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The western side of the mound, not affected by this process of erosion and deposition, reveals the height of the mound as it appeared during its use by the Hohokam. The raised ground level of the eastern plaza and the lowered eastern edge of the mound makes Mesa Grande appear smaller from the east.

Rains from the southwest rounded off and lowered the eastern edge of the mound and removed soil to fill up the eastern plaza. Have you noticed that the Mesa Grande mound looks much taller when viewed from the west side (the parking lot area) than it does from the east side? Like other ancient mounds, Mesa Grande erodes in an asymmetrical pattern that is determined by the prevalent direction of wind and rain. Eventually, through periods of cutting and filling, the gullies reach a relatively stable state and further erosion continues only at a very slow rate, minimizing the damaging effects of natural weathering.Ĭross section of Mesa Grande from the Eastern Plaza over the mound, showing the effects of mound erosion. Periods of deposition followed when sediments partially filled the channels. When the gullies first formed, largely by monsoon rains, they cut deeply into the mound. The gullies give the impression that the mound is eroding away before our very eyes! But excavations into these features tell a different story. Visitors viewing the Mesa Grande mound quickly notice the deep erosion gullies cutting into the mound. Today, even they have fallen from natural erosion and perhaps the impact of early visitors to the site.Įrosion Gully on the East Side of the Mound.Įrosion Gully on the West Side of the Mound. When John Bartlett first visited Mesa Grande in the 1850s and drew a picture of it, only a few standing walls remained. When archaeologists uncover buried walls their tops are badly eroded, reflecting the time when they were exposed to weathering. The remaining stubs of the walls were eventually buried and hidden from view. This process, known as basal erosion, weakens the base of the wall, which ultimately becomes unstable and collapses. This moisture then evaporates out of the face of the wall, leaving behind salt crystals that loosen particles of soil. The walls wick or carry up moisture from the ground. The greatest threat to walls occurred at their base. Temperature changes and wet and dry cycles caused expansion and contraction of walls, a process that can create cracks. Wind, especially high winds carrying small particles of soil, also creates erosion. Water ran down wall surfaces and cut grooves. Water penetrated the face of the wall, causing chemical weathering and the leaching or physical removal of materials. As rain drops struck the walls, they dislodged small grains of soil. Rain was the strongest agent of weathering. As the wooden roofs of rooms gave way, the standing walls began to deteriorate. When Mesa Grande fell into disuse in the 1400s the natural forces of wind and rain began a cycle of erosion. It was known as the Mesa Pond.Įrosion undercutting a wall at Mesa Grande. Early Mesa residents filled the pit with water from the historic canal to the north. Now only a small remnant survives but at one time it was a large open pit. A very large barrow pit was located to the northwest of the mound, just beyond the large tamarisk tree where today is located the reproduction of a Hohokam ballcourt. The Hohokam excavated large holes in the ground, referred to as barrow pits, to extract the caliche. This material is so durable that finger impressions of the Hohokam workers who built the walls are often still preserved in the plaster. By sifting out the course material, builders produced a fine plaster and used it to coat the walls of the mound. The ancient builders used caliche in the walls of the mound and to create the floors in rooms. It contains calcium carbonates, minerals that are leached down through the soil by rain and deposited as a "hard pan." The Hohokam mined this material and mixed it with water for use as a building material. Natural layers of caliche in arid desert soils from Texas.Ĭaliche is a hard, white deposit that forms under the ground.