EXHIBIT 5.1

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


                                December 8, 2003


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

     Re:  Registration Statement on Form S-4

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 assumed 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.

     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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