Harry R. Schwartz

In general, the geometric figures are formed by a ditch approximately 11m wide, currently 1-3m deep, with adjacent 0.5-1m high earthen banks, formed by deposition of the excavated soil. Ring ditches have diameters that vary from 90 to 300m. The circular structures are more common in the south, while composite and rectangular structures become more frequent as one moves north (see Figure 2). When there are two or more structures, they are usually connected by embanked roads. Some of the single rectangular structures may have short roads coming out of their mid-sides or corners. Composite figures include a rectangle inside a circle or vice versa.

—Pre-Columbian geometric earthworks in the upper Purús: a complex society in western Amazonia, Pärssinen, Schaan & Ranzi

Pretty cool paper; they used Google Earth to find geoglyphs!