overburden

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive verb To burden with too much weight; overload.
• transitive verb To subject to an excessive burden or strain; overtax.
• noun An excessive burden; an overload.
• noun Material overlying a useful mineral deposit.
• noun Sedimentary rock covering older crystalline layers.
• noun Archaeology A sterile stratum overlying a stratum bearing traces of the culture being studied.

from The Century Dictionary.

• To load with too great burden or weight; overload; overtask: as, trees overburdened with fruit.
• noun Detrital material or rock which has to be removed, as being of no value, in order to get at some valuable substance beneath, which it is intended to mine or quarry: used in reference to quarrying or excavating clay and similar materials.
• noun An excessive load; a burden or load greater than should be carried.

from the GNU version of the Collaborative International Dictionary of English.

• noun The waste which overlies good stone in a quarry.
• transitive verb To load with too great weight or too much care, etc.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb To overload or overtax
• noun geology The rock and subsoil that lies above a mineral deposit such as a coal seam.
• noun archaeology A sterile stratum that lies above the stratum being investigated

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb burden with too much work or responsibility
• verb load with excessive weight
• noun the surface soil that must be moved away to get at coal seams and mineral deposits
• noun an excessive burden