HORN EXPEDITION PHYSICAL GKOriRAPIIY. 23 



all directions, cr.-u-ks as imicli as a ([uartei' to lialf an incli wide making their 

 appearance and sepai'atini;' tlie more Of less rectangular masses from eacli other. 



The following explanation of the method of their formation has suggested 

 itself to us. 1'hey naturally occur only where the country is flat, i.e., wliere the 

 slope of the ground is not decided enough in any particular direction to cause the 

 surface water to flow in that direction. There is therefore a tendency for the 

 water to lie on the surface, or rather to be gathereil into slight depressions, which 

 are sure to exist even on otherwise almost level surfaces. At first the water that 

 was gatliered into these slight depi'essions would almost immediately percolate the 

 porous strata, but in doing so it would leave behind a deposit of silt. Tiiis would 

 happen with every subsequent heavy fall of rain, until tiie silt suspended in the 

 water and carried into the depression and deposited there was in sufficient cpjantity 

 to prevent further percolation. 



The claypan lias now become established and will retain watei' for ;i longer or 

 shorter period, and as there is now very little percolation thr-ough the bottom tiiere 

 will 1)(^ no furthei' settling of the floor as there may have been in the eaily stages. 

 The depth of the claypan in many cases will, however, bo increased by tlie growth 

 of fhe rim by the deposition at the water's edge of the fine particles of saiul, etc., 

 driven along the surface of tli(> ground by the wind. In this m.anner the rim may 

 be added t(j from time to time, and t\w. holding capacity of the claypan theieby 

 increased. 



The largest exam})le of a claypan seen during the Expedition is one that 

 occurs a few miles to the south of lleavitree CJap, aiul known locally as (Jonlon 

 Lagoon. In its greatest dimensions it is a quartei- of a mile wide aiul three- 

 quarters of a mile long. The depth of the bottom of the lagoon below the 

 surrounding country does not exceed four or five feet, and as tiiere was at the time 

 it was visited not more than three feet of water in it, it was already in process of 

 drying up. Where dry the bottom was seen to be formed of a clayey sand. 

 Conlon Lagoon lies between two ranges trending east and west, the drainage 

 from wiiich finds its way into the lagoon. There is, however, no indication of an 

 outflow eastwards to the Todd (the only possible direction), tlu; lagoon .and the 

 ranges on the nortii and south forming a miniature internal drainage .system of 

 their own. 



F.— STONY PLAINS. 



The whole of the area from the noithern extiemity of Flinders Range north- 

 wards to within a few miles of .Mount, P)arrell Cattle Station on the Jlugli Hiver, 



