46 Transactions of the Academy of Science of St. Louis 
Differential resistance has important influences upon the 
other aspects of the physical environment. In the Ozarks with 
their humid continental climate, the action of stream erosion 
upon the different rocks has produced various topographic ex- 
pressions. Where the differential resistance between two adjacent 
rock masses is low, relatively low angles of slope are typical, 
regardless of the individual durability of the rocks. However, 
where the differential resistance is high, the land slopes may be 
either steep or gentle, depending upon the order of superposition 
of the beds. If a durable stratum overlies a weaker, dissection 
produces high angles of slope; but if the less resistant is the 
cap, low angles of slope are typical. In addition to this, the 
realtive positions of resistant and non-resistant strata influence 
the rate of topographic development. For example, where the 
more durable layer forms the cap, level uplands are dissected 
slowly; whereas, where the weaker layer is uppermost, the up- 
lands soon become rounded and broad valleys may develop. 
Structure, together with relative resistance, plays an im- 
portant part in determining topographic features, patterns of 
streams and outcrops, and the areal distribution of minerals. 
Over most of the Ozarks dendritic streams acting upon near- 
horizontal strata have produced intaglio patterns of outcrops, 
but in the southeast, steeper dips have resulted in banded out- 
crops, cuestas, and the rudiments of trellis drainage. 
The relative porosity and permeability of bed and mantle 
rock influence topographic texture, valley forms, and drainage 
character. Impermeable materials typically develop fine topo- 
graphic textures; permeable commonly have coarse. Where pet- 
meable mantle rock overlies limestone, drainage through the 
regolith leads to the development “of solution valleys which lack 
permanent surface streams. Permeable bed rock increases sub- 
surface drainage, and, where the rock is soluble limestone, may 
result in the development of karst topography. 
