NAVIGATION:  BACK TO MODULE TWO INTRODUCTION

Where's the Soil - Weathering-Limited vs. Transport-Limited Surfaces

Look Ma, no soil!  There are two general types of " slopes (surfaces) that often coexist on the Colorado Plateau (however, more than likely only weathering nerds - Like Brandon and John - and GPH211 students notice them).  The climate and geomorphic and geologic conditions are just right to have both.  The two types of slopes are those that are weathering-limited and those that are transport-limited.  The main difference between the two is soil or sediment cover - areas with an abundance of weathered material just laying around are transport limited surfaces whereas areas with no sediment laying around are weathering-limited.  

On the weathering limited areas in Figure One, the rock is too steep (and the climate is not just right) for the weathered material (sand grains) to collect.  The sand that weathers away from the rock face falls or is somehow transported down to the transport limited area...  Here (in the valley floor and along the base of the scarp), the rate of erosion is less than the rate of sediment production.  Erosive processes (running water, wind) can't keep the sand "swept away" in the flat topographic lows (valley floor) like it can on the steeper rock surfaces (in the alcove and along the steep faces of rock).

Figure One - Weathering-limited areas on this image have no soil - the particles are removed by erosion BEFORE soil can develop.  The areas with soil (the sandy section with sage bushes) are transport-limited; that is, the material that has weathered is not being carried away (transported) before being integrated into the soil.

To summarize the two types of slopes, 

• Weathering-Limited Slopes:  Rates of soil/regolith production are less that rates of erosion 
• Transport-Limited Slopes:  Rates of weathering are more rapid that rates to transport