EXHIBIT 5.1

                           Kirkpatrick & Lockhart LLP
                            Henry W. Oliver Building
                              535 Smithfield Street
                              Pittsburgh, PA 15222


                                October 31, 2003


Arch Western Finance, LLC
One CityPlace Drive, Suite 300
St. Louis Missouri 63141

         Re:  Registration Statement on Form S-4
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Ladies and Gentlemen:

         We have acted as counsel to Arch Western Finance, LLC, a Delaware
limited liability company (the "Company"), Arch Western Resources, LLC, a
Delaware limited liability company ("Arch Western Resources"), Arch of Wyoming,
LLC, a Delaware limited liability company ("Arch of Wyoming"), Mountain Coal
Company, L.L.C., a Delaware limited liability company ("Mountain Coal"), and
Thunder Basin Coal Company, L.L.C., a Delaware limited liability company
("Thunder Basin," and together with Arch Western Resources, Arch of Wyoming and
Mountain Coal collectively, the "Guarantors") in connection with the
Registration Statement on Form S-4 (File No. 333-107569) (the "Registration
Statement") filed by the Company and the Guarantors with the Securities and
Exchange Commission under the Securities Act of 1933, as amended (the
"Securities Act"), relating to the registration by the Company of $700,000,000
aggregate principal amount of its 6 3/4% Senior Notes due 2013 (the "Exchange
Notes") and the guarantees of the Exchange Notes by the Guarantors (the
"Guarantees"). The Exchange Notes and the Guarantees are proposed to be issued
in accordance with the provisions of the Indenture (the "Indenture"), dated as
of June 25, 2003, by and among the Company, the Guarantors and The Bank of New
York, as Trustee.

         In connection with rendering the opinions set forth below, we have
examined the Registration Statement, the Prospectus contained therein, the
Indenture, which is filed as an exhibit to the Registration Statement, the
respective Certificates of Formation and Limited Liability Company Agreements of
the Company and the Guarantors and resolutions adopted by the Board of Directors
of the Company, and we have made such other investigation as we have deemed
appropriate. We have examined and relied on certificates of public officials. We
have not independently established any of the facts so relied on.

         For the purposes of this opinion letter we have made the assumptions
that are customary in opinion letters of this kind, including the assumptions
that each document submitted to us is



accurate and complete, that each such document that is an original is authentic,
that each such document that is a copy conforms to an authentic original, and
that all signatures (other than signatures on behalf of the Company or the
Guarantors) on each such document are genuine. We have further assumed the legal
capacity of natural persons, and we have assumed that each party to the
documents we have examined or relied on (other than the Company and the
Guarantors) has the legal capacity or authority and has satisfied all legal
requirements that are applicable to that party to the extent necessary to make
such documents enforceable against that party. We have not verified any of those
assumptions.

         We are opining herein as to the effect of the laws of the State of New
York and the Delaware Limited Liability Company Act. We are not opining on, and
we assume no responsibility for, the applicability to or effect on any of the
matters covered herein of any other laws, the laws of any other jurisdiction, or
the local laws of any jurisdiction.

         Based on the foregoing, and subject to the foregoing and the additional
qualifications and other matters set forth below, it is our opinion that the
Exchange Notes and the Guarantee, when (a) the Company's outstanding 6 3/4%
Senior Notes Due 2013 have been exchanged in the manner described in the
Registration Statement, (b) the Exchange Notes and the Guarantees have been duly
executed, authenticated, issued and delivered in accordance with the terms of
the Indenture, (c) the Indenture has been duly qualified under the Trust
Indenture Act of 1939, as amended, and (d) all applicable provisions of "blue
sky" laws have been complied with, will constitute valid and binding obligations
of the Company and the Guarantors, respectively, enforceable against the Company
and the Guarantors, respectively, in accordance with their terms, under the laws
of the State of New York which are expressed to govern the same, except as the
enforcement thereof may be limited by bankruptcy, insolvency, reorganization,
moratorium (including, without limitation, all laws relating to fraudulent
transfers), other similar laws relating to or affecting enforcement of
creditors' rights generally, general principles of equity (regardless of whether
enforcement is considered in a proceeding in equity or at law) and limitations
of the waiver of rights under usury laws, and will be entitled to the benefits
of the Indenture.

          We express no opinion as to the validity, legally binding effect or
enforceability of any related provisions of the Indenture or the Exchange Notes
that require or relate to payment of any interest at a rate or in an amount
which a court would determine in the circumstances under applicable law to be
commercially unreasonable or a penalty or a forfeiture. In addition, we express
no opinion as to the validity, legally binding effect or enforceability of the
waiver of rights and defenses contained in the Indenture.


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         The foregoing opinions are rendered as of the date of this letter. We
assume no obligation to update or supplement any of such opinions to reflect any
changes of law or fact that may occur.

         We hereby consent to the reference to us in the Registration Statement
under the caption "Legal Matters."

                                      Yours truly,

                                      /s/ KIRKPATRICK & LOCKHART LLP






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