TOPOGRAPHY ALONG ESTRADA CENTRAL OF PERNAMBUCO 77 

 TOPOGRAPHY IN DETAIL AND THE ROCK EXPOSURES 



From the central station, in the city of Pernambuco, to within half a 

 kilometer of Areias station, the railway passes over a flat country, much 

 of it covered with mangrove swamps. Half a kilometer east of Areias 

 the road cuts a sand bank 5 or 6 meters in height. This bank is the 

 margin of the flat sand- covered plain around Areias station. A few 

 hundred meters east of Areias station the railway crosses a narrow 

 steep-sided, flat-bottomed valle}'. West of Areias and east of Tigipio 

 station is another valley of similar shape, draining into the Capibaribe. 

 The valleys around the margin of this sand plain are dendritic in form 

 and belong to a single type. The following sketch shows a cross-section 

 of one of them: 



These peculiar features are interpreted to mean that the Areias plain 

 formerly stood at a greater elevation than at present, and at the time of 

 this elevation steep-sided 



V 



Figure 12.— Profile of the Hills at Areias. 



gullies or narrow winding 

 valleys were cut by the 

 streams around the mar- 

 gins of the plain. A sub- 

 sequent depression carried the bottoms of these narrow valleys beneath 

 the salt water, whereupon they were immediately silted up. 



Three hundred meters west of Areias station the railway line cuts the 

 colored Tertiary sedimentary beds. These beds are red ; most of them 

 contain. small quartz pebbles that are scattered through the strata rather 

 than arranged in well-marked bands. At Tigipio a cut on the north 

 side of the road shows waterworn pebbles in approximately horizontal 

 bands. 



At this same station the bottom of the creek valley is flat, as if be- 

 longing to the dendritic group around the Areias plain. The low hills 

 east of Tigipio are of about even height. West of Tigipio the hills are 

 at once higher — perhaps 70 meters higher than the railway. The Ter- 

 tiary beds, such as are exposed at Dois Irmaos, Caxanga, and Macacos, 

 on the north side of the open plain about Pernambuco, are but little 

 exposed along the line of the Estrada Central. Beginning a short dis- 

 tance west of Areias, the}^ are exposed to and at Tigipio station and end 

 2 or 3 kilometers west of there; even so they appear mostly in the tops 

 of the hills at this western end of the beds. Two kilometers west of 

 Tigipio a railway cut exposes a horizontal bed of waterworn gravel. 

 Between 2 and 3 kilometers west of Tigipio decayed crystalline rocks 

 are exposed in a cut on the south side of the track. These rocks, how- 

 ever, are exposed only in the lower portion of the cut. They are over- 



