22 HORN KXPEDITIOX — PHYSICAL GEOGRAPHY. 



a few inches. On each side of the lake the sandhills rise to a height of fifty feet, 

 and have a nearly east and west trend. 



K— CLAYPANS. 



After the description of the lakes it is necessary to say a few words about 

 those miniature lacustrine features known as " claypans." They are usually in the 

 form of flat shallow depressions, often nearly circular, but in the majority of cases 

 of irregular outline, and usually devoid of vegetation. They are generally 

 surrounded by loamy plains or sandhills, and while they are more frequently met 

 with throughout the Cretaceous area, they are still not uncommonly found on the 

 plains and in the valleys of the ranges within the Ordovician area. Mr. Streich 

 suggests that claypans owe their existence* to an ascent of subterranean water at 

 the junction between the "sedimentary and the metamorphic formations," as he 

 found them numerous in the neighbourhood of the junction line. We, however, 

 observed no such relation, and give below our explanation of their formation. 



Claypans vary in diameter from a few feet up to as much as three-quarters of 

 a mile, a conmion size being from fifty to one hundred yards. They are exceedingly 

 shallow, the depth of the bottom below the general level of the surrounding sand- 

 hills or plains is seldom greater than live feet, and usually much less, being in the 

 majority of cases not more than two to three feet. In some cases the edge of the 

 depression is ill-detined, the plain merging almost imperceptibly into the claypan, 

 and the only indication of the circumferential limits is the ring marking the edge 

 of the deposit of line silt which covers the bottom and sides of the claypan. In 

 other cases the sandhills come to the very edge, and form a well-defined rim to it. 

 The area drained by them is limited to a very narrow peripheral belt. As a 

 general rule water does not remain in them very long ; some of the best hold water 

 for three or four months, but in the great majority of cases, especially of the smaller 

 ones, the water disappears from them at the end of a month or two. There are 

 exceptional instances, however, of which Conlon Lagoon is an example, in which 

 water would remain for very much longer periods. 



The water of the claypans has generally a reddish-yellow colour, due to the 

 presence of a quantity of very fine mud of that colour held in suspension, which 

 on evaporation of the water is deposited on the bottom of the claypan as a fine silt 

 with a peculiar glazed surface, due perhaps to the extreme fineness of the last 

 portion of the sediment deposited. On drying, this mud loses water and splits in 



" Trans. Roy. Soc. of South Australia, vol. xvi., part ii., p. 90. 



