were also common. Their form is thought to derive from archaic reed-built shrines. Carved from stone, the columns were highly decorated with carved and painted hieroglyphs, texts, ritual imagery and natural motifs. Egyptian columns are famously present in the Great Hypostyle Hall of Karnak (circa 1224 BC), where 134 columns are lined up in 16 rows, with some columns reaching heights of 24 metres. One of the most important type are the papyriform columns. The origin of these columns goes back to the 5th Dynasty. They are composed of