ALTAMAHA GRIT REGION OF GEORGIA 27 



for instance, thus describes a station for Ceratiola ericoides: 

 "Near Murphy's Bridge on the Edisto it covers a space of three 

 of four hundred yards in width, and two or three miles long, 

 which appears to have been a sand-bank formed by some of the 

 ancient freshets of that river, and on which only lichens, and a 

 few stunted oaks (Q. Catesbczi and nigra) are found intermingled 

 with it." 



At the base of most sand-hills in our territory is a densely 

 wooded area known as a " hammock. " 1 A hammock can scarcely 

 be classed as a topographic feature, however, since it is character- 

 ized by its vegetation rather than by topography. The vegeta- 

 tion of hammocks, and of the peculiar intermediate forms known 

 as sand-hammocks, will be discussed at the proper place. 



Although the topography in the Altamaha Grit country, as 

 in most other parts of the world, has doubtless been determined 

 almost entirely by erosion, yet there is almost no trace of any 

 erosion going on there at the present time. There are several 

 reasons for this. Before the Columbia period there must have 

 been times when the Lafayette loam which then formed the sur- 

 face was being worn down quite rapidly in places, giving the 

 topography approximately the form it has to-day. But now 

 the porous Columbia sand allows rain-water to sink into the 

 ground almost immediately without disturbing the surface, 

 while at the same time the impervious Lafayette just beneath 

 protects the underlying rocks from decay. Furthermore, the 

 smaller streams are usually so filled with trees, shrubs, and the 

 humus derived from them that they cannot deepen their chan - 

 nels appreciably. Thus we have a topography of unusual 

 stability. The impotence of erosive forces is shown by the 

 appearance of the streams. The branches, creeks, and rivers 

 rising in this region (and in other parts of the coastal plain cov- 

 ered with the Columbia sands) are rarely or never muddy. The 

 only sediment they carry ordinarily is finely divided vegetable 

 matter, which gives the water a blackish appearance, just as in 

 the rivers of the glaciated region where analogous soil conditions 



1 For a discussion of the orthography, definition, and geographical dis- 

 tribution of this word see Science II. 22: 400-402. Sept. 29, 1905. 



