151. Enormous changes in materials and techniques of construction within the last few generations have made it possible to enclose space with much greater ease and speed and with a minimum of material.
152. Progress in this area can be measured by the difference in weight between buildings built now and those of comparable size built one hundred ago.
153. In the past, whole cities grew from the arduous task of cutting and piling stone upon.
154. The doorways and windows are made possible by placing over the open spaces thick stone beams that support the weight from above.
155. A structural invention had to be made before the physical limitations of stone could be overcome and new architectural forms could be created.
156. It works in compression to divert the weight above it out to the sides, where the weight is borne by the vertical elements on either side of the arch.
157. The doorways and windows are made possible by placing over the open spaces thick stone beams that support the weight from above.
158. This region has a semiarid climate, and for 50 years after its settlement, it supported a low-intensity agricultural economy of cattle ranching and wheat farming.
159. In the early twentieth century, however, it was discovered that much of the High Plains was underlain by a huge aquifer (a rock layer containing large quantities of groundwater).
160. The Ogallala aquifer is a sandstone formation that underlies some 583,000 square kilometers of land extending from northwestern Texas to southern South Dakota.
